Attention-deficit/hyperactivity disorder (ADHD) diagnosis is based on reported symptoms, which carries the potential risk of over- or under-diagnosis. A biological marker that helps to objectively define the disorder, providing information about its pathophysiology, is needed. A promising marker of cognitive states in humans is pupil size, which reflects the activity of an ‘arousal’ network, related to the norepinephrine system. We monitored pupil size from ADHD and control subjects, during a visuo-spatial working memory task. A sub group of ADHD children performed the task twice, with and without methylphenidate, a norepinephrine–dopamine reuptake inhibitor. Off-medication patients showed a decreased pupil diameter during the task. This difference was no longer present when patients were on-medication. Pupil size correlated with the subjects’ performance and reaction time variability, two vastly studied indicators of attention. Furthermore, this effect was modulated by medication. Through pupil size, we provide evidence of an involvement of the noradrenergic system during an attentional task. Our results suggest that pupil size could serve as a biomarker in ADHD. This condition is characterized by inattention, impulsiveness and hyperactivity, and is becoming increasingly recognized that many patients continue having these types of difficulties in adulthood3. Most of ADHD symptoms are related to problems in behavioral and cognitive control, and have been attributed to a deficient dopaminergic signaling5. First-line treatment for ADHD is given by stimulants, mainly methylphenidate, and the usual second-line treatment is atomoxetine, a noradrenaline reuptake inhibitor6. Both kinds of drugs increase the catecholamine availability at synapses. This, together with evidence of a weak but consistent link between genetic polymorphisms associated to the catecholaminergic system and ADHD, prompted the widely accepted hypothesis that in this condition there is an underlying deficit in catecholaminergic neurotransmission7.Attention-deficit/hyperactivity disorder (ADHD) is the most prevalent childhood neuropsychiatric disorder8. Like any complex disorder, ADHD patients display clear heterogeneities at the clinical and biological levels. These differences probably explain why some patients do not respond to medication9. A biological marker that helps to objectively define the disorder, providing information about its pathophysiology and potential responses to medication, is needed10.Currently, diagnosis is being performed solely on the basis of observed behavior and reported symptoms, which carries the potential risk of over-diagnosis or under-diagnosis15. After controlling for stimulus luminance, pupil size correlates with task difficulty, emotional valence, physical effort, motor output and arousal states21. Independent findings suggest that these fluctuations in pupil size reflect the state of the brain norepinephrine (NE) system26. This system originates in the locus-coeruleus (LC) and projects throughout the cerebral cortex, hippocampus, thalamus and midbrain, among others29. Brain areas associated with attentional processing  receive particularly dense LC-NE innervations25. Data from animal models, both rodents and non-human primates, have shown a central role for the LC in selective attention25.A promising potential marker of cognitive states in humans is pupil sizeNowadays, pupil size can be monitored in a completely noninvasive way in humans, using a remote camera and infrared light. We hypothesized that if pupil size reflects the activity of the LC-NE system, which is one of the important attentional systems in humans, then pupil size is a potential and unexplored marker for attentional states in ADHD. If this is the case, changes in pupil size during an attentional task should reflect the behavioral differences observed between ADHD patients and control subjects.31. Indeed, unaffected siblings present deficits in this type of task, which are intermediate between ADHD and controls31, a finding that has led some authors to propose it as an endophenotype32. In our task, three images plus a distractor were sequentially presented and then a ‘probe’ image was shown. Subjects had to evaluate if the probe image had been presented before. ADHD patients performed the task in two separate sessions, on- and off-medication, which in all cases was methylphenidate. We found that pupil size changed during the probe presentation, the attentionally most relevant cue of each trial, and covaried with the subject’s variability in response time and performance. Furthermore, this effect was modulated by medication.We monitored pupil size from ADHD and control subjects during a visuo-spatial working memory task see Fig. . DeficitAs a first step, we performed the analysis using all the data  of the 28 ADHD children who performed the task twice, on-medication and off-medication). Here we described the data and compared between groups, both the behavioral and the physiological measures. As a second step, we specifically focused on the subset of subjects which performed the task on- and off-medication. In this way, we were able to assess for medication-associated changes on the same subject.2(d.f.=2) = 13.4, p = 0.0012). Post-hoc test performed through the Bonferroni method, showed differences only between ADHD and Control groups . We obtained the same differences via Tukey’s Honestly Significant Difference Procedure. When trials were parsed by cognitive load , the data confirmed that 2-dot arrays were harder for the subjects, across all groups. There were significant differences in performance levels for low and high cognitive loads, as assessed by a Wilcoxon signed-rank test . The plot in Fig. To assess for performance differences between groups, we first conducted a Kruskal-Wallis analysis of variance, which gave significant differences (χ2(d.f.=2) = 15.57, p = 0.0004). Post-hoc tests conducted using the Bonferroni method showed significant differences between ADHD and mADHD, and also between ADHD and Control, but not between mADHD and Control groups . We obtained the same differences via Tukey’s Honestly Significant Difference Procedure. As such, medication tended to “normalize” the pupil response of patients during the attentional task. Pupil diameter average magnitudes for the first three arrays presentations are shown in Supplementary Fig. Average pupil curves across trials and subjects Fig.  reliablyWe then evaluated associations between pupil diameter and two of the most relevant behavioral variables: performance and variability in reaction time. In all the combinations, these variables showed consistent associations, as assessed statistically by the Spearman correlation coefficient see Fig. . We usedFor a total of 17 subjects, we were able to collect one off-medication session (children suspended their medication 24 hours before performing the task) and a matching on-medication session for the same subject. We used these data to evaluate differences between the on-medication and off-medication conditions for the experimental variables, both physiological (pupil diameter) and behavioral .During on-medication sessions, children displayed a larger pupil diameter during probe presentation compared to off-medication sessions, as assessed by a Wilcoxon signed-rank test  see Fig. . PerformFinally, we analyzed the relations between pupil size and performance, and between pupil size and reaction time, specifically in the subset of subjects which performed the task in on- and off-medication conditions. To do this, we subtracted the mean variable values of the off-medication sessions from the mean values of on-medication sessions, separately for each subject. We added the prefix ‘delta’ (Δ) to the new variables representing the on-medicated minus off-medicated values. We then plotted these differences against each other See Fig. . For theThese results show that there could be an effect of medication on the observed pupil and reaction time changes, but in our data there is a potential confound with training, measured as session order.27. Specifically, pupil diameter covaries with neuronal activity in the LC-NE system, the so-called ‘arousal’ or ‘alerting’ network. The LC-driven norepinephrine modulation, which reaches almost the entire cortical surface, has a direct readout in pupil size23. This confluence of evidence from NE involvement in attentional circuits and control of pupil size prompted us to explore pupil size as a biological marker in ADHD.We present evidence supporting pupil changes during an attentional task as a promising biomarker for ADHD. It has been recently shown in animal experiments that pupil diameter tracks arousal and attentional states23, these results support the involvement of the cerebral NE system in ADHD.We tested ADHD and control children in a visuo-spatial working memory task Fig.  previous34. In this study, the correlations between these markers and pupil size were weaker in the on-medication group and display their lowest values in the control group. This association between pupil size and behavior seems to be restricted to the period after probe onset. For example, the sustained baseline increase across array presentations , while the non-ADHD group had a mean IQ of 105 (SD =  ± 7.3). All ADHD subjects were diagnosed as ADHD-Combined Subtype by a trained child neurologist according to the DSM-IV criteria40. All participants were also screened for neurologic or psychiatric comorbid conditions using a protocol that included parents’ interview, M.I.N.I. Kid test41, and general psychological and physical assessment of the children. The Conner’s Abbreviated Parent–Teacher Questionnaire41 is usually utilized to screen for symptoms of ADHD in the clinical setting. In the present study, it was used as an additional symptoms-counting tool. Participants with ADHD and those from the non-ADHD group were evaluated using this instrument as a controlling variable. Parents granted informed consent for their children’s participation, and children signed an informed assent form. The procedures in the study were approved by the Ethics Committee of the School of Medicine of the Pontificia Universidad Católica de Chile (Protocol number 11082), in accordance with the Declaration of Helsinski.A total of 50 subjects participated in the study. 28 subjects were patients diagnosed with ADHD . All ADHD children were being treated with methylphenidate. 22 subjects were non-ADHD assigned to our control group . A subgroup of 17 ADHD patients  performed the task twice, on- and off-medication (time between sessions: 180.23 ± 125.17 days). The order of the sessions (on- or off-medication) was randomized to avoid carry-over effects. In the on-medication session, ADHD subjects took their regularly prescribed dose 2–2.5 hr before the task, which is the time corresponding to the maximum drug concentration after oral administration42. The memoranda were 1- or 2-dot arrays, with the dots located variably in any of the sixteen spaces of a 4 × 4 grid  A task-related dot array; (2) a neutral natural image; and (3) an emotional image. Distractors were constructed and modified from public domain images, adapted from Vidal, Ossandon There were also two trial types, according to cognitive load: in low-load trials, only one dot was presented on each image, whereas in high-load trials, two dots were presented on each image. Therefore, in low-load trials subjects had to retain the location of three dots (one per image) and in high-load trials they had to retain the location of six dots (two per image).A total of 160 trials were presented on each session, separated in 8 blocks of 20 trials. Sessions usually lasted 30 minutes.Pupil diameter data was acquired with Eyelink 1000 , with a 1 kHz sampling frequency. Subjects sat in front of a table containing the computer screen for image presentation and the eye tracker device. During the task, the subjects kept their head in a forehead/chin rest (SR Research Ltd.). Subjects were placed at a viewing distance of 30 cm from the display monitor. We measured illuminance data with a lux-meter positioned in the same chin-rest used by the subjects, at the same distance from the screen. The only modestly large differences in illuminance occurred between delay and array displays. The distractors presented more variability in illuminance and had lower illuminance values, on average , and subjects delivered their responses using a keypad. Alongside checking for correct responses, we also recorded the reaction time as the time delay between the probe presentation and the subject’s response. This allowed us to calculate the variability in reaction time, a parameter known to correlate with cognitive performance in ADHD children46, implemented through Matlab function spline.Pupil diameter analysis was performed using Matlab® software with in-house functions. First of all, periods of blinks, in which no pupil diameter information was available, were detected by the Eyelink software. Pupil data surrounding blinks were removed from the time series used in the analyses. Pupil diameter during these periods was estimated using a cubic spline interpolation29, and a high-pass filter was applied to detrend the basal slow change of the pupil diameter across trials. Furthermore, outliers, defined as periods of pupil change (derivative function) higher than 3 standard errors from the mean were discarded. Finally, all trials with more than 50% of missing data (due to blinks or outliers) were not considered in the analysis. Unless otherwise stated, data analysis was restricted to the 8-s trial period . In this case, we first computed the t-statistic for the actual (non-permuted) data. We then implemented the permutations and calculated the t-statistic for each permutation. To assess significance, we compared the non-permuted t-value with the distribution of permuted t-values and calculated if it lied within 95% of the distribution . All tests were performed using the subject as the unit of analysis .All our statistical tests were implemented in Matlab® (release 2016a). Due to the non-normal nature of the data, we used non-parametric tests to assess statistically for differences. For paired comparisons, we used the Wilcoxon signed-rank test, with Matlab function The authors declare that the data gathered for this study are available within the paper. We also state that the data are available from the authors upon reasonable request.Supplementary Information


ADHD is a neurobiological disorder with a large worldwide prevalence causing significant impairment in children, adolescents, and adults. While there is general agreement about genetic contributions toward the disorder, progress in leveraging genetics to learn more about the biology and risk factors for ADHD has been limited. In this perspective, we identified 105 genes from the literature showing at least nominal statistical significance in association with ADHD. We analyzed these genes for enrichment in biological pathways and in known interacting biological networks. We also analyzed the expression patterns of candidate genes across brain regions and across periods of human development. From our analysis, we identify 14 genes that cluster within an interactive gene network, with enrichment in nitric oxide synthase and alpha-1 adrenergic pathways. Furthermore, these genes show enrichment for expression in the cerebellum during childhood through young adulthood, and in the cortex in adolescence and young adulthood. Gene discovery holds great potential for elucidating the unknown biological underpinnings of ADHD. Genome-wide sequencing efforts are underway and are likely to provide important insights that can be leveraged for new treatments and interventions. Attention-deficit/hyperactivity disorder (ADHD) is the most common neurodevelopmental disorder in childhood and is characterized by persistent and developmentally inappropriate hyperactivity, impulsivity and inattention . There aADHD etiology is believed to be multifactorial. It is known that genetics plays a significant role in the disorder; twin studies indicate the heritability value of ADHD to be 0.76 . In addiThis perspective will begin with a brief overview of treatment and structural neuroimaging findings, then will amalgamate and succinctly outline findings from genetic research studies of ADHD thus far. We will then incorporate these findings into a new analysis showing enrichment of ADHD genetic variants in particular biological pathways and networks. We advocate for continued updating of such analyses in the future, as more high confidence genetic risk variants are uncovered. These analyses will elucidate essential spatial, temporal, and biological network characteristics of ADHD. This knowledge is a prerequisite for advancing treatment and prevention efforts.Best Practice recommendations for ADHD treatment generally depend on the age of the individual. For preschool-aged children, cognitive behavioral therapy (CBT) is recommended as a first-line treatment, while for children aged 6 years and above, pharmacological treatment is advised . CommonlMost MRI studies of ADHD have been conducted on males, most likely due to the gender disparity of the disorder. There is evidence that the ADHD brain has a smaller global volume  and an op ≤ 5 × 10−8) via GWAS. This is not entirely surprising; given the heterogeneity and multifactorial nature of the disorder, very large cohorts are likely required to achieve this stringent level of significance. In 2017, Demontis et al. performed a meta-analysis of prior GWAS studies, which included 20,183 ADHD cases and 35,191 controls, to identify 12 loci surpassing the threshold of genome-wide significance (Genome-wide association studies (GWAS) are hypothesis-free analyses that have been used to scan the genomes of children, adolescents and adults in search of genetic markers of ADHD . This tyificance . The majificance . Additioificance . Furthera priori hypotheses regarding the function of the genes and their presumed relationship to the disorder . It is important to note, however, that these genes did not necessarily achieve genome-wide significance.Supplementary Table http://apps.broadinstitute.org/genets, default settings, InWeb3 background meta-network). ADHD genes in Supplementary Table p < 2E-03). Furthermore, 14 genes were found to organize within three “communities.” Communities are sets of genes that are more connected to one another than they are to other groups of genes; they can help identify functional modules . Results are shown in Supplementary Table p = 7.3E-04) and NMDA receptor trafficking (p = 3.3E-05). These are notable findings, with convergent evidence from pharmacology and basic science. For example, bupropion is used off-label in the United States as an alternative to stimulants to treat ADHD; this medication is a non-competitive antagonist of nicotinic acetylcholine receptors , with localization at the synapse (p = 7E-05–1.6E-06)  and alpha-1 adrenergic receptor signaling (p = 1E-03). In animal studies, low expression of NOS has been associated with an inattentive phenotype, and methylphenidate administration has been associated with increased NOS expression (p = 1.1E-03). A downstream action of lithium is to attenuate NMDA receptor activity   , 73. We tool-2/) . When litool-2/) . The certool-2/) , and redtool-2/) , 76. Furtool-2/) , 78. Stutool-2/) , 80. BasRisk gene identification in ADHD has tremendous potential to teach us about underlying biology, including susceptible periods during human development. In this perspective, we demonstrate this potential using the best available evidence for gene association from the literature. However, these association studies are often underpowered and have largely focused on pre-selected candidate genes of interest, which can potentially bias downstream findings. Further efforts toward genome-wide and statistically robust risk gene identification are crucial for leveraging genetics to learn about biology. These studies are underway in our lab and others, with the hope that the findings will lead to improved treatments and interventions that will improve the lives of those with ADHD.Both authors (VH and TF) contributed to the conceptualization, data analysis, and writing of this manuscript.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


Introduction. Neurofeedback (NF) is an adjuvant or alternative therapy for children with Attention Deficit Hyperactivity Disorder (ADHD). This study intended to compare the efficacy of two different NF protocols on clinical and cognitive symptoms of ADHD. Materials and Methods. In this clinical trial, sixty children with ADHD aged 7 to 10 years old were randomly grouped to receive two different NF treatments . Clinical and cognitive assessments were conducted prior to and following the treatment and also after an eight-week follow-up. Results. Both protocols alleviated the symptoms of ADHD in general (p < 0.001), hyperactivity (p < 0.001), inattention (p < 0.001), and omission errors (p < 0.001); however, they did not affect the oppositional and impulsive scales nor commission errors. These effects were maintained after an eight-week intervention-free period. The only significant difference between the two NF protocols was that high-frequency alpha enhancement protocol performed better in suppressing omission errors (p < 0.001). Conclusion. The two NF protocols with theta suppression/beta enhancement and theta suppression/alpha enhancement have considerable and comparable effect on clinical symptoms of ADHD. Alpha enhancement protocol was more effective in suppressing omission errors. Attention Deficit Hyperactivity Disorder (ADHD) is a serious health issue and a proper treatment is necessary to treat or prevent behavioral, social, and academic problems , 2.Different treatments which are employed to improve cognitive performance in children with ADHD have certain pros and cons . AlthougNeurofeedback (NF) has been introduced to treat ADHD recently and is able to improve the attention level and alleviate the hyperactivity symptoms –12. The Despite therapeutic advantages of NF for patients with ADHD , 11, 19,According to previous findings, this study aims to compare the efficacy of two NF protocols including theta suppression/beta enhancement and theta suppression/high-frequency alpha enhancement regarding their effect on cognitive functioning of children with ADHD.IRCT201404122660N4, http://en.search.irct.ir/view/17747). After a thorough explanation of the study purpose, caregivers of participants signed the consent form for participating in the study.Sixty children with ADHD were randomly selected from patients who were referred to the specialized psychiatric clinics in Tabriz, northwest of Iran. This study was verified by the Scientific and Ethics Committee of Tabriz University of Medical Sciences as a doctoral thesis . The proAll of drug naïve children and adolescents who meet the DSM-5 diagnostic criteria  for combined ADHD were eligible. The diagnosis was made through a semistructured diagnostic interview with parents using SADS-K-PL by a child and adolescent psychiatrist. Exclusion criteria were history of severe head injury, neurological disorders, genetic disorders, psychiatric disorders other than ADHD, and intellectual disability. Children who had received psychotherapy within the past one year were also excluded. After enrolment by the evaluating psychiatrist, selected children were randomly divided into two groups by a schedule generated by RandList  by a nonThe NF protocol included theta suppression/beta enhancement in one group and theta suppression/high-frequency alpha enhancement in the other group. Children received forty sessions of NF .Clinical symptoms intensity and mental activity  were measured right before the intervention, after 40 sessions of NF, and after an 8-week intervention-free follow-up.CPRS was used to assess behavior of participants and the intensity of the symptoms of ADHD . Similarly, the computerized version of Conners' continuous performance task II (CPT-II)  was used to analyze problems of inattention. Scores of omission and commission indicate the participants' performance in this test.This questionnaire is a semistructured diagnostic interview which has been designed based on DSM-IV criteria for current and past episodes of psychopathology. The interview includes children and their parents. Reliability and validity of the questionnaire have been confirmed in Iran .There are 18 symptoms of ADHD in this scale and its questions are answered based on 4 Point Likert Scale. Reliability and validity have been confirmed elsewhere . This scThis test is used to assess core symptoms of ADHD and certain comorbidities such as oppositional disorder and conduct disorder. It is used for children and adolescents (3–17 years) and is able to separately assess inattention, hyperactivity, and impulsivity. This scale is used for both screening and monitoring the treatment results. Validity and reliability of the Persian version of CSR have been confirmed in Iran .This test is very popular in assessing cognitive aspects of ADHD. Its main objective is analyzing the sustained attention as well as impulsivity. The Persian version of the test, which is computer based, has 150 Farsi digits which are considered as stimuli, out of which 30 stimuli (20%) are target stimuli and the remaining 80% are considered as nontarget stimuli. The interval between offering two stimuli is 500 ms and each stimulus is offered for 150 ms. The test period, considering the practical phase, which is carried out in order to enhance the subject's understanding before the main phase, is 200 s. Two types of errors are scored: omission and commission errors. The error of omission occurs when the subject fails to respond to the target stimulus indicating that the subject has had problem in understanding the stimulus, whereas the error of commission occurs when the subject responds to the nontarget stimulus. This reflects the deficiency in inhibiting the impulse. In this test, these two errors are counted by software. Correct responds and reaction time of the participant are also calculated. Reliability and validity of the Farsi version of CPT are reported in previous studies .EEG was recorded with international 10-10 system in the same clinic where psychiatric evaluations were made. Data were recorded using Nihon Kohden Amplifier and with the sampling rate of 500 Hz, power-line notch-filtered at 50 z, and band-pass filtered at 0.5–60 Hz. EEG was recorded with open and closed eye, each for 10 minutes. Segmentation of EEG was considered as 2-second epochs. Artifacts caused by eyeball movements were refined using Gratton et al. algorithm (1983). Neuriguid software and Fourier transformation software were used to quantify EEG data. The relative power among all components resulting from quantitative analysis of the cortex activity was used.Two protocols were applied for training NF: (1) enhancing high-frequency alpha protocol in frontocentral areas of brain and suppressing theta waves and (2) enhancing beta in frontocentral areas of brain and suppressing theta waves. The system was a Procomp Infinity from Thought Technology Ltd.  running Biograph Infinity software. The session included 5-minute baseline record and 20-minute intervention in frontal area  and 20 bipolar intervention therapies in central areas. In alpha protocol, participants needed to suppress theta waves (4–7 Hz) and to enhance the high-frequency alpha waves (10–12 Hz). In beta protocol, participants needed to suppress theta waves and to enhance low-frequency beta waves .Feedback by the device was only provided when individual could distinguish the increase in high alpha or SMR.Data were analyzed by SPSS (version 17). A General Linear Model (GLM) with repeated measurements was used to evaluate effects of the interventions on performance of participants. The type of intervention as between-subjects factor (group) and time of measurements as the within-subjects factor (time) were considered in clinical symptoms and cognitive performance. Descriptive information is presented as mean ± standard deviation and the level of significance was considered at 0.05.Each treatment group included 30 children and adolescents with ADHD. Six children failed to complete the course of treatment and were excluded from the analysis. The final sample consisted of 54 members with mean age (standard deviation) of 8.51 (1.44) years, including 28 children in alpha enhancement group  and 26 children in beta enhancement group (beta group).The two groups were matched in terms of gender, age, and disease duration. All of quantitative indices recorded by qEEG were also compared between the two groups prior to neurofeedback sessions and there was no significant difference.No side adverse event was reported from participants or parents.F = 330.63, p < 0.001), hyperactivity , inattention , and omission , but effect was not significant on oppositional behavior. The group/time interaction was significant for omission errors  and alpha enhancement protocol made more suppression. These results were maintained after an 8-week follow-up.F = 19.37, p < 0.001), high-frequency alpha band , and low-frequency beta band .Groups showed different changes in delta band  and low-frequency bands  in both anterior and posterior areas.The results suggested that there was not a significant difference between the two groups in terms of intensity of theta  and beta activity (15–181 Hz), as well as medication (methylphenidate), has positive effect on improving inattention and speed and precision of continuous attention . PreviouBased on high overlap of sensorimotor rhythm (12–15 Hz) with high-frequency alpha band activity as well as the strong correlation between high-frequency alpha band and cognitive performance improvement, Hanslmayr et al. supposed that the efficacy of NF on increasing sensorimotor rhythm protocol is due to the enhanced power of high-frequency alpha band . They deAnother open label study reports that the enhancement of upper alpha power is effective in improving several measures of clinical outcome and cognitive performance in ADHD .Results of our study are compatible with these reports and show efficacy of theta suppression/alpha enhancement on hyperactivity, inattention, and also omission errors. In the present study none of two protocols had an effect on oppositional and impulsive behaviors as well as the error of commission which reflect impulsivity. Leins et al. compared the NF treatment with theta/beta frequencies protocol and training slow cortical potentials and did not find any significant difference between the two considerably effective protocols in terms of behavioral or perceptual results . HoweverHowever, nonspecific factors may contribute to the positive effects induced by NF and improvement of the core symptoms of ADHD . There aAbsence of a control group might be the main limitation of the study. However, as stated by previous studies, ethical principles are against using a waiting or sham group for our treatment naïve patients. An active control group is suggested instead, but this will not solve the problem here, as the effect of confounding factors will not be distinguished. This is the reason for absence of a control group in several previous studies , 39, 40,Both NF protocols were equally effective in alleviating the clinical symptoms of ADHD, as reported by parents. However, increasing high-frequency alpha protocol was associated with lower errors of omission.


Impulsive behaviours are common symptoms of attention-deficit hyperactivity disorder (ADHD). Although previous studies have suggested functional models of impulsive behaviour, a full explanation of impulsivity in ADHD remains elusive. To investigate the detailed mechanisms behind impulsive behaviour in ADHD, we applied an economic intertemporal choice task involving gains and losses to adults with ADHD and healthy controls and measured brain activity by functional magnetic resonance imaging. In the intertemporal choice of future gains, we observed no behavioural or neural difference between the two groups. In the intertemporal choice of future losses, adults with ADHD exhibited higher discount rates than the control participants. Furthermore, a comparison of brain activity representing the sensitivity of future loss in the two groups revealed significantly lower activity in the striatum and higher activity in the amygdala in adults with ADHD than in controls. Our preliminary findings suggest that an altered size sensitivity to future loss is involved in apparent impulsive choice behaviour in adults with ADHD and shed light on the multifaceted impulsivity underlying ADHD. One indication of impulsive behaviour is ‘impulsive choice’, that is, choosing a smaller but immediate reward over a larger but delayed reward from the intertemporal choices presented. The intertemporal choice paradigm has been widely used to evaluate an individual’s temporal preference in economic surveys8 and neuroimaging studies20, as well as in clinical studies involving individuals with ADHD30. A recent meta-analysis of studies on impulsive choice in individuals with ADHD provided robust evidence that choice behaviour is more impulsive in individuals with ADHD than in control participants31. Some studies on impulsive choice in ADHD have indicated that individuals with ADHD show a strong negative affect when delay is imposed and, in light of this, ‘choosing immediate gains to avoid the slightest delay’ is the cause of the impulsive choice observed in ADHD32.Attention-deficit hyperactivity disorder (ADHD) is characterized by inattention, impulsivity, and hyperactivity and affects 3.4% of children35. A clinical study found that children with ADHD are prone to drug, alcohol, and gambling addictions as adults36. Another model of impulsive choice in ADHD that does take into account the specific features of both gains and losses38 proposes that individuals with ADHD cannot resist the temptation of immediate gains because of a reduced sensitivity to future negative outcomes caused by choosing immediate gains. This means that not only delay aversion to gains, but also reduced sensitivity to future negative outcomes, can cause impulsive choice behaviour.The delay aversion model, however, is concerned with gains alone and omits the factor of losses. Previous findings indicated that individuals with ADHD exhibit a dysfunctional response to losses. For example, children with ADHD require a larger response cost (monetary loss) to inhibit their reactions in the Go/no-go task or Stop task compared with control children39, and in the dopamine system, leading to aberrant sensitivity to the size of both gains and losses40. Given these previous findings on brain function, we hypothesized that the brain areas (including the striatum) receiving projections from dopaminergic neurons, as well as the frontal areas  that have connections to the striatum, are involved in the delay discounting of gains and losses and that the brain activity associated with size sensitivity in these areas shows a difference specifically in loss prediction between ADHD and healthy individuals.Previous studies have shown differences in brain activity between adults with ADHD and healthy adults. For example, adults with ADHD show abnormalities in the corticostriatal loops, which are involved in delay discounting41. Although ADHD has traditionally been considered a disorder of childhood and impulsive choice has been reported in children and adolescents with ADHD42, adults diagnosed as having ADHD in childhood report difficulties in social and economic activities, such as higher school dropout rates, underemployment, compromised job performance, difficulty maintaining employment, and problems maintaining friendships43. This suggests that investigation of impulsive choice in adults with ADHD is needed to understand fully the mechanism of impulsive behaviour in ADHD.A recent study suggested the importance of investigating delay discounting in adults with ADHD10. Here, by measuring brain activity using functional magnetic resonance imaging (fMRI) during an intertemporal choice task with gain/loss, we investigated the effects of the ‘delay’ and ‘gain/loss’ factors on behavioural and neural levels in patients with ADHD.Thus, to understand the mechanism of the impulsive behaviour observed in ADHD, it is necessary to evaluate the effects of both ‘delay’ and ‘gain/loss’ factors. In our previous study, we developed an intertemporal choice task that can estimate an individual’s temporal preference for both gains and losses10 for future gains (GAIN condition) and losses (LOSS condition)  participants  × g(delay), where u is the utility function and g is the discount function, which decreases with an increasing length of the delay. The ‘indifference point’ method was used to calculated each participant’s degree of discounting based on his or her individual choice patterns45. The delay length of the larger-later option (DL) was plotted against the delay length of the smaller-sooner option (DS) for each choice. Logistic regression analysis was used to fit the probability of choosing the larger-later option (PL) as follows11:In economic theory, intertemporal choice is explained by the delay discounting modelPL = 0.5, this equation is transformed towhen βL, βS, and β0). Figure Logistic regression analysis provides the coefficients (g(D) = 1/(1 + khD) can be transformed to the same form as the indifference line equation in the following way:By applying the method of indifference points to the specific discount model, we can compute an individual discount rate from the estimated intercept. The theoretical model of hyperbolic discounting u(40) and u(10) are the utility for 40 yen and 10 yen, respectively. We adopted a linear utility function here; thus, u(40) = 40 and u(10) = 10. kh is the discount rate and is included in the intercept. The intercept in Equation . A 2 × 2 mixed analysis of variance (ANOVA) was used to analyse the inverse discount rates for gains and losses, with condition (GAIN/LOSS) as the within-subject factor and group (NC/ADHD) as the between-subject factor. A significant interaction  and a significant condition effect  were found. Multiple comparison analysis of inverse discount rates revealed a significant difference between the two groups in the LOSS condition; a smaller inverse discount rate  was found in adults with ADHD than in the NCs . In the GAIN condition, no significant difference was found between the groups . These results suggest that adults with ADHD show a smaller discounted value for future losses than healthy adults, even though their choice behaviour stays the same in intertemporal choice tasks involving gains.Figure P < 0.05, Tukey–Kramer method): NCs showed a larger inverse discount rate  in the LOSS condition than in the GAIN condition. No such significant difference was seen in adults with ADHD . This result suggests that NCs exhibited asymmetry in the discounting of gains and losses, whereas adults with ADHD did not.Multiple comparison analysis also revealed a significant difference between the GAIN and LOSS conditions in NCs . In the BOLD signal representing size sensitivity in the caudate, we observed a significant condition effect . On multiple comparison, BOLD signals representing size sensitivity were significantly weaker in adults with ADHD than in NCs in the LOSS condition only . These results suggest that neural activity representing size sensitivity is reduced in adults with ADHD and that this reduction occurs only in the LOSS condition. We also found significantly greater activity representing size sensitivity in the LOSS condition than in the GAIN condition in NCs alone . This result suggests that the NCs exhibited asymmetry in the size sensitivity in gains and losses, whereas adults with ADHD did not, as observed in discounting behaviour.We evaluated the effect of condition (gain/loss) at the neural level. We subtracted event-related blood-oxygen-level dependent (BOLD) signals for choosing smaller-sooner options from event-related BOLD signals for choosing larger-later options, where a small difference between the BOLD signals would indicate weak gain/loss size sensitivity at the neural level. First, we simply performed a group comparison using a two-sample t-test in the GAIN and LOSS condition separately. In the LOSS condition, we found a significantly lower activation in adults with ADHD than in NCs in the caudate . This result was the opposite of the behavioural and activity in the caudate.Next, to evaluate an interactive effect between gain/loss and group, we employed a factorial analysis with condition and group factors. A significant interaction was found in temporal areas, including the amygdala . Factorial analysis revealed no significant interaction . This suggests that brain activity representing delay length did not differ significantly between the two groups in either the GAIN or LOSS condition.We also evaluated the effect of delay at the neural level. We defined the correlation between the event-related BOLD signals and the delay length of the chosen options as delay-related activity. A group comparison using a two-sample t-test for the GAIN and LOSS conditions separately found no significant difference between ADHD and NCs .At the neural level, we obtained data indicating striatal insensitivity to loss in our adult participants with ADHD. Many previous studies have demonstrated that specific brain areas, such as the prefrontal cortex, insula, and striatum, are involved in temporal discounting of future appetitive outcomes, such as drinks and monetary rewards54. We performed ROI analysis of the nucleus accumbens  and interaction (P > 0.1) in the size sensitivity of the nucleus accumbens, the activation pattern was similar to that of the caudate. We found a significant difference between the GAIN and LOSS conditions in NCs (P = 0.0055), but not in the participants with ADHD (P > 0.1). This result also supports our finding that ADHD patients did not exhibit the sign effect in the neural level in the caudate and nucleus accumbens. However, we did not observe a significant difference between groups in the LOSS condition (P > 0.1). This result suggests that the caudate is more sensitive to the LOSS in NCs than the nucleus accumbens and that a significant difference between groups in the LOSS was observed.A recent meta-analysis revealed that the striatum, including the nucleus accumbens and caudate, is involved in delay discounting55. Previous studies found amygdala hyperactivation in ADHD during emotional processing of negative outcomes, including anticipation and experience of monetary loss57. De Martino et al. demonstrated that individuals with amygdala lesions exhibit a distinct reduced sensitivity to monetary losses compared with matched controls. On the other hand, hyperactivation of the amygdala has been reported in delayed outcome processing in adults with ADHD60, suggesting a link between a neural alteration in the amygdala in delayed outcome processing and behavioural delay aversion in ADHD. Based on these previous findings, we suggest that our finding of hyperactivation of the amygdala in loss size sensitivity in ADHD is involved not only in loss processing, but also in delay processing in our loss discounting task. Interestingly, we found the opposite activation pattern of loss size sensitivity in the caudate and amygdala, namely, hypoactivation and hyperactivation in adults with ADHD. This different involvement of the caudate and amygdala in loss discounting in ADHD should be further examined.Our result suggests a difference in the involvement of the amygdala in loss discounting between adults with ADHD and healthy participants, with ADHD individuals hypersensitive to the loss. The amygdala has been suggested as one of the important loci in ADHD5. Our finding is that adults with ADHD, in contrast to the controls, did not exhibit the sign effect. Also, loss size insensitivity in the caudate in our adult participants with ADHD indicates that they had weaker ‘loss aversion’6 compared with healthy adults, because loss aversion is suggested to be the main cause of the sign effect7. Along these lines, because healthy individuals show loss aversion, future losses are predicted to be larger than the actual losses, which lowers apparent discount rates and inhibits impulsive behaviour. This would mean that our adult participants with ADHD did not predict future losses to be larger than the actual losses because of weaker loss aversion.Here, the control participants discounted future losses less than future gains. In behavioural economics, this observed asymmetry in discounting between gains and losses is called the ‘sign effect’10. The present results showed no difference in delay-related activity in the loss condition, thereby eliminating the possibility of ‘enhanced loss delay sensitivity’ and allowing us to identify ‘reduced loss size sensitivity’ in the ADHD group. However, because we used only two different sizes in each condition, we could evaluate only one type of size sensitivity in this study. To understand to what extent size sensitivity is reduced or whether the absolute reward values influence the intensity of size sensitivity, it is necessary to use an experimental paradigm that allows for assessment of size-related and delay-related systems separately by manipulating size and delay independently, as seen in studies using rats61.In the theoretical model of temporal discounting, not only ‘reduced loss size sensitivity’, but also ‘enhanced loss delay sensitivity’ may be causal factors for the sign effect or seemingly high discount rates for losses. If a future loss is perceived as a more distant event than it is in actuality, its discounted value at the time of decision-making becomes smaller, leading to the larger apparent discount rate. The analytical method used in this study to reveal the brain activity associated with size sensitivity and delay allowed us to investigate the two causal factors separately for seemingly high discount rates for future losses. Indeed, in our previous study, we observed ‘reduced loss size sensitivity’ in the insula and ‘enhanced loss delay sensitivity’ in the striatum in the individuals who did not exhibit the sign effectIn conclusion, we used a neuroeconomics approach with an intertemporal choice task for gains and losses to demonstrate that altered loss size sensitivity results in apparent impulsive choice behaviour in adults with ADHD. Our findings shed light on the multifaceted impulsivity underlying ADHD. A limitation of this study was the modest sample size; thus, larger studies are required to evaluate the present findings and reveal the detailed mechanism of impulsive behaviour in ADHD and other psychiatric disorders.62 criteria and the Mini-International Neuropsychiatric Interview (MINI)63 for comorbid psychiatric conditions by an experienced child psychiatrist . The diagnosis of ADHD was confirmed through subsequent visits to the outpatient clinic, with supplementary use of information from parents, school report cards (assessments and comments by teachers regarding the child’s behaviour at school), and data from the Adult ADHD Self-Report Scale (ASRS)64, Wender Utah Rating Scale (WURS)66, and AQ. The final consensus diagnosis was made through a clinical conference among child psychiatrists and psychologists . Subsequently, Conners’ Adult ADHD Diagnostic Interview for DSM-IV™ (CAADID) and Conners’ Adult ADHD Rating Scale (CAARS) became available in Japan in 2012 and were applied to 8 of the 14 participants, confirming the diagnosis of ADHD.Twenty-two Japanese right-handed adults with ADHD were recruited from the outpatient clinic at the Department of Neuropsychiatry, The University of Tokyo Hospital, Japan. The diagnostic procedure included an approximately 2-h interview based on the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV)67 or 3rd Edition (WAIS-III)68. We also excluded one participant with ADHD due to a young age (16 years old).The presence of psychopathological co-morbidity was screened using MINI and six patients had dysthymia, three had generalized anxiety disorder, and one had obsessive-compulsive disorder . The following exclusion criteria were applied: (1) comorbid current major depressive disorder; (2) history of manic episode; (3) schizophrenia; (4) any sign of neurological disorder; (5) history of electroconvulsive therapy; (6) alcohol or substance abuse or dependence as defined by DSM-IV; (7) use of a psychotropic drug within 4 weeks of MRI data acquisition, and (8) a full-scale IQ < 85 on the Wechsler Adult Intelligence Scale–Revised (WAIS-R)Twenty-two Japanese age-, sex-, dominant handed-, and IQ-matched healthy NCs participated as the control group. The presence of ADHD symptoms and other psychopathological symptoms and diseases was evaluated in a manner similar to the patient group evaluation. The NCs were interviewed by a trained psychiatrist (A.T.) to screen for neuropsychiatric disorders. The exclusion criteria applied to the control group were: (1) ADHD symptom scale scores exceeding the cutoff value; (2) neurological illness; (3) traumatic brain injury with any known cognitive consequences or loss of consciousness for more than 5 min; (4) alcohol/substance abuse or addiction; and (5) history of psychiatric disease. We also excluded one NC who chose only the larger-later option in the GAIN condition because we could not estimate any discount parameter.We excluded one adult with ADHD and two NCs from the fMRI data analysis because there was a problem with the synchronization between the fMRI scanner and the PC running the experimental task. As a result, behavioural data were acquired from 15 medication-naïve adults with ADHD  and 19 NC  and imaging data were acquired from 14 medication-naïve adults with ADHD  and 17 NC .69 with a laterality index of 0.8 as the cutoff for right-handedness. IQ was assessed with the full-scale WAIS-R or III for the adults with ADHD and with the Japanese version of the National Adult Reading Test46 for the NCs. Level of social functioning was evaluated using the Global Assessment of Functioning (GAF)62. The demographic characteristics of the participants are shown in Table All participants were right-handed, as determined by the Edinburgh InventoryThe Ethics Committee of the Faculty of Medicine, The University of Tokyo, approved this study (No. 3048). After receiving a complete explanation of the study, all participants provided written informed consent. All methods were carried out in accordance with the approved guidelines and regulations.10. In economic studies, the ‘intertemporal choice task’ has been commonly used to estimate a participant’s discount rate for both gains and losses8. In an economic intertemporal choice task, the participant chooses a smaller-sooner monetary gain/loss or a larger-later monetary gain/loss in the form of a questionnaire. A typical questionnaire contains several choices with a fixed delay length and different monetary amounts. To measure neural activity when computing the discounted value, we conducted a repeated intertemporal choice task11 with monetary gains and losses10. In the repeated intertemporal choice task, which we have described previously10, the participants chose between a smaller-sooner and a larger-later option in each trial. At the start of each trial, a white square  and a yellow square (larger-later option), occluded by a varying number of black patches indicating the monetary payoff delays, were shown side-by-side on a display for 2,000 ms. In the GAIN condition, the choice was between a white square indicating a small gain (10 yen) with a short delay (1–7 s) and a yellow square indicating a large gain (40 yen) with a longer delay (6–26 s). In the LOSS condition, participants chose between a small loss (−10 yen) with a short delay and a large loss (−40 yen) with a longer delay. The participant indicated their choice by pressing a button; then, the black patches were removed from that square, one-by-one, at 400-ms intervals . When the square was completely exposed, the monetary payoff was displayed on the screen for 1,000 ms. After a 5,000–7,000-ms interval, the next trial began. The white and yellow squares were displayed with novel mosaic patterns. The initial number of coloured patches was randomly chosen from uniform distributions . The number of black patches removed at each step was also randomly drawn from a uniform distribution (5 ± 2). Each GAIN and LOSS session contained six blocks, and each block was finished when a total of 180 steps was achieved. In total, each GAIN and LOSS session lasted approximately 20 min. The order of the GAIN/LOSS conditions was counterbalanced across participants. Sixteen participants  performed six GAIN blocks first and then performed six LOSS blocks. Fifteen participants  performed six LOSS sessions first and then performed six GAIN blocks.We used a repeated intertemporal choice with monetary gains and lossesAll participants were instructed to maximize monetary gains in the GAIN condition and to minimize monetary losses within the time limitation. Thus, at the beginning of each trial, the participants were required to choose between a sooner but smaller reward/loss (white) and a later but larger reward/loss (yellow) by comparing the number of black patches on the two squares. Because the participants were required to respond within 2,000 ms, decision-making was based on the visual impression of darkness on the white and yellow squares, rather than on an explicit count of the numbers of black patches and their division by the numbers of patches filled at each step. All participants received 7,500 yen as an initial monetary reward, with the total amount varying depending on their task performance. Rewards were disbursed later by direct deposit.In this experiment, the duration of the task was fixed as described above. In detail, each block was finished when the sum of the duration (or steps) from choice to outcome in each trial exceeded the threshold (180 steps = 180 × 0.4 = 72 s). The participants were informed of this time limitation rule. Thus, each participant tried to maximize their gain per unit time and also tried to minimize their loss per unit time. If the delay for larger loss was too long, then the loss per unit time was quite small, which was the motivation for the participants to choose a larger-later loss over a smaller-sooner one. This experimental paradigm using time limitation can prevent participants from choosing sooner options independent of the size of the outcome based on delay aversion, that is, their motivation to finish the experiment as soon as possible.The task presentation during the scan was controlled by Presentation software . Visual stimuli were delivered to a translucent screen by an electromagnetic noise-shielded projector located at the foot of the bed. The participants viewed this screen through a tilted reflection mirror attached to the head coil. During the scan, participants held a response box and were instructed to use their right index finger to make a choice in each trial.A 3.0-T scanner  was used to acquire structural T1-weighted images and T2*-weighted echo planar images  with BOLD contrast. SPM12  was used for preprocessing and statistical analyses. We discarded the first six image volumes to avoid T1 equilibrium effects. The images were corrected for slice timing, realigned to the first image as a reference, spatially normalized with respect to the Montreal Neurological Institute (MNI) echo planar imaging (EPI) template, and spatially smoothed with a Gaussian kernel .In the fMRI data analyses , three event-related regressors were included in the GAIN and LOSS condition separately: stimulus display timing, reward display timing, and timing of each step (every 400 ms from the stimulus display until the reward display). To evaluate the effect of ‘delay’ and ‘gain/loss’ factors at the neural level, we focused on delay-related activity and outcome-related activity for both GAIN and LOSS outcomes as follows. We prepared two event-related regressors at the time of stimulus presentation for choosing a smaller-sooner option or a delayed option separately, and each event-related regressor included parametric modulation of the delay length for the chosen option. In total, we included six variables (regressors) for each session: (1) an event-related regressor for the timing of stimulus presentation when the participants chose the smaller-sooner option; (2) parametric modulation of regressor 1 with delay length; (3) an event-related regressor for the timing of stimulus presentation when the participants chose the larger-later option; (4) parametric modulation of regressor 3 with delay length; (5) an event-related regressor for the timing of each step; and (6) an event-related regressor for the timing of reward presentation.We excluded the trials in which participants failed to press the button within the effective duration (2 s) from all regressors. All explanatory variables were convolved with a canonical hemodynamic response function and entered into a general linear model. We created images of parameter estimates for the event-related activity at the timing of stimulus displays as the interest contrasts for each participant in each condition.In the group-level analysis, we employed two types of design. First, to evaluate group differences, we simply performed a group comparison with a two-sample t-test in the GAIN and LOSS conditions separately. Second, to evaluate an interactive effect between condition (GAIN/LOSS) and group (ADHD/NC), we employed a factorial design, in which size sensitivity or delay correlation was set as observation data, and a design matrix corresponding to factorial analysis with the condition (GAIN/LOSS) and group (ADHD/NC) as a factor was created. For visualization, we show a brain activation map with a relatively moderate threshold in Figs 10. We focused on the caudate because the most frequently replicated findings in neuroimaging studies in ADHD patients are structural and functional abnormalities in the caudate71 and a recent meta-analysis revealed that not only the nucleus accumbens, but also the caudate are involved in delay discounting54. We defined the left and right head of the caudate  by the PickAtlas (http://fmri.wfubmc.edu/software/pickatlas). In the ROI analysis of the amygdala, we used a functionally defined ROI with a threshold of P < 0.05 uncorrected . In both ROI analyses, we again used a general linear model with the averaged BOLD signal of all voxels within ROIs at the single-subject level and computed the value of the parameter estimates for ‘size sensitivity’ and ‘delay correlation’ averaged across participants for each GAIN and LOSS condition: ‘size sensitivity’ was defined as the subtraction of event-related activity at the timing of stimulus display for choosing smaller-sooner options from event-related activity at the timing of stimulus display for choosing delayed options, and ‘delay correlation’ was defined as a parametric modulation of delay length. We used the MarsBaR toolbox for ROI analysis. In the statistical comparisons of behavioural and ROI data analyses, we used a 2 × 2 mixed ANOVA with condition (GAIN/LOSS) as the within-subject factor and group (NC/ADHD) as the between-subject factor and also used multiple comparison analysis with the Tukey–Kramer method after ANOVA.To precisely evaluate the relationship between condition and group in the activation representing size sensitivity, we performed ROI analyses in the caudate and amygdala. In the ROI analysis of the caudate, we used an anatomically defined ROI of the caudate that included the results of our present work and our previous studySupplementary Information


Emerging evidence suggests that children with attention deficit and hyperactivity disorder (ADHD) present more difficulties in standing and walking balance than typically developing children. Most of previous studies have assessed these functions using postural and sensory organization tests showing differences in balance performance between control and ADHD children. However, to date, it is unknown whether these balance alterations are accompanied with vestibular dysfunction. The principal aim of this study is to evaluate vestibular otolith function in ADHD and matched control children.We assessed vestibular otolith function in children with ADHD and controls using the subjective visual vertical (SVV) bucket test and cervical vestibular-evoked myogenic potentials (cVEMPs). In addition, gait and balance were evaluated using the dynamic gait index (DGI) and computerized posturography.p < 0.001), while computerized postural recordings showed significant differences for the limit of stability between groups (p = 0.02). cVEMPs in response to 500 Hz tone bursts presented at 100 dB were absent or reduced in children with ADHD, as revealed by differences in P1 and N1 peak-to-peak amplitudes between groups (p < 0.01).Non-significant differences between groups were obtained in SVV evaluation. DGI results show lower scores for overall test performance in children with ADHD (These findings suggest that vestibular brainstem reflexes are altered in a subset of children with ADHD. We propose to include cVEMP reflexes in the clinical evaluation of ADHD patients. Attention deficit and hyperactivity disorder (ADHD) is a neuropsychiatric condition characterized by the presence of inattention, hyperactivity, and impulsivity  associatPrevious evidence indicates that a subset of children with ADHD have abnormal gait and balance –9. As thHowever, in addition to the cerebellum, the vestibular system is one of the most important neural networks involved in the control of balance and gait. There are five different vestibular receptors, including two otolith organs (utricle and saccule) and three semicircular canals. Semicircular canals detect angular acceleration and project mainly to ocular-motor nuclei in the brain stem, stabilizing the visual field while compensating for head movements through vestibular–ocular reflexes . On the The principal aim of this study was to assess vestibular otolith function using cervical vestibular-evoked myogenic potentials (cVEMPs) and the perception of subjective visual vertical (SVV) in ADHD and matched control children. In addition, postural and gait performances were also measured and correlated with cVEMP responses.Thirteen children with ADHD  and 13 age-matched healthy controls  were enrolled in the study. ADHD participants were selected from the Pediatric and Diagnosis Center Ceril, while control children were recruited from schools with similar socioeconomic level to that of ADHD children in Santiago. All children were known to be full-term born infants and currently assisting regularly to school. All ADHD participants matched the DSM-5 criteria for the combined type  main classroom form, designed for children from ages 5 to 12 . The SPMThe SVV perception comprises the integration of different sensory modalities: visual, vestibular, and somesthetics  within tThe DGI is a performance-based assessment tool developed by Shumway-Cook and Wollacott  to quant®). Static posturography uses a stable platform set to record body pressure center and body sway velocity (SW). Children were instructed to stand on the platform wearing a pair of virtual reality goggles, while completing 10 given tasks. For the first task (task 0), an area for LOS was established, where the child, standing with both feet on platform and wearing the goggles, is instructed to shift body center of pressure (COP) as far away as possible, in all directions (as indicated), without losing balance and without moving feet. The next five tasks were performed under the condition of “restricted vision,” with the goggles showing a black screen. Tasks under this condition include (1) standing as still as possible on two feet, (2–3) on one foot and then the other foot, (4–5) on tandem switching the foot in front. The following three tasks, (6) standing as still as possible on both feet, (7–8) then on one foot and then the other, are performed under the condition of “visual distractions” . In the last task (9), the child is presented with a virtual environment and instructed to explore through trunk and head movements but keeping both feet stable on platform. For each of the nine tasks, COP, in square centimeters, and SW, in centimeters per second, were recorded. As children  cannot maintain attention during long-lasting evaluations, we had to abbreviate the posturography protocol, and COP areas and SW velocities were not measured with foam.Postural control during balance was assessed using the balance rehabilitation unit  signal was amplified and band-pass filtered , and the rectified EMG signal was measured to obtain valid trials with muscle activation. The stimulus consisted of 500 Hz tone bursts, presented at 5.1 Hz rate through earphones at 100 dB nHL. To obtain cVEMPs waveforms, 200 trials were averaged, and P1–N1 amplitudes and P1 latencies were measured. To rule out middle ear conductive alterations, an otoscopic examination of the tympanic membrane was performed and middle ear impedance was measured.Cervical VEMPs were recorded using surface electrodes placed over the sternocleidomastoid (SCM) muscles, while reference and ground electrodes were placed on the upper sternum and in the midline of the forehead, respectively. Subjects sat comfortably on a chair, keeping the SCM activated and tense through head rotation. We used the Eclipse, EP25-InteracousticsU tests were used to compare SVV, DGI, and posturography tasks. VEMP amplitudes and latencies were compared with t-tests analyses. Spearman correlation tests were used to evaluate possible association between the different variables. p-Values <0.05 were considered as significant in all analyses.Descriptive statistics including mean ± SEM or median and interquartile range (IQR) were computed for all variables in ADHD and control groups. Normal distribution of data was evaluated using the Shapiro–Wilk test. Non-parametric Mann–Whitney U Statistic = 0.0, p < 0.001). Regarding SVV perception, there were non-significant differences in the average of SVV between controls  and ADHD children    , while children diagnosed with ADHD had a mean total SPM score of 32.5 ± 1.8 points, which was significantly worse for ADHD children than for controls  and control children   , but not in the ADHD group . In addition, we measured the COP and SW in nine situations with different motor and visual conditions . Remarkably, in 3 of the 13 ADHD children the cVEMP responses were bilaterally absent, while cVEMP responses were always obtained in the control group. On the other hand, there were no significant differences in P1 latencies between ADHD and controls groups  and in the ADHD group . Similarly, we found no significant correlations between individual average cVEMP amplitudes and height in the control  and ADHD  groups.Next, we studied possible correlations between cVEMP amplitudes and age and height. We found non-significant correlations between age and individual average cVEMP amplitudes (left + right cVEMP amplitudes/2) in the control group (R(26) = −0.719, p < 0.001; Figure Finally, we studied a possible relation between cVEMP responses and the behavioral symptoms severity using the SPM score. We found a significant negative correlation between the individual average amplitude of bilateral cVEMPs and total SPM score [Spearman, The main findings of this study are (i) no differences in SVV perception between controls and ADHD children, (ii) lower DGI scores and LOS in ADHD patients, and (iii) cVEMP amplitude reduction in a subset of ADHD children. Together, these results highlight the importance of including balance and vestibular assessments in the clinical evaluation of ADHD patients.In our study, controls and ADHD children showed no differences in their ability to correctly orient the SVV during the bucket test. These results could be explained through the use of visual and somesthetic cues, for instance, Lee and Aronson  found thIn healthy adults, the range of absolute deviations of SVV  using the bucket test has been reported with values around 0.9 ± 0.7° (mean ± SD) . ResultsFurther research on the subjective vertical development and perception is needed to identify importance of each sensory system involved at different ages. Varying proprioception and/or postural alignment during the bucket test might offer more insight as to what degree this ability relies on somesthetic input .Significant differences were found in total DGI score, due to particular difficulties in performance for tasks 2, 3, 4, and 6 in ADHD children. For controls, DGI tasks seemed quite simple to perform, with only a few occasional “mild” errors associated to slowing down gait speed while performing tasks. Most controls ranging from ages 8 to 10 obtained the maximum of 24 points, while the range of younger children aged 5–7 tended to slow down gait speed particularly in tasks 3 and 4. These results are in agreement with findings by Lubetzky-Vilnai et al. , which aAttention deficit and hyperactivity disorder children not only showed more frequent errors but also most of these would fall under the category of “moderate” changes in gait, indicating more visible disruptions in gait pattern, direction, and speed. In the younger age range, 5–7 years, every ADHD child altered their gait during task 3, and only one managed to perform task 4 without changes in gait pattern. Another observation made during this test is that children with ADHD showed task disruptions not seen in controls, for example, in task 6 tripping over the obstacle, and in task 3 deviating gait directions with head movement. It is important to note that none in the ADHD group achieved the maximum score in the DGI, and that an altered DGI is not a reliable measure of a vestibular dysfunction . TherefoThe aim of this assessment was to evaluate postural control and static balance abilities with and without visual aid. All tasks were performed under either occluded vision (black screen) or altered visual reality (distractors). This was a challenging test for all children, since it required a large amount of sustained physical (resistance) and mental effort (concentration). Our results show that ADHD children presented more difficulties (larger COP areas and SW velocities) than controls, while keeping balance with both feet on platform with black screen (task 1) and when balancing on one foot (non-dominant side) with black screen (task 3). Similarly, LOS areas were significantly smaller (worse) in the ADHD group, indicating less freedom of body center oscillation before losing balance. This might translate into a lack of postural control and stability during body movements reflected in a higher probability of staggering, falling, and clumsiness in ADHD children. The positive correlations between LOS and age and height in the control group and the lack of correlation in the ADHD group could suggest a delayed or interfered development of posture control in ADHD children.In tasks 1–8, children were required to maintain their standing posture stable and as still as possible. Controls were able to sway their center of mass with lower speeds and to keep their COP area smaller than the group of ADHD children. These findings are consistent with previous work that used posturography in ADHD children, where surface areas for COP and SWs in ADHD children were significantly larger and higher than those observed in controls , 32. In A speculative explanation is that when directed back to task, they would not elaborate any strategy or new movement that would prevent another fall. In addition, it is important to remark that none of the control children reached the point of falling off the platform. All control children managed to complete the task sequence fluently without interruptions; however, some ADHD children tended to complain they felt tired and some needed a short break half way through the test before continuing. This might reflect that overall for the ADHD group this test was more challenging and required much more effort, because of a lack of postural control abilities, or concentration abilities, or both. Although we cannot rule out the possibility that the poor performance in balance tasks in ADHD children is also influenced by inattention and hyperactivity, these behavioral symptoms should not affect objective measures of otolith function like cVEMP responses.The cVEMP is an objective measure of a brainstem reflex, which comprise an ipsilateral neural circuit, including the saccule, inferior vestibular nerve, vestibular nuclei, medial vestibulospinal tract, accessory nucleus and nerve, and the SCM muscle . The cVEHere, we found that an important subset of children diagnosed with ADHD, and without any other neuropsychiatric comorbidity or pharmacological treatment, have altered saccular reflexes measured with cVEMPs. The amplitude reduction of cVEMPs in ADHD children reflects an alteration in the receptor or in the neural circuit that generates cVEMP responses. Moreover, cVEMP amplitudes correlated with high SPM scores, and ADHD children had low DGI values and reduced LOS areas. Therefore, the otolith dysfunction could be contributing to deficits in postural and balance performance in ADHD children , 12, 32.The gravisensing otolith organs make direct and indirect connections with the flocculus and the nodulus in the cerebellum, which constitute the vestibulo–cerebellum , 43. RegThe vestibular inputs contribute to a variety of cognitive processes including visuospatial ability, memory, attention, and executive functions . ProjectSince cVEMPs of reduced amplitudes correlated with dysfunctional behaviors assessed with the SPM form, some open questions are whether the presence of ADHD-like symptoms indicates possible vestibular dysfunction? Are altered saccular reflexes a part of the neural basis of ADHD disorders? Another possibility is that vestibular alterations constitute a completely different diagnosis, which tends to present itself with similar ADHD behaviors in the pediatric population? In the present work, we are not able to answer these questions, but we show that balance and otolith assessments are important to consider in the clinical evaluation of ADHD patients.In this study, the most novel result is that cVEMP amplitudes are reduced in ADHD children. Moreover, 11 out of 13 ADHD children could be classified with 100% specificity using a cVEMP amplitude criterion. These findings suggest that vestibular otolith function is altered in a significant number of children exhibiting ADHD symptoms. Future studies in utricular and semicircular canal functions using the video head impulse test and the possible pharmacological modulation of cVEMP responses are needed for a more complete understanding of a vestibular dysfunction in ADHD children.VI and PD designed research; VI and DO performed research; VI, DO, FA, and PD analyzed data; VI, FA, and PD wrote the manuscript.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.


We report on a compact, simple and robust high brightness entangled photon source at room temperature. Based on a 30-mm-long periodically-poled potassium titanyl phosphate crystal, the source produces non-collinear, type-0, phase-matched, degenerate photons at 810 nm with spectral brightness as high as ~0.41 ± 0.02 (~0.025 ± 0.02) MHz/mW/nm for multi (single) mode fiber coupling. So far, this is the highest number of degenerate photons generated using a continuous-wave laser pumped bulk crystal and detected using multimode fiber. We have studied the dependence of pump focusing on the brightness of the generated photons collected using both multimode, and single mode fibers. For a fixed pump power and crystal parameters, the SPDC source has an optimum pump waist radius producing maximum number of paired photons. Combining the crystal in a novel system architecture comprised with Sagnac interferometer and polarizing optical elements, the source produces polarization entangled photon states with high spectral brightness. Even in the absence of any phase compensation, the entangled photon states detected using single mode fiber have a Bell’s parameter, S = 2.63 ± 0.02, violating the Bell’s inequality by nearly 32 standard deviations and fidelity of 0.975. The compact footprint, robust design, and room temperature operation, make our source ideal for various quantum communication experiments. However, the realization of next generation envisaged projects towards the implementation of world-wide quantum network through ground-to-satellite and or inter-satellite links5 require development of compact and robust entangled photon sources with high brightness, and entanglement purity. Over decades, a variety of schemes have been proposed and implemented7 for entangled photons, however, the polarization entangled photon sources realized through the spontaneous parametric down-conversion (SPDC) in second order, (χ2), bulk nonlinear crystals remain the most appropriate choice.Entangled photon sources, a basic ingredient for many quantum optical experiments, are of paramount importance not only for the fundamental research9 and waveguide10 structures, different phase-matching geometries including type-II15, type-I11 and type-017, and different experimental schemes. Given that the length and nonlinearity are two important crystal parameters greatly influencing the overall gain of the SPDC process for a given pump laser intensity, the brightness of the entangled photon sources have been significantly improved over the years through the use of long periodically poled crystals engineered for high effective nonlinearity.Since, the parametric gain of SPDC process is low, efforts have been made to improve the brightness of the entangled photon sources by exploring different nonlinear crystals in bulk17. On the other hand, PPKTP crystals in type-II phase-matching have been extensively used in Sagnac-loop to produce narrow band entangled photons19 without exploiting the full advantage of high nonlinear co-efficient of the QPM crystals. Therefore, despite the generation of broadband entangled photons, the type-0 phase-matching is more favored over other phase-matching geometries to produce bright entangled photons. While one can expect further enhancement in the paired photons rate by operating the source at degeneracy, however, the separation and detection of individual photons of the collinear, co-polarized paired photons at degenerate wavelength is significantly difficult proposition. Attempt has been made to overcome such problem with the use of non-collinear, type-0, third order QPM in periodically poled lithium tantalate (PPSLT) crystal20, however, the use of third order QPM resulted in moderately low rate of paired photons (98.5 kHz/mW). On the other hand, the requirement of active temperature control for non-critical phase-matching in periodically poled crystals increases the overall complexity of the sources. However, the success of future quantum communication experiments, in addition to other obstacles, demands engineering of high brightness entangled photon sources with minimal system complexities. Here, we demonstrate a high brightness entangled photon source producing non-collinear, degenerate photons at 810 nm with spectral brightness as high as ~0.41 ± 0.02 MHz/mW/nm. Based on a single, 30-mm long PPKTP crystal configured in a polarization Sagnac interferometer25, the source produces entangled photon states violating the Bell’s inequality by nearly 32 standard deviations and a Bell state fidelity of 0.975. The Table As such, use of type-0, quasi-phase-matching (QPM) in periodically poled potassium titanyl phosphate (PPKTP) crystal in crossed-crystal geometry has produced collinear, non-degenerate paired photons at a spectral brightness as high as 0.278 MHz/mW/nm2 in aperture, single grating, PPKTP  crystal of period, Λ = 3.425 µm, is used for type-0 (e → e + e) phase-matched down conversion of pump beam at 405 nm. A convex lens of focal length, f = 300 mm, is used to focus the pump beam at the center of the crystal. To generate entangled photons, we have devised a novel experimental scheme based on polarization Sagnac interferometer consisting of a dual wavelength polarizing beam splitter cube (D-PBS), a dual wavelength half-wave plate (D-λ/2) plate, and two high reflecting (R > 99%) mirrors, M, at both 405 nm and 810 nm. The working principle of the scheme can be understood as follows. The λ/2 plate at 405 nm placed before the D-PBS controls the polarization of the pump beam in such a way that the reflection  and transmission  ports of the D-PBS have equal laser powers. The V polarized pump photons travelling in clock-wise (CW) direction in the Sagnac interferometer, generate V polarized SPDC photons at 810 nm owing to type-0, non-collinear phase-matching in PPKTP crystal. The D-λ/2 plate transforms the polarization of both pump and the SPDC photons from H to V and vice versa. Therefore, the V polarized SPDC photons propagating in CW direction of the Sagnac interferometer transformed into H polarized photons and pass through the D-PBS. At the same time, the H polarized pump photons travelling in the counter clockwise direction (CCW) in the Sagnac interferometer transformed into V polarization after passing through the D-λ/2 plate and generate V polarized non-collinear SPDC photons in the same PPKTP crystal. The D-PBS combines the SPDC photons generated in both CW and CCW directions of Sagnac interferometer. Since both the CW and CCW pump beams follow the same path in opposite directions, and the PPKTP crystal is placed at a position symmetric to the D-PBS, the present experimental scheme is robust against any optical path changes to produce SPDC photons in orthogonal polarizations with ultra-stable phase. Using high reflecting mirrors, M, and two interference filters (IF) of different transmission bandwidths (2 nm and 10 nm) centered at 810 nm, the SPDC photons are collected and subsequently detected with the help of single photon counting module, SPCM  . The analyzer, comprises a PBS and a λ/2 plate, is used to study the polarization entanglement of the generated photons. The lens pair, f1 and f2, images the crystal plane to the fiber tip used for photon collection. For all the measurements presented throughout the manuscript we have used a coincidence window of 8.1 ns.The schematic of the experimental setup is shown in Fig. f = 50 mm in f-f optical system configuration26. Since each pixel represents a particular transverse momentum of the SPDC photons, using the recorded spatial distribution of the SPDC photons we calculated the emission angle  of the photons at different crystal temperatures with the results shown in Fig. 27. Using the Sellmeier equations28 of PPKTP crystals and the equations for non-collinear phase-matching we have theoretically calculated (solid line) the emission angle of the SPDC photons in good agreement with the experimental data (dots). The non-collinear generation and variation of the SPDC ring diameter with crystal temperature is also evident from the CCD images of the intensity distribution of the SPDC photons as shown by the inset of Fig. 28, confirming a reasonable agreement between the experimental data and theoretical calculations. Figure To verify the generation of degenerate SPDC photons in non-collinear, type-0 phase matching, we pumped the crystal at an input power of 40 mW. Using a CCD camera  along with an interference filter of spectral width ~10 nm centered at 810 nm, we have recorded the angular spectrum or transverse momentum distribution of the SPDC photons in the Fourier plane using a convex lens of focal length, f1 and f2, in 2f1-2f2 configuration to image the crystal plane to the tip of the fiber connected to SPCM. For comparison, we have used both multimode(MMF) and single mode fibers(SMF) separately to measure the spectral brightness of the photons collected from two diametrically opposite points on the SPDC ring filtered with the interference filter (IF) of bandwidth ~2 nm at different pump beam focusing and pump power. The results are shown in Fig. f1 = 50 mm and f2 = 5 mm, vary from 1.98 ± 0.02 × 105 Hz/mW/nm to 2.73 ± 0.02 × 105 Hz/mW/nm with the pump beam waist radius varying from 4 µm to 139.5 µm clearly showing a maximum spectral brightness of 4.19 ± 0.03 × 105 Hz/mW/nm at optimum pump beam waist radius of 59 µm. We have also measured the dependence of the spectral brightness with the pump power at two different crystal temperatures, 22 °C and 29 °C. The results are shown in Fig. 5 Hz/mW/nm, and Nc = 3.92 ± 0.01 × 105 Hz/mW/nm at crystal temperature 22 °C (open circle) and 29 °C (closed circle) respectively. Using the photon numbers, N1~N2 recorded from two diametrically opposite points on the SPDC ring and their spectral brightness, Nc in the equation, NPair = (N1 × N2)/Nc20, we can estimate the spectral brightness to be as high as ~8.05 ± 0.03 MHz/mW/nm and ~10.16 ± 0.02 MHz/mW/nm at 22 °C and 29 °C respectively. Such a high number of non-collinear degenerate paired photons as compared to previous reports, can be attributed to high parametric gain resulting from the use of long crystal length and first order grating period of the PPKTP crystal. To the best of our knowledge, this is so far, the highest detected non-collinear, degenerate paired photons from a bulk crystal based SPDC source. Given that the parametric gain is almost constant near the degeneracy and the SPDC is a very feeble nonlinear process, one can, in principle, expect the number of detected paired photons to be constant with crystal temperature. However, as evident from Fig. After successful generation of degenerate, non-collinear SPDC photons using type-0 phase-matched PPKTP crystal, we have  characterized the source in terms of spectral brightness and efficiency. In order to collect photons from two diametrically opposite points of the SPDC ring we have used the imaging systems consist of two convex lenses of focal lengths, f1 = 100 mm and f2 = 2 mm for different pump beam waist radius. As evident from Fig. 4 Hz/mW/nm to 0.336 ± 0.006 × 104 Hz/mW/nm with the pump beam waist radius varying from 4 µm to 139.5 µm clearly showing a maximum spectral brightness of 2.50 ± 0.02 × 104 Hz/mW/nm at optimum pump beam waist radius of 41 µm. Keeping the pump beam waist radius at optimum value of 41 µm, we have measured the variation of spectral brightness with the pump power. As evident from Fig. 4 Hz/mW/nm resulting a maximum paired photons of 6.18 ± 0.01 × 103 Hz/nm at a pump power of 0.25 mW. The corresponding maximum number of paired photons produced in the SPDC process can be calculated to be NPair = 0.74 ± 0.01 MHz/mW/nm. The coupling efficiency is almost constant at ~18.45% for all the pump powers.Further, we have characterized the SPDC source using single mode fibers. Pumping the crystal at a constant power of 0.25 mW and temperature ~29 °C we have measured the photon spectral brightness collected using the single mode fiber and the imaging systems comprise lenses of With the successful generation of paired photons with high brightness, we study the source for the generation of polarized entangled photons. Since in type-0 phase-matching geometry, the generated paired photons have the same state of polarization as that of the pump photons, we placed the PPKTP crystal along with a D-λ/2 plate inside the polarization Sagnac interferometer and pumped in both CW and CCW directions with same state of polarization see Fig. , verticaf1 = 150 mm and f2 = 5 mm in 2f1-2f2 configuration, an interference filter of bandwidth ~2 nm, polarization analyser, multimode fiber, and SPCM. The results are shown in Fig. 5 Hz/mW/nm) presented in Fig. f1 = 150 mm, of the detection system to avoid mechanical constraint in the experimental setup.For quantitative analysis of the polarized entanglement of the source, we pumped the crystal with a total power of ~1 mW at crystal temperature ~29 °C and aligned the system to maintain perfect overlapping of the SPDC rings. Using standard coincidence measurement technique we recorded the two-photon interference in terms of photon coincidence between the twin photons distributed in two diametrically opposite points of the SPDC ring under two non-orthogonal projection bases, H/V  and D/A . The photon detection system comprised with two lenses of focal length, 29. Similarly, we have studied the polarization entanglement using single mode fiber with the results shown in Fig. 4 Hz/mW/nm at 0.5 mW of pump power. However, the interference fringe visibility in H (solid circles) and D (open circles) projections are measured to be 97.4 ± 0.2% and 96.9 ± 0.3%. Using the coincidence counts we have calculated the Bell’s parameter to be S = 2.63 ± 0.02, clearly violating the Bell’s inequality by nearly 32 standard deviations. Such study clearly shows the generation of high quality entangled photons with high brightness even in the absence of any phase compensation schemes, as commonly used for SPDC based entangled photon sources. The Table The interference fringe visibility in H and D projections are estimated to be 98.2 ± 0.3% and 72.8 ± 0.4% respectively. The measured fringe visibility under both bases are higher than 71%, sufficient to violate Bell’s inequality. Using the coincidence rates, we measured the Bell’s parameter to be S = 2.19 ± 0.03, violating the Bell’s inequality by nearly 6 standard deviations, a clear indication of polarization entanglement of the generated two-photon states. The lower value of fringe visibility in diagonal projection can be attributed to the relative phase among the higher order spatial modes in the SPDC process collected by the multimode fiber, which can be improved through proper phase compensation schemes12, here, we have also observed that the fringe visibility in both H and D projections decrease with the pump power resulting a fringe visibility of 70.8% and 68.9% in H and D projections respectively, below the fringe visibility limit of 71% for entanglement, even for the pump power as low as 5 mW. Such degradation in the entanglement quality with the increase of pump power can be attributed to the multi-pair generation due to high gain in the PPKTP crystal. To verify the robustness of our entangled photon source, we have measured the fluctuation in the spectral brightness at H-H projection collected using single mode fiber. Keeping the pump power constant at 0.5 mW, we have recorded the coincidence of the paired photons with an exposure of one second over 3600 seconds with the results shown in Fig. 4 Hz/nm over the average spectral brightness of 1.97 × 104 Hz/nm for 0.5 mW, corresponding to an estimated passive fluctuation as low as ~3%. Similarly, in D-A projection we have measured the fluctuation in spectral brightness at a standard deviation of 0.074 × 104 Hz/nm with an estimated passive fluctuation of ~4.7%, slightly higher than that of H-H projection. Such instability of the SPDC source can further be improved with proper isolation of the source from the temperature fluctuation and air turbulence of the laboratory environment. To find the degree of entanglement of the photon states we have constructed the density matrix of the state using linear tomographic technique30. The inset of Fig. 25, we can transform the output state, We have also studied the effect of pump power on the entanglement quality of the paired photons by measuring the visibility of quantum interference fringes at different pump powers with the results shown in Fig. In conclusion, we have demonstrated a simple, compact and robust source of entangled photons at high brightness. Based on non-collinear, degenerate, type-0 phase-matched single PPKTP crystal at room temperature in a Sagnac interferometer, the source produces a detected paired photons rate of 0.41 ± 0.02 MHz/mW/nm. To the best of our knowledge, this is the highest number of degenerate paired photons detected with the help of multimode fiber from a bulk crystal pumped with a continuous-wave laser. The polarization correlation study and quantum tomography measurement using single mode fibers reveals that the source produces entangled photon states violating the Bell’s inequality by nearly 36 standard deviations and a Bell state fidelity of 0.975. Such high brightness entangled photon source in compact and rugged architecture is ideal for the many present and future experiments in the field of quantum optics especially in quantum communications.