Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> The first electrophysiological recordings from animal1 and human2 taste nerves gave clear evidence of thermal sensitivity, and studies have shown that as many as half of the neurons in mammalian taste pathways respond to temperature3,4,5,6. Because temperature has never been shown to induce sensations of taste, it has been assumed that thermal stimulation in the gustatory system is somehow nulled6. Here we show that heating or cooling small areas of the tongue can in fact cause sensations of taste: warming the anterior edge of the tongue (chorda tympani nerve) from a cold temperature can evoke sweetness, whereas cooling can evoke sourness and/or saltiness. Thermal taste also occurs on the rear of the tongue (glossopharyngeal nerve), but the relationship between temperature and taste is different there than on the front of the tongue. These observations indicate the human gustatory system contains several different types of thermally sensitive neurons that normally contribute to the sensory code for taste. <s> BIB001 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> We tested whether auditory sequences of beeps can modulate the tactile perception of sequences of taps (two to four taps per sequence) delivered to the index fingertip. In the first experiment, the auditory and tactile sequences were presented simultaneously. The number of beeps delivered in the auditory sequence were either the same as, less than, or more than the number of taps of the simultaneously presented tactile sequence. Though task-irrelevant (subjects were instructed to focus on the tactile stimuli), the auditory stimuli systematically modulated subjects' tactile perception; in other words subjects' responses depended significantly on the number of delivered beeps. Such modulation only occurred when the auditory and tactile stimuli were similar enough. In the second experiment, we tested whether the automatic auditory-tactile integration depends on simultaneity or whether a bias can be evoked when the auditory and tactile sequence are presented in temporal asynchrony. Audition significantly modulated tactile perception when the stimuli were presented simultaneously but this effect gradually disappeared when a temporal asynchrony was introduced between auditory and tactile stimuli. These results show that when provided with auditory and tactile sensory signals that are likely to be generated by the same stimulus, the central nervous system (CNS) tends to automatically integrate these signals. <s> BIB002 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> The principles of cross-modal integration were investigated with an auditory-tactile illusion in sighted and con-genitally blind adults. Participants had to judge the number of rapidly presented tactile stimuli, which were presented together with task-irrelevant sounds. When one tactile stimulus was accompanied by more than one tone, participants reported perceiving more than a single touch. This illusion was more pronounced in sighted than congenitally blind participants. Given that the congenitally blind were more precise in judging the number of tactile stimuli in a control condition without tones, the present data are in accordance with a modality-appropriateness account suggesting that interference by a task-irrelevant modality is reduced if processing accuracy of the task-relevant modality is high. <s> BIB003 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> Abstract When eating or drinking, the individual experiences a multitude of sensations, including taste, smell, touch, temperature, sight, sound, and sometimes pain/irritation. This multi-faceted sensory experience is the underpinning of perceived flavor, although certainly some sensations contribute more than others. This paper reviews how all these sensations interact, both on a perceptual and a physical level, and discusses the resulting impact each has on flavor ratings. Interactions between taste and smell, and interactions of the remaining sensations will be discussed. Finally, practical implications of these interactions for sensory evaluation are discussed. <s> BIB004 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> Abstract We report a series of three experiments in which participants made unspeeded ‘Which modality came first?’ temporal order judgments (TOJs) to pairs of auditory and tactile stimuli presented at varying stimulus onset asynchronies (SOAs) using the method of constant stimuli. The stimuli were presented from either the same or different locations in order to explore the potential effect of redundant spatial information on audiotactile temporal perception. In Experiment 1, the auditory and tactile stimuli had to be separated by nearly 80 ms for inexperienced participants to be able to judge their temporal order accurately (i.e., for the just noticeable difference (JND) to be achieved), no matter whether the stimuli were presented from the same or different spatial positions. More experienced psychophysical observers (Experiment 2) also failed to show any effect of relative spatial position on audiotactile TOJ performance, despite having much lower JNDs (40 ms) overall. A similar pattern of results was found in Experiment 3 when silent electrocutaneous stimulation was used rather than vibrotactile stimulation. Thus, relative spatial position seems to be a less important factor in determining performance for audiotactile TOJ than for other modality pairings (e.g., audiovisual and visuotactile). <s> BIB005 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> We report two experiments designed to assess the consequences of posture change on audiotactile spatiotemporal interactions. In Experiment 1, participants had to discriminate the direction of an auditory stream (consisting of the sequential presentation of two tones from different spatial positions) while attempting to ignore a task-irrelevant tactile stream (consisting of the sequential presentation of two vibrations, one to each of the participant’s hands). The tactile stream presented to the participants’ hands was either spatiotemporally congruent or incongruent with respect to the sounds. A significant decrease in performance in incongruent trials compared with congruent trials was demonstrated when the participants adopted an uncrossed-hands posture but not when their hands were crossed over the midline. In Experiment 2, we investigated the ability of participants to discriminate the direction of two sequentially presented tactile stimuli (one presented to each hand) as a function of the presence of congruent vs incongruent auditory distractors. Here, the crossmodal effect was stronger in the crossed-hands posture than in the uncrossed-hands posture. These results demonstrate the reciprocal nature of audiotactile interactions in spatiotemporal processing, and highlight the important role played by body posture in modulating such crossmodal interactions. <s> BIB006 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> In this study we investigated the effect of the directional congruency of tactile, visual, or bimodal visuotactile apparent motion distractors on the perception of auditory apparent motion. Participants had to judge the direction in which an auditory apparent motion stream moved (left-to-right or right-to-left) while trying to ignore one of a range of distractor stimuli, including unimodal tactile or visual, bimodal visuotactile, and crossmodal (i.e., composed of one visual and one tactile stimulus) distractors. Significant crossmodal dynamic capture effects (i.e., better performance when the target and distractor stimuli moved in the same direction rather than in opposite directions) were demonstrated in all conditions. Bimodal distractors elicited more crossmodal dynamic capture than unimodal distractors, thus providing the first empirical demonstration of the effect of information presented simultaneously in two irrelevant sensory modalities on the perception of motion in a third (target) sensory modality. The results of a second experiment demonstrated that the capture effect reported in the crossmodal distractor condition was most probably attributable to the combined effect of the individual static distractors (i.e., to ventriloquism) rather than to any emergent property of crossmodal apparent motion. <s> BIB007 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> We report two experiments designed to investigate the nature of any cross-modal interactions between olfactory and tactile information processing. In Experiment 1, we assessed the influence of olfactory cues on the tactile perception of fabric softness using computer-controlled stimulus presentation. The results showed that participants rated fabric swatches as feeling significantly softer when presented with a lemon odor than when presented with an animal-like odor, demonstrating that olfactory cues can modulate tactile perception. In Experiment 2, we assessed whether this modulatory effect varied as a function of the particular odors being used and/or of the spatial coincidence between the olfactory and tactile stimuli. The results replicated those reported in Experiment 1 thus further supporting the claim that people’s rating of tactile stimuli can be modulated by the presence of an odor. Taken together, the results of the two experiments reported here support the existence of a cross-modal interaction between olfaction and touch. <s> BIB008 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> Sequences of auditory beeps and tactile taps were simultaneously presented and participants were instructed to focus on one of these modalities and to ignore the other. We tested whether (i) the two sensory channels bias one another and (ii) the interaction depends on the relative reliability of the channels. Audition biased tactile perception and touch biased auditory perception. Lowering the reliability of the auditory channel (i.e. the intensity of the beeps) decreased the effect of audition on touch and increased the effect of touch on audition. These results show that simultaneous auditory and tactile stimuli tend to be automatically integrated in a reliability-dependent manner. <s> BIB009 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> To study the effects of touch on auditory processing, we examined whether uninformative and irrelevant tactile stimuli presented together with task-relevant sounds can improve auditory detection (Experiment 1), and enhance perceived loudness (Experiment 2). We demonstrated that irrelevant tactile signals facilitate the detection of faint tones, and increase auditory intensity ratings. These crossmodal facilitation effects were found for synchronous when compared to asynchronous auditory–tactile stimulation, and were stronger for weaker than for louder sounds. They are interpreted in terms of a multisensory integration mechanism that increases the strength of auditory signals, and adheres to the rules of inverse effectiveness and temporal (but not spatial) co-occurrence. This integration might be mediated by auditory–tactile multisensory neurons in regions of auditory association cortex that are also involved in auditory detection and loudness discrimination. <s> BIB010 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> Perceptual interactions between odour and oral texture were explored in a study in which a cream odour was presented ortho- or retronasally at well-defined moments whilst milk-like foods with different viscosities, produced by adding a thickener, were present in the mouth. Gaseous (odour) and liquid (texture) pulses were presented using a specially-developed computer-controlled system of air-dilution olfactometry and pumps. Odour pulses, lasting 2 s, were presented either during a 3-s period in which a liquid filled the oral cavity, during a 3-s period in which the liquid was manipulated orally or during the swallowing of the liquid. Subjects rated the intensity of overall flavour, thickness and creaminess. Perceived flavour intensity was reduced with increasing viscosity of the liquid, irrespective of whether or not the odour was presented ortho- or retronasally. The odour stimulus increased the intensities of thickness and creaminess, but only when the odour was presented retronasally that is as if the odour would have originated from the liquid. Furthermore, this enhancement was most pronounced when odours coincided with swallowing, less pronounced when odours coincided with oral manipulation and absent when presented during mouth filling. The results suggest that cross-modal interactions are the rule rather than the exception, provided that multi-modal sensory integration has occurred. <s> BIB011 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> In the present study, we examined the potential modulatory effect of relative spatial position on audiotactile temporal order judgments (TOJs) in sighted, early, and late blind adults. Pairs of auditory and tactile stimuli were presented from the left and/or right of participants at varying stimulus onset asynchronies (SOAs) using the method of constant stimuli. The participants had to make unspeeded TOJs regarding which sensory modality had been presented first on each trial. Systematic differences between the participants emerged: While the performance of the sighted participants was unaffected by whether the two stimuli were presented from the same or different positions (replicating the results of several recent studies), the blind participants (regardless of the age of onset of blindness) were significantly more accurate when the auditory and tactile stimuli were presented from different positions rather than from the same position. These results provide the first empirical evidence to suggest a spatial modulation of audiotactile interactions in a temporal task performed by visually impaired humans. The fact that the performance of the blind participants was modulated by the relative spatial position of the stimuli is consistent with data showing that visual deprivation results in an improved ability to process spatial cues within the residual tactile and auditory modalities. These results support the hypothesis that the absence of visual cues results in the emergence of more pronounced audiotactile spatial interactions. <s> BIB012 </s> Tactual perception: a review of experimental variables and procedures <s> Touch and audition <s> Following on from ecological theories of perception, such as the one proposed by [Gibson, J. J. (1966). The senses considered as perceptual systems. Boston: Houghton Mifflin] this paper reviews the literature on the multisensory interactions underlying the perception of flavor in order to determine the extent to which it is really appropriate to consider flavor perception as a distinct perceptual system. We propose that the multisensory perception of flavor may be indicative of the fact that the taxonomy currently used to define our senses is simply not appropriate. According to the view outlined here, the act of eating allows the different qualities of foodstuffs to be combined into unified percepts; and flavor can be used as a term to describe the combination of tastes, smells, trigeminal, and tactile sensations as well as the visual and auditory cues, that we perceive when tasting food. <s> BIB013
Audiotactile interactions have been reported in two main domains: temporal and spatial. The temporal domain is related to a large group of research evaluating the perception of temporal order in tactile and auditory stimuli when presented with different time intervals (e.g., BIB012 . The spatial domain focuses on the perceived simultaneity of tactile and auditory stimuli presented at a same location in space and concentrates on the apparently automatic integration of this type of information (e.g., BIB005 . It has been shown that beep sounds can modulate the counting of tactile taps BIB002 . Participants were asked to concentrate in the number of taps and to ignore the beeps; nonetheless, numerosity judgements were biased by simultaneous beep presentation. As the interval between tap and beep was increased, the modulation effect decreased, suggesting that simultaneously presented audiotactile information tends to be integrated BIB002 . In a later study with similar procedure, the authors concluded that not only auditory signals affected tactile processing, but also tactile taps affected auditory counting and reported that diminishing the intensity of sounds generated less interference on tactile counting, as well as affected the impact of touch on the auditory task BIB009 . As such, not only audiotactile integration was shown to be automatic, but also it was revealed to be signal-reliability dependent. BIB010 reported tactile enhancement of auditory perception with two experiments evaluating detectability and intensity of auditory stimuli. In the first experiment, participants had to signal if an auditory stimulus was presented in one of two time intervals, defined by a visual mark. Results showed that tactile stimuli facilitated the detection of sound and that this effect was larger on simultaneous trials (in which tactile and auditory stimuli were presented at the same time) than on asynchronous trials (in which tactile and auditory stimuli were presented with a time interval between them). The second experiment implied a stimuli intensity task, consisted in the evaluation, in a nine-point scale, of tactile and auditory stimuli. Tactile stimulation led to higher intensity judgements of the auditory signal. Again, simultaneous presentation resulted in better integration between modalities, revealing that temporal synchrony is an essential condition for audiotactile integration. Interactions between audition and touch were found to be dependent on hand position in a study reported by BIB006 . Participants were asked to place their hands either crossed or over the midline and had to discriminate the direction of an auditory array (sequence of sounds presented in different spatial locations) while receiving spatiotemporal congruent or incongruent tactile stimulation in the hands. Impaired performance was reported for incongruent tactile stimulation, but only for participants who did not have crossed hands. In a second experiment, the authors used the same procedure but asked participants to focus on the tactile stream and not the auditory one and found that the interaction effect was stronger in the crossed hands condition. The authors concluded that body posture could affect crossmodal interaction, and audiotactile influences seemed to be reciprocal BIB006 . In an exploration of audiotactile integration in blind and sighted participants, BIB003 asked participants to estimate the number of rapidly presented tactile stimuli while ignoring non-relevant sounds during the task. When a tactile stimulus was presented in conjunction with more than one sound, participants reported feeling more than one touch, and this effect was stronger for sighted than blind participants. Crossmodal perception between touch and audition has also been tested with other paradigms like the saltation illusion, when one target stimulus is systematically displaced in the direction of another stimulus that is presented either at a near position or at the same time (e.g.,  ); the ventriloquist effect, when stimuli from two sensory modalities presented from a same spatial location are attributed to the same source (e.g., Bruns and Röder 2010); and apparent motion phenomenon, when a sequence of presented stimuli is perceived as one stimulus in motion (e.g., BIB007 . In all these paradigms, the presence of an automatic integration of auditory and tactile stimuli is reported, and the effect seems to be stronger when both stimuli are presented simultaneously, especially when the stimuli are presented in the same spatial location, indicating a natural form of integrated perception between modalities. Touch, smell and taste Olfaction and taste are research topics that have been studied mainly in the food industry, and within this context, research on touch has been essentially restricted to the relevance of perceived textures and temperature in the tongue while tasting food. The connections between touch, smell and taste were already mentioned in the literature many years ago.  suggested the definition of flavour as a specific sense that involved inputs from taste, smell and touch.  also mentioned the relevance of a combined perception of temperature, touch, taste and smell in the ingestion of coffee or fruit. A review on the multisensory perception of flavour BIB004 ) enhanced the role of touch influencing perceived flavour through temperature and texture perception. The interaction between touch and taste was clear in a study that revealed a tactile-taste illusion caused by thermal stimulation BIB001 . In this illusion, after putting an ice cube on the side of the tongue, a salty taste was perceived, originating a taste perception after temperature stimulation. Besides the strong salty sensation, the authors were also able to induce perceptions of sweet (by warming the anterior edge of the tongue after a cold temperature), sour and bitter (by cooling the tongue). Another reported interaction between touch and taste was that the perceived viscosity of a liquid can alter the intensity of its flavour to the taster BIB011 ). In a more recent review on this topic, BIB013 also argued in favour of a multisensory perception of flavour, derived from taste, smell and tactile cues, which continuously affect each other when we are eating. The connections between touch and smell are also explored in the literature.  conducted a study in which women judged the quality of silk stockings. Without the knowledge of the participants, the stockings were impregnated with light fragrances, and women consistently evaluated identical stockings as being better when they presented a narcissus fragrance, and justified their evaluations with properties as durability or sheen, but never attending to olfactory characteristics. The presence of odours can also affect the evaluation of fabric softness, as was shown in a study in which fabrics associated with lemon odours were evaluated as softer than fabrics associated with animal odours BIB008 . The understanding of the apparently automatic processes that allow interchange of information between sensory modalities, enabling reciprocal modulation in ongoing tasks, is crucial in cognitive sciences. The need to control for any multisensory interactions when trying to access processes in a specific sensory modality, as well as the exploration of these natural connections between the senses in memory processes, is clear. The nature of sensory representations, either in their suggested amodal or higher order processing, or concerning intra-modal particularities needs to be acknowledged in order to strengthen experimental designs and procedures.