What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Eye movements of 24 subjects, including untrained laymen, medical students, radiology residents, and staff radiologists, were recorded while the subjects viewed normal and abnonnal chest radiographs. Evolution of search patterns from the localized central pattern of the untrained observer to the circumferential pattern of the radiologist is discussed. It is postulated that development of the pattern is secondary to development of knowledge about the location and characteristic features of normal and abnormal. Improvement in search patterns comes about as a result of concentrating on clearly defining the abnormal feature that is the object of search and the normal background that envelops it. <s> BIB001 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Viewers briefly glimpsed pictures presented in a sequence at rates up to eight per second. They recognized a target picture as accurately and almost as rapidly when they knew only its meaning given by a name (for example, a boat) as when they had seen the picture itself in advance. <s> BIB002 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Ten radiologists were shown a series of 10 normal and 10 abnormal chest films under two viewing conditions: a 0.2-second flash and unlimited viewing time. The results were compared in terms of verbal content, diagnostic accuracy, and level of confidence. The overall accuracy was surprisingly high (70% true positives) considering that no search was possible. Performance improved as expected with free search (97% true positives). These data support the hypothesis that visual search begins with a global response that establishes content, detects gross deviations from normal, and organizes subsequent foveal checking fixations to conduct a detailed examination of ambiguities. The total search strategy then consists of an ordered sequence of interspersed global and checking fixations. <s> BIB003 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Search is composed of survey and examination fixations which, when coupled, form basic perceptual-cognitive decision units. These basic perceptual-cognitive units feed the interpretative decision-making process. Search stages depend on the effectiveness of perceptual-cognitive units. The survey or screening aspect of the perceptual-cognitive unit contributes mainly to global and discovery search stages. The examination aspect of the perceptual-cognitive unit contributes mainly to the reflective search stage. Contrast affects the efficiency of the perceptual-cognitive unit by decreasing surveying and screening and shifting the analysis to the examination side of visual search. <s> BIB004 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Abstract Global context plays an important, but poorly understood, role in visual tasks. This study demonstrates that a robust memory for visual context exists to guide spatial attention. Global context was operationalized as the spatial layout of objects in visual search displays. Half of the configurations were repeated across blocks throughout the entire session, and targets appeared within consistent locations in these arrays. Targets appearing in learned configurations were detected more quickly. This newly discovered form of search facilitation is termed contextual cueing. Contextual cueing is driven by incidentally learned associations between spatial configurations (context) and target locations. This benefit was obtained despite chance performance for recognizing the configurations, suggesting that the memory for context was implicit. The results show how implicit learning and memory of visual context can guide spatial attention towards task-relevant aspects of a scene. <s> BIB005 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Many experiments have shown that the human visual system makes extensive use of contextual information for facilitating object search in natural scenes. However, the question of how to formally model contextual influences is still open. On the basis of a Bayesian framework, the authors present an original approach of attentional guidance by global scene context. The model comprises 2 parallel pathways; one pathway computes local features (saliency) and the other computes global (scene-centered) features. The contextual guidance model of attention combines bottom-up saliency, scene context, and top-down mechanisms at an early stage of visual processing and predicts the image regions likely to be fixated by human observers performing natural search tasks in real-world scenes. <s> BIB006 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> This paper investigates the temporal dependencies of natural vision by measuring eye and hand movements while subjects made a sandwich. The phenomenon of change blindness suggests these temporal dependencies might be limited. Our observations are largely consistent with this, suggesting that much natural vision can be accomplished with “just-in-time” representations. However, we also observe several aspects of performance that point to the need for some representation of the spatial structure of the scene that is built up over different fixations. Patterns of eye-hand coordination and fixation sequences suggest the need for planning and coordinating movements over a period of a few seconds. This planning must be in a coordinate frame that is independent of eye position, and thus requires a representation of the spatial structure in a scene that is built up over different fixations. <s> BIB007 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Extensive cognitive research has been devoted to the sensitivity of the visual system to invariant statistical information. For example, many studies have shown that performance improves when a visual display is presented repeatedly. But what allows humans to connect the current visual input to previous memory? Is the connection made only when the entire incoming display matches with a previous memory, or can retrieval rely on an incomplete match between the input and a learned display? Using a visual search task, we show that (1) once a repeated display is learned, subjects can retrieve it even when an incoming display only matches it in 3-4 locations; (2) however, early during learning, repetition of a small proportion of a display is not enough to establish a strong memory trace for the repeated locations. We suggest that the retrieval of a well-established visual memory can proceed even if an incoming display partly matches the previous memory. <s> BIB008 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> We examined the prioritization of abruptly appearing and disappearing objects in real-world scenes. These scene changes occurred either during a fixation (transient appearance/disappearance) or during a saccade (nontransient appearance/disappearance). Prioritization was measured by the eyes’ propensity to be directed to the region of the scene change. Object additions and deletions were fixated at rates greater than chance, suggesting that both types of scene change are cues used by the visual system to guide attention during scene exploration, although appearances were fixated twice as often as disappearances, indicating that new objects are more salient than deleted objects. New and deleted objects were prioritized sooner and more frequently if they occurred during a fixation, as compared with during a saccade, indicating an important role of the transient signal that often accompanies sudden changes in scenes. New objects were prioritized regardless of whether they appeared during a fixation or a saccade, whereas prioritization of a deleted object occurred only if (1) a transient signal was present or (2) the removal of the object revealed previously occluded objects. <s> BIB009 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Four observer groups with different levels of expertise were tested to investigate the nature of expert performance. The task was the detection and localisation of significant pulmonary nodules in postero-anterior views of the chest. One hundred and twenty digitised chest images were used. The observer groups were 8 experienced radiologists, 5 experienced radiographers before and after six months training in chest image interpretation, and 8 undergraduate radiography students. Eye tracking was carried out to investigate differences in visual search strategies between observers. Detection performance was measured with an Alternate Free Response Operating Characteristic technique. Performance measures showed the experienced group of radiologists plus radiographers after training were better at the task than the remainder (t-test p = 0.046). Differences were shown in the eye-tracking parameters between the groups: saccadic amplitude (ANOVA p = 0.00047), number of fixations before and after training (t-test p = 0.041), and scrutiny time per decision and per film for the experienced versus the inexperienced observers (t-test p = 0.02). Visual coverage reduced with increasing level of experience but this result did not reach significance. Generally there were distinct differences in the search strategies between the experienced and inexperienced observers and we discuss the significance of these findings. We believe the results support some recent theoretical models of expert performance and that the findings may prove to be helpful in ‘fast-track’ educational programmes of image interpretation for non-radiology practitioners. <s> BIB010 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> How do bottom-up and top-down guidance signals combine to guide search behavior? Observers searched for a target either with or without a preview (top-down manipulation) or a color singleton (bottom-up manipulation) among the display objects. With a preview, reaction times were faster and more initial eye movements were guided to the target; the singleton failed to attract initial saccades under these conditions. Only in the absence of a preview did subjects preferentially fixate the color singleton. We conclude that the search for realistic objects is guided primarily by top-down control. Implications for saliency map models of visual search are discussed. <s> BIB011 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> PURPOSE ::: To test the hypothesis that rapid and accurate performance of the proficient observer in mammogram interpretation involves a shift in the mechanism of image perception from a relatively slow search-to-find mode to a relatively fast holistic mode. ::: ::: ::: MATERIALS AND METHODS ::: This HIPAA-compliant study had institutional review board approval, and participant informed consent was obtained; patient informed consent was not required. The eye positions of three full-time mammographers, one attending radiologist, two mammography fellows, and three radiology residents were recorded during the interpretation of 20 normal and 20 subtly abnormal mammograms. The search time required to first locate a cancer, as well as the initial eye scan path, was determined and compared with diagnostic performance as measured with receiver operating characteristic (ROC) analysis. ::: ::: ::: RESULTS ::: The median time for all observers to fixate a cancer, regardless of the decision outcome, was 1.13 seconds, with a range of 0.68 second to 3.06 seconds. Even though most of the lesions were fixated, recognition of them as cancerous ranged from 85% (17 of 20) to 10% (two of 20), with corresponding areas under the ROC curve of 0.87-0.40. The ROC index of detectability, d(a), was linearly related to the time to first fixate a cancer with a correlation (r(2)) of 0.81. ::: ::: ::: CONCLUSION ::: The rapid initial fixation of a true abnormality is evidence for a global perceptual process capable of analyzing the visual input of the entire retinal image and pinpointing the spatial location of an abnormality. It appears to be more highly developed in the most proficient observers, replacing the less efficient initial search-to-find strategies. <s> BIB012 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> A core skill in diagnostic pathology is light microscopy. Remarkably little is known about human factors that affect the proficiency of pathologists as light microscopists. The cognitive skills of pathologists have received relatively little attention in comparison with the large literature on human performance studies in radiology. One reason for this lack of formal visual search studies in pathology has been the physical restrictions imposed by the close positioning of a microscope operator's head to the microscope's eyepieces. This blocks access to the operator's eyes and precludes assessment of the microscopist's eye movements. Virtual slide microscopy now removes this barrier and opens the door for studies on human factors and visual search strategies in light microscopy. The aim of this study was to assess eye movements of medical students, pathology residents, and practicing pathologists examining virtual slides on a digital display monitor. Whole histopathology glass slide digital images, so-called virtual slides, of 20 consecutive breast core biopsy cases were used in a retrospective study. These high-quality virtual slides were produced with an array-microscope equipped DMetrix DX-40 ultrarapid virtual slide processor (DMetrix, Tucson, Ariz). Using an eye-tracking device, we demonstrated for the first time that when a virtual slide reader initially looks at a virtual slide his or her eyes are very quickly attracted to regions of interest (ROIs) within the slide and that these ROIs are likely to contain diagnostic information. In a matter of seconds, critical decisions are made on the selection of ROIs for further examination at higher magnification. We recorded: (1) the time virtual slide readers spent fixating on self-selected locations on the video monitor; (2) the characteristics of the ways the eyes jumped between fixation locations; and (3) x and y coordinates for each virtual slide marking the sites the virtual slide readers manually selected for zooming to higher ROI magnifications. We correlated the locations of the visually selected fixation locations and the manually selected ROIs. Viewing profiles were identified for each group. Fully trained pathologists spent significantly less time (mean, 4.471 seconds) scanning virtual slides when compared to pathology residents (mean, 7.148 seconds) or medical students (mean, 11.861 seconds), but had relatively prolonged saccadic eye movements (P < .0001). Saccadic eye movements are defined as eye movements between fixation locations. On the other hand, the pathologists spent significantly more time than trainees dwelling on the 3 locations they subsequently chose for zooming. Unlike either the medical students or the residents, the pathologists frequently choose areas for viewing at higher magnification outside of areas of foveal (central) vision. Eye movement studies of scanning pathways (scan paths) may be useful for developing eye movement profiles for individuals and for understanding the difference in performances between novices and experts. They may also be useful for developing new visual search strategies for rendering diagnoses on telepathology virtual slides. <s> BIB013 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> What role does the initial glimpse of a scene play in subsequent eye movement guidance? In 4 experiments, a brief scene preview was followed by object search through the scene via a small moving window that was tied to fixation position. Experiment 1 demonstrated that the scene preview resulted in more efficient eye movements compared with a control preview. Experiments 2 and 3 showed that this scene preview benefit was not due to the conceptual category of the scene or identification of the target object in the preview. Experiment 4 demonstrated that the scene preview benefit was unaffected by changing the size of the scene from preview to search. Taken together, the results suggest that an abstract (size invariant) visual representation is generated in an initial scene glimpse and that this representation can be retained in memory and used to guide subsequent eye movements. <s> BIB014 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Rationale and Objectives Use data collected independently at three institutions to compare time to first fixate the true lesion in searching for cancers on mammograms. Examine the fit of the results to a holistic model of visual perception. Materials and Methods The time required to first fixate a cancer on a mammogram was extracted from 400 eye-tracking records collected independently from three institutions. The time was used as an indicator of the initial perception of cancer. The distribution of first fixation times was partitioned into two normally distributed components using mixture distribution analysis. The true-positive fraction of each component was calculated. Results About 57% of the cancers had a 95% chance of being fixated in the first second of viewing. The remainder took longer (range, 1.0 to 15.2 seconds). The true-positive fraction was larger for the lesions hit immediately for most of the readers (TPF = 0.63 vs. 0.52, F = 5.88, P = .02) in 68% (13/19) of the readers. Conclusions The initial detection occurs before visual scanning and, therefore, must be the result of a parallel “global” analysis of the image resulting in an initial holistic, gestalt-like perception. The development of expertise in medical image analysis may consist of a shift in the recognition mechanism from scan-look-detect to look-detect-scan. <s> BIB015 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Historically, radiology research has been dominated by chest and breast screening. Few studies have examined complex ::: interpretative tasks such as the reading of multidimensional brain CT or MRI scans. Additionally, no studies at the time ::: of writing have explored the interpretation of stroke images; from novices through to experienced practitioners using eye ::: movement analysis. Finally, there appears a lack of evidence on the clinical effects of radiology reports and their ::: influence on image appraisal and clinical diagnosis. A computer-based, eye-tracking study was designed to assess ::: diagnostic accuracy and interpretation in stroke CT and MR imagery. Eight predetermined clinical cases, five images per ::: case, were presented to participants (novices, trainee, and radiologists; n=8). The presence or absence of abnormalities ::: was rated on a five-point Likert scale and their locations reported. Half cases of the cases were accompanied by clinical ::: information; half were not, to assess the impact of information on observer performance. Results highlight differences in ::: visual search patterns amongst novice, trainee and expert observers; the most marked differences occurred between ::: novice readers and experts. Experts spent more time in challenging areas of interest (AOI) than novices and trainee, and ::: were more confident unless a lesion was large and obvious. The time to first AOI fixation differed by size, shape and ::: clarity of lesion. 'Time to lesion' dropped significantly when recognition appeared to occur between slices. The ::: influence of clinical information was minimal. <s> BIB016 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> An understanding of radiologists’ visual and interpretive behaviors, explored in conjunction with the capabilities of innovative advanced technologies, may provide useful new paradigms for interpreting results from medical imaging examinations. <s> BIB017 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> This meta-analysis integrates 296 effect sizes reported in eye-tracking research on expertise differences in the comprehension of visualizations. Three theories were evaluated: Ericsson and Kintsch’s (Psychol Rev 102:211–245, 1995) theory of long-term working memory, Haider and Frensch’s (J Exp Psychol Learn Mem Cognit 25:172–190, 1999) information-reduction hypothesis, and the holistic model of image perception of Kundel et al. (Radiology 242:396–402, 2007). Eye movement and performance data were cumulated from 819 experts, 187 intermediates, and 893 novices. In support of the evaluated theories, experts, when compared with non-experts, had shorter fixation durations, more fixations on task-relevant areas, and fewer fixations on task-redundant areas; experts also had longer saccades and shorter times to first fixate relevant information, owing to superiority in parafoveal processing and selective attention allocation. Eye movements, reaction time, and performance accuracy were moderated by characteristics of visualization (dynamics, realism, dimensionality, modality, and text annotation), task (complexity, time-on-task, and task control), and domain (sports, medicine, transportation, other). These findings are discussed in terms of their implications for theories of visual expertise in professional domains and their significance for the design of learning environments. <s> BIB018 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> How does one find objects in scenes? For decades, visual search models have been built on experiments in which observers search for targets, presented among distractor items, isolated and randomly arranged on blank backgrounds. Are these models relevant to search in continuous scenes? This article argues that the mechanisms that govern artificial, laboratory search tasks do play a role in visual search in scenes. However, scene-based information is used to guide search in ways that had no place in earlier models. Search in scenes might be best explained by a dual-path model: a ‘selective' path in which candidate objects must be individually selected for recognition and a ‘nonselective' path in which information can be extracted from global and/or statistical information. <s> BIB019 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Gaze tracking is a common method to assess perceptual processes when reading medical images. However, little ::: attention has yet been paid to multi-slice images. The present study examines the gaze data of four experienced radiologists reading 15 cranial Computer Tomography scans (CCT), five of which contain lesions. The participants navigated freely through the slices, while their eye position was tracked. Participants' visual search performance was ::: examined in terms of: time per case, scrolling pattern including the number of runs through each case and number of oscillations within each case, fixation duration, time to first fixate a lesion and the initial dwell time on a lesion. The ::: results of the study indicate that performance and reading strategy differ between radiologists. The greatest behavioral ::: differences occurred between the two readers, who performed best. One of them, participant 4, showed extremely short periods of inspection, few oscillations between the slices, short initial dwells on lesions and short time to first fixation, ::: whereas participant 2 performed equally as well, but took longer to read individual cases, went through the slices with many more oscillations, showed longer time to first fixation and initial dwell times on lesions. The behavior displayed by participant 4 is consistent with expert behavior reading 2-dimensional images. In contrast, participant 2's behavior resembles that of a novice, namely because of the systematic search pattern employed. The results hint that expertise may be characterized by various and diverse strategies. <s> BIB020 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> We examined how the ability to detect lung nodules in chest x-ray inspection is reflected in experience-related differences in visual search and decision making, and whether the eye-tracking metric time-to-first hit showed systematic decreases across expertise levels are examined. In the study decision making improved with expertise, however, time-to-first fixate a nodule showed only a non-significant trend to decrease with expertise. Surprisingly, naive and expert observers allocated less visual attention at nodules compared with first and third year radiography students. This similarity in visual attention at nodules but not in decision making was explained by the fact that naive observers were more likely to fixate and make errors on distracter regions. Time-to-first hit has been linked to expert performance in mammography, but in this study was not sufficiently sensitive to demonstrate clear linear improvements across expertise groups. This brings into question the use of this metric as an indirect measure of rapid initial holistic processing. <s> BIB021 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Repeated display configurations improve visual search. Recently, the question has arisen whether this contextual cueing effect (Chun & Jiang, 1998) is itself mediated by attention, both in terms of selectivity and processing resources deployed. While it is accepted that selective attention modulates contextual cueing (Jiang & Leung, 2005), there is an ongoing debate whether the cueing effect is affected by a secondary working memory (WM) task, specifically at which stage WM influences the cueing effect: the acquisition of configural associations (e.g., Travis, Mattingley, & Dux, 2013) versus the expression of learned associations (e.g., Manginelli, Langer, Klose, & Pollmann, 2013). The present study re-investigated this issue. Observers performed a visual search in combination with a spatial WM task. The latter was applied on either early or late search trials—so as to examine whether WM load hampers the acquisition of or retrieval from contextual memory. Additionally, the WM and search tasks were performed either temporally in parallel or in succession—so as to permit the effects of spatial WM load to be dissociated from those of executive load. The secondary WM task was found to affect cueing in late, but not early, experimental trials—though only when the search and WM tasks were performed in parallel. This pattern suggests that contextual cueing involves a spatial WM resource, with spatial WM providing a workspace linking the current search array with configural long-term memory; as a result, occupying this workspace by a secondary WM task hampers the expression of learned configural associations. <s> BIB022 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> In a series of experiments, we investigated the dependence of contextual cueing on working memory resources. A visual search task with 50 % repeated displays was run in order to elicit the implicit learning of contextual cues. The search task was combined with a concurrent visual working memory task either during an initial learning phase or a later test phase. The visual working memory load was either spatial or nonspatial. Articulatory suppression was used to prevent verbalization. We found that nonspatial working memory load had no effect, independent of presentation in the learning or test phase. In contrast, visuospatial load diminished search facilitation in the test phase, but not during learning. We concluded that visuospatial working memory resources are needed for the expression of previously learned spatial contexts, whereas the learning of contextual cues does not depend on visuospatial working memory. <s> BIB023 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Very fast extraction of global structural and statistical regularities allows us to access the “gist”—the basic meaning—of real-world images in as little as 20 ms. Gist processing is central to efficient assessment and orienting in complex environments. This ability is probably based on our extensive experience with the regularities of the natural world. If that is so, would experts develop an ability to extract the gist from the artificial stimuli (e.g., medical images) with which they have extensive visual experience? Anecdotally, experts report some ability to categorize images as normal or abnormal before actually finding an abnormality. We tested the reality of this perception in two expert populations: radiologists and cytologists. Observers viewed brief (250- to 2,000-ms) presentations of medical images. The presence of abnormality was randomized across trials. The task was to rate the abnormality of an image on a 0–100 analog scale and then to attempt to localize that abnormality on a subsequent screen showing only the outline of the image. Both groups of experts had above-chance performance for detecting subtle abnormalities at all stimulus durations (cytologists d' ≈ 1.2 and radiologists d' ≈ 1), whereas the nonexpert control groups did not differ from chance (d' ≈ 0.23, d' ≈ 0.25). Furthermore, the experts’ ability to localize these abnormalities was at chance levels, suggesting that categorization was based on a global signal, and not on fortuitous attention to a localized target. It is possible that this global signal could be exploited to improve clinical performance. <s> BIB024 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> The present eye-movement study assessed the effect of expertise on eye-movement behaviour during image perception in the medical domain. To this end, radiologists, computed-tomography radiographers and psychology students were exposed to nine volumes of multi-slice, stack-view, axial computed-tomography images from the upper to the lower part of the abdomen with or without abnormality. The images were presented in succession at low, medium or high speed, while the participants had to detect enlarged lymph nodes or other visually more salient abnormalities. The radiologists outperformed both other groups in the detection of enlarged lymph nodes and their eye-movement behaviour also differed from the other groups. Their general strategy was to use saccades of shorter amplitude than the two other participant groups. In the presence of enlarged lymph nodes, they increased the number of fixations on the relevant areas and reverted to even shorter saccades. In volumes containing enlarged lymph nodes, radiologists' fixation durations were longer in comparison to their fixation durations in volumes without enlarged lymph nodes. More salient abnormalities were detected equally well by radiologists and radiographers, with both groups outperforming psychology students. However, to accomplish this, radiologists actually needed fewer fixations on the relevant areas than the radiographers. On the basis of these results, we argue that expert behaviour is manifested in distinct eye-movement patterns of proactivity, reactivity and suppression, depending on the nature of the task and the presence of abnormalities at any given moment. <s> BIB025 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Summary The goal of this study was to examine and characterize changes in the ways that pathology residents examine digital whole slide images as they progress through the residency training. A series of 20 digitized breast biopsy whole slide images (half benign and half malignant biopsies) were individually shown to 4 pathology residents at four points in time—at the beginning of their first, second, third, and fourth years of residency. Their task was to examine each image and select three areas that they would most want to zoom in on in order to view the diagnostic detail at higher resolution. Eye position was recorded as they scanned each whole slide image at low magnification. The data indicate that with each successive year of experience, the residents' search patterns do change. Overall, with time, it takes significantly less time to view an individual slide and decide where to zoom, significantly fewer fixations are generated overall, and there is less examination of nondiagnostic areas. Essentially, the residents' search becomes much more efficient. These findings are similar to those in radiology, and support the theory that an important aspect of the development of expertise is improved pattern recognition (taking in more information during the initial Gestalt or gist view) as well as improved allocation of attention and visual processing resources. Progression in improvements in visual search strategies was similar, but not identical, for the 4 residents. <s> BIB026 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> A central question about spatial attention is whether it is referenced relative to the external environment or to the viewer. This question has received great interest in recent psychological and neuroscience research, with many but not all, finding evidence for a viewer-centered representation. However, these previous findings were confined to computer-based tasks that involved stationary viewers. Because natural search behaviors differ from computer-based tasks in viewer mobility and spatial scale, it is important to understand how spatial attention is coded in the natural environment. To this end, we created an outdoor visual search task in which participants searched a large (690 square feet), concrete, outdoor space to report which side of a coin on the ground faced up. They began search in the middle of the space and were free to move around. Attentional cuing by statistical learning was examined by placing the coin in one quadrant of the search space on 50% of the trials. As in computer-based tasks participants learned and used these regularities to guide search. However, cuing could be referenced to either the environment or the viewer. The spatial reference frame of attention shows greater flexibility in the natural environment than previously found in the lab. <s> BIB027 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Visual memory has been demonstrated to play a role in both visual search and attentional prioritization in natural scenes. However, it has been studied predominantly in experimental paradigms using multiple two-dimensional images. Natural experience, however, entails prolonged immersion in a limited number of three-dimensional environments. The goal of the present experiment was to recreate circumstances comparable to natural visual experience in order to evaluate the role of scene memory in guiding eye movements in a natural environment. Subjects performed a continuous visual-search task within an immersive virtual-reality environment over three days. We found that, similar to two-dimensional contexts, viewers rapidly learn the location of objects in the environment over time, and use spatial memory to guide search. Incidental fixations did not provide obvious benefit to subsequent search, suggesting that semantic contextual cues may often be just as efficient, or that many incidentally fixated items are not held in memory in the absence of a specific task. On the third day of the experience in the environment, previous search items changed in color. These items were fixated upon with increased probability relative to control objects, suggesting that memory-guided prioritization (or Surprise) may be a robust mechanisms for attracting gaze to novel features of natural environments, in addition to task factors and simple spatial saliency. <s> BIB028 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> This study provides insight into visual perception in experienced and inexperienced readers who view three-dimensional CT colonography images; in our study, readers pursued polyps in 97% of observations, which indicated that reader errors occurred in either discovery or image recognition, but rarely in the scanning phase. <s> BIB029 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Objective:Eye tracking in three dimensions is novel, but established descriptors derived from two-dimensional (2D) studies are not transferable. We aimed to develop metrics suitable for statistical comparison of eye-tracking data obtained from readers of three-dimensional (3D) “virtual” medical imaging, using CT colonography (CTC) as a typical example.Methods:Ten experienced radiologists were eye tracked while observing eight 3D endoluminal CTC videos. Subsequently, we developed metrics that described their visual search patterns based on concepts derived from 2D gaze studies. Statistical methods were developed to allow analysis of the metrics.Results:Eye tracking was possible for all readers. Visual dwell on the moving region of interest (ROI) was defined as pursuit of the moving object across multiple frames. Using this concept of pursuit, five categories of metrics were defined that allowed characterization of reader gaze behaviour. These were time to first pursuit, identification and assessment time, ... <s> BIB030 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> OBJECTIVE ::: We aimed to identify the effect of computer-aided detection (CAD) on visual search and performance in CT Colonography (CTC) of inexperienced and experienced readers. ::: ::: ::: METHODS ::: Fifteen endoluminal CTC examinations were recorded, each with one polyp, and two videos were generated, one with and one without a CAD mark. Forty-two readers (17 experienced, 25 inexperienced) interpreted the videos during infrared visual search recording. CAD markers and polyps were treated as regions of interest in data processing. This multi-reader, multi-case study was analysed using multilevel modelling. ::: ::: ::: RESULTS ::: CAD drew readers' attention to polyps faster, accelerating identification times: median 'time to first pursuit' was 0.48 s (IQR 0.27 to 0.87 s) with CAD, versus 0.58 s (IQR 0.35 to 1.06 s) without. For inexperienced readers, CAD also held visual attention for longer. All visual search metrics used to assess visual gaze behaviour demonstrated statistically significant differences when "with" and "without" CAD were compared. A significant increase in the number of correct polyp identifications across all readers was seen with CAD (74 % without CAD, 87 % with CAD; p < 0.001). ::: ::: ::: CONCLUSIONS ::: CAD significantly alters visual search and polyp identification in readers viewing three-dimensional endoluminal CTC. For polyp and CAD marker pursuit times, CAD generally exerted a larger effect on inexperienced readers. ::: ::: ::: KEY POINTS ::: • Visual gaze is attracted by computer-assisted detection (CAD) marks on polyps • Inexperienced readers' gaze is affected more by CAD than experienced readers. • CAD marks could mean that the unannotated endoluminal surface is relatively neglected. • Correct polyp identification is increased significantly by CAD. <s> BIB031 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> RATIONALE AND OBJECTIVES ::: To examine the effect of changes in utilization and advances in cross-sectional imaging on radiologists' workload. ::: ::: ::: MATERIALS AND METHODS ::: All computed tomography (CT) and magnetic resonance imaging (MRI) examinations performed at a single institution between 1999 and 2010 were identified and associated with the total number of images for each examination. Annual trends in institutional numbers of interpreted examinations and images were translated to changes in daily workload for the individual radiologist by normalizing to the number of dedicated daily CT and MRI work assignments, assuming a 255-day/8-hour work day schedule. Temporal changes in institutional and individual workload were assessed by Sen's slope analysis (Q = median slope) and Mann-Kendall test (Z = Z statistic). ::: ::: ::: RESULTS ::: From 1999 to 2010, a total of 1,517,149 cross-sectional imaging studies (CT = 994,471; MRI = 522,678) comprising 539,210,581 images (CT = 339,830,947; MRI = 199,379,634) were evaluated at our institution. Total annual cross-sectional studies steadily increased from 84,409 in 1999 to 147,336 in 2010, representing a twofold increase in workload (Q = 6465/year, Z = 4.2, P < .0001). Concomitantly, the number of annual departmental cross-sectional images interpreted increased from 9,294,140 in 1990 to 94,271,551 in 2010, representing a 10-fold increase (Q = 8707876/year, Z = 4.5, P < .0001). Adjusting for staffing changes, the number of images requiring interpretation per minute of every workday per staff radiologist increased from 2.9 in 1999 to 16.1 in 2010 (Q = 1.7/year, Z = 4.3, P < .0001). ::: ::: ::: CONCLUSIONS ::: Imaging volumes have grown at a disproportionate rate to imaging utilization increases at our institution. The average radiologist interpreting CT or MRI examinations must now interpret one image every 3-4 seconds in an 8-hour workday to meet workload demands. <s> BIB032 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> The role of memory in guiding attention allocation in daily behaviors is not well understood. In experiments with two-dimensional (2D) images, there is mixed evidence about the importance of memory. Because the stimulus context in laboratory experiments and daily behaviors differs extensively, we investigated the role of memory in visual search, in both two-dimensional (2D) and three-dimensional (3D) environments. A 3D immersive virtual apartment composed of two rooms was created, and a parallel 2D visual search experiment composed of snapshots from the 3D environment was developed. Eye movements were tracked in both experiments. Repeated searches for geometric objects were performed to assess the role of spatial memory. Subsequently, subjects searched for realistic context objects to test for incidental learning. Our results show that subjects learned the room-target associations in 3D but less so in 2D. Gaze was increasingly restricted to relevant regions of the room with experience in both settings. Search for local contextual objects, however, was not facilitated by early experience. Incidental fixations to context objects do not necessarily benefit search performance. Together, these results demonstrate that memory for global aspects of the environment guides search by restricting allocation of attention to likely regions, whereas task relevance determines what is learned from the active search experience. Behaviors in 2D and 3D environments are comparable, although there is greater use of memory in 3D. <s> BIB033 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Rapid scene recognition is a global visual process we can all exploit to guide search. This ability is thought to underpin expertise in medical image perception yet there is no direct evidence that isolates the expertise-specific contribution of processing scene previews on subsequent eye movement performance. We used the flash-preview moving window paradigm (Castelhano & Henderson, 2007) to investigate this issue. Expert radiologists and novice observers underwent 2 experiments whereby participants either saw a 250ms scene preview or a mask before searching for a target. Observers looked for everyday objects from real-world scenes (Experiment 1), and searched for lung nodules from medical images (Experiment 2). Both expertise groups exploited the brief preview of the upcoming scene to more efficiently guide windowed search in Experiment 1, but there was only a weak effect of domain-specific expertise in Experiment 2, with experts showing small improvements in search metrics with scene previews. Expert diagnostic performance was better than novices in all conditions but was not contingent on seeing the scene preview, and scene preview actually impaired novice diagnostic performance. Experiment 3 required novice and experienced observers to search for a variety abnormalities from different medical images. Rather than maximising the expertise-specific advantage of processing scene previews, both novices and experienced radiographers were worse at detecting abnormalities with scene previews. We discuss how restricting access to the initial glimpse can be compensated for by subsequent search and discovery processing, but there can still be costs in integrating a fleeting glimpse of a medical scene. <s> BIB034 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Purpose To investigate the development of chest radiograph interpretation skill through medical training by measuring both diagnostic accuracy and eye movements during visual search. Materials and Methods An institutional exemption from full ethical review was granted for the study. Five consultant radiologists were deemed the reference expert group, and four radiology registrars, five senior house officers (SHOs), and six interns formed four clinician groups. Participants were shown 30 chest radiographs, 14 of which had a pneumothorax, and were asked to give their level of confidence as to whether a pneumothorax was present. Receiver operating characteristic (ROC) curve analysis was carried out on diagnostic decisions. Eye movements were recorded with a Tobii TX300 (Tobii Technology, Stockholm, Sweden) eye tracker. Four eye-tracking metrics were analyzed. Variables were compared to identify any differences between groups. All data were compared by using the Friedman nonparametric method. Results The average area under the ROC curve for the groups increased with experience (0.947 for consultants, 0.792 for registrars, 0.693 for SHOs, and 0.659 for interns; P = .009). A significant difference in diagnostic accuracy was found between consultants and registrars (P = .046). All four eye-tracking metrics decreased with experience, and there were significant differences between registrars and SHOs. Total reading time decreased with experience; it was significantly lower for registrars compared with SHOs (P = .046) and for SHOs compared with interns (P = .025). Conclusion Chest radiograph interpretation skill increased with experience, both in terms of diagnostic accuracy and visual search. The observed level of experience at which there was a significant difference was higher for diagnostic accuracy than for eye-tracking metrics. (©) RSNA, 2016 Online supplemental material is available for this article. <s> BIB035 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Learning of spatial inter-item associations can speed up visual search in everyday life, an effect referred to as contextual cueing (Chun & Jiang, 1998). Whereas previous studies investigated contextual cueing primarily using 2D layouts, the current study examined how 3D depth influences contextual learning in visual search. In two experiments, the search items were presented evenly distributed across front and back planes in an initial training session. In the subsequent test session, the search items were either swapped between the front and back planes (Experiment 1) or between the left and right halves (Experiment 2) of the displays. The results showed that repeated spatial contexts were learned efficiently under 3D viewing conditions, facilitating search in the training sessions, in both experiments. Importantly, contextual cueing remained robust and virtually unaffected following the swap of depth planes in Experiment 1, but it was substantially reduced (to nonsignificant levels) following the left–right side swap in Experiment 2. This result pattern indicates that spatial, but not depth, inter-item variations limit effective contextual guidance. Restated, contextual cueing (even under 3D viewing conditions) is primarily based on 2D inter-item associations, while depth-defined spatial regularities are probably not encoded during contextual learning. Hence, changing the depth relations does not impact the cueing effect. <s> BIB036 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> Although the facilitation of visual search by contextual information is well established, there is little understanding of the independent contributions of different types of contextual cues in scenes. Here we manipulated 3 types of contextual information: object co-occurrence, multiple object configurations, and background category. We isolated the benefits of each contextual cue to target detectability, its impact on decision bias, confidence, and the guidance of eye movements. We find that object-based information guides eye movements and facilitates perceptual judgments more than scene background. The degree of guidance and facilitation of each contextual cue can be related to its inherent informativeness about the target spatial location as measured by human explicit judgments about likely target locations. Our results improve the understanding of the contributions of distinct contextual scene components to search and suggest that the brain's utilization of cues to guide eye movements is linked to the cue's informativeness about the target's location. (PsycINFO Database Record <s> BIB037 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> AIM ::: To investigate image interpretation performance by diagnostic radiography students, diagnostic radiographers and reporting radiographers by computing eye gaze metrics using eye tracking technology. ::: ::: ::: METHODS ::: Three groups of participants were studied during their interpretation of 8 digital radiographic images including the axial and appendicular skeleton, and chest (prevalence of normal images was 12.5%). A total of 464 image interpretations were collected. Participants consisted of 21 radiography students, 19 qualified radiographers and 18 qualified reporting radiographers who were further qualified to report on the musculoskeletal (MSK) system. ::: ::: ::: OUTCOME MEASURES ::: Eye tracking data was collected using the Tobii X60 eye tracker and subsequently eye gaze metrics were computed. Voice recordings, confidence levels and diagnoses provided a clear demonstration of the image interpretation and the cognitive processes undertaken by each participant. A questionnaire afforded the participants an opportunity to offer information on their experience in image interpretation and their opinion on the eye tracking technology. ::: ::: ::: RESULTS ::: Reporting radiographers demonstrated a 15% greater accuracy rate (p≤0.001), were more confident (p≤0.001) and took a mean of 2.4s longer to clinically decide on all features compared to students. Reporting radiographers also had a 15% greater accuracy rate (p≤0.001), were more confident (p≤0.001) and took longer to clinically decide on an image diagnosis (p=0.02) than radiographers. Reporting radiographers had a greater mean fixation duration (p=0.01), mean fixation count (p=0.04) and mean visit count (p=0.04) within the areas of pathology compared to students. Eye tracking patterns, presented within heat maps, were a good reflection of group expertise and search strategies. Eye gaze metrics such as time to first fixate, fixation count, fixation duration and visit count within the areas of pathology were indicative of the radiographer's competency. ::: ::: ::: CONCLUSION ::: The accuracy and confidence of each group could be reflected in the variability of their eye tracking heat maps. Participants' thoughts and decisions were quantified using the eye tracking data. Eye tracking metrics also reflected the different search strategies that each group of participants adopted during their image interpretations. This is the first study to use eye tracking technology to assess image interpretation skills between various groups of different levels of experience in radiography, especially on a combination of the MSK system, chest cavity and a variety of pathologies. <s> BIB038 </s> What do we know about volumetric medical image interpretation?: a review of the basic science and medical image perception literatures <s> How are scene regularities learned in volumetric images? <s> As a promising imaging modality, digital breast tomosynthesis (DBT) leads to better diagnostic performance than traditional full-field digital mammograms (FFDM) alone. DBT allows different planes of the breast to be visualized, reducing occlusion from overlapping tissue. Although DBT is gaining popularity, best practices for search strategies in this medium are unclear. Eye tracking allowed us to describe search patterns adopted by radiologists searching DBT and FFDM images. Eleven radiologists examined eight DBT and FFDM cases. Observers marked suspicious masses with mouse clicks. Eye position was recorded at 1000 Hz and was coregistered with slice/depth plane as the radiologist scrolled through the DBT images, allowing a 3-D representation of eye position. Hit rate for masses was higher for tomography cases than 2-D cases and DBT led to lower false positive rates. However, search duration was much longer for DBT cases than FFDM. DBT was associated with longer fixations but similar saccadic amplitude compared with FFDM. When comparing radiologists’ eye movements to a previous study, which tracked eye movements as radiologists read chest CT, we found DBT viewers did not align with previously identified “driller” or “scanner” strategies, although their search strategy most closely aligns with a type of vigorous drilling strategy. <s> BIB039
In a typical laboratory search task, the observer might be asked to indicate whether a target is present or absent in a display that consists of randomly ordered objects on a blank background. In contrast, real-world scenes are rich with context, and neighboring objects are often closely related to each other. A toothbrush near the bathroom sink will be identified more quickly than a toothbrush placed on a piano or floating in midair BIB006 . This form of top-down guidance is referred to as scene grammar  . To a knowledgeable observer, medical images are also highly structured and contextual. For example, gallstones always occur in the gallbladder, which is situated under the liver. This greatly constrains the regions of an abdominal CT scan that need to be evaluated for gallstones. This type of top-down knowledge is thought to alter the areas of chest radiographs that are attended over the course of training, leading to qualitatively different search patterns between experts and novices BIB001 BIB010 . Importantly, this effect seems to develop organically without any explicit instructions on how to search through chest radiographs, which suggests it is strongly driven by top-down knowledge about where abnormalities are likely to occur rather than training on specific search strategies. Although our knowledge of the world allows us to make an educated guess about where to find a toothbrush in a stranger's house, we will likely find a toothbrush more quickly in our own bathroom due to repeated experience. In the laboratory, the response time benefit from repeated exposures to the same search array is referred to as contextual cueing BIB005 . Although these effects are typically observed in highly artificial search tasks, contextual cueing is also found in dynamic tasks where targets and distractors repeatedly move with a certain trajectory, 3D depth displays, outdoor environments, and virtual apartments  BIB027 BIB028 BIB033 BIB036 . Furthermore, although object-based information is a strong contextual cue BIB037 , contextual guidance does not necessarily depend on objects in a scene; this information can be extracted from statistical regularities in low-level visual features BIB006 . In addition, contextual cueing is tolerant to a number of changes between exposures BIB008 . Given the large size of volumetric medical images BIB017 BIB032 , it is undoubtedly important for radiologists to lean on some of the aforementioned mechanisms to narrow the search area down to relevant regions of space. It is simply not practical to search every pixel of a large CT scan (Fig. 1) and it is likely this top-down guidance is one of the biggest advantages of expertise (for reviews, see BIB018 . However, unlike 2D medical image interpretation, the influence of top-down knowledge on the observer's search strategy over the course of training when reading volumetric images is largely unknown, particularly as it relates to scrolling through depth. Typically, expertise studies approach these questions by analyzing indirect measures, such as image coverage or time to first hit, across levels of experience (e.g., BIB021 . However, it can be difficult to disentangle the influence of medical knowledge versus learned statistical regularities using these indirect measures of top-down processing. A complementary approach to these indirect measures might be to train novice observers on artificial volumetric displays and determine how search behavior changes with experience. What are the characteristics of expertise in volumetric image interpretation? The advantage of regularities in our environment is that we can form detailed scene representations, known as schemas, to guide visual search behavior. For example, contextual cueing appears to rely on spatial working memory resources for the expression, but not acquisition, of learned displays BIB022 BIB023 . It is thought that spatial working memory rapidly links the current search configuration to schemas held in long-term memory, making the observer sensitive to statistical irregularities in their environment. In fact, familiar scenes presented for a fraction of a second can be accurately categorized BIB002 , guide subsequent eye-movements BIB014 , and increase the detectability of novel objects in the scene BIB009 BIB011 . This phenomenon is referred to in the literature as "gist", "holistic", or "global" processing, and it is frequently studied using a flash moving-window paradigm BIB014 . In this paradigm, observers are shown a brief preview of the scene followed by a mask and a subsequent target cue. The search task is performed using a gaze contingent window, which eliminates the influence of online parafoveal processing and isolates the effect of scene preview (i.e., the initial holistic impression) on search behavior. In radiology, the beneficial effects of scene preview appear to be more modest than those observed in the visual search literature. Scene previews before a lung cancer detection task were associated with small improvements in search time and fewer overall fixations BIB034 . However, these benefits did not correspond with an increase in accuracy and were only weakly associated with expertise. Furthermore, scene previews appeared to be harmful if the pathology varied between trials. Nonetheless, there is strong evidence that radiologists are able to rapidly detect statistical anomalies in medical images. BIB003 found that 70% of lung nodules were detected after chest radiographs were viewed for only 200 ms. Similarly, research has shown that mammographers can classify images as normal or abnormal at a rate above chance after viewing them for only 250 ms BIB024 . However, the ability to localize these lesions was at chance (though see  . In addition, the majority (57%) of breast cancers and a large portion (33%) of lung cancers are fixated on in the first second of viewing, which is simply not enough time to perform a thorough search BIB021 BIB012 BIB015 . In addition, eye-tracking demonstrates that expertise is associated with substantial differences in search behavior: experts exhibit more circumferential scan patterns, shorter time to first fixation, greater fixation duration, a smaller fixation count, less image coverage, and reduced variability in gaze BIB001 BIB038 . Notably, expert-like scan patterns may pre-date expert decisionmaking BIB035 . These findings have led to a series of models on medical image perception, which all feature holistic processing as a prominent component of expertise Swennson, 1980) . Swennson proposed a two-stage model. The first stage involves a pre-attentional filter, similar to feature integration theory (FIT), which rapidly selects certain areas of the image for processing. In the second stage, the areas marked during the first stage receive further scrutiny. Similarly, BIB004 proposed a global-focal search model. During an initial global impression, the image is rapidly compared to the observer's schema of a normal image. In the next stage, perturbations between the image and the mental representation are further evaluated using focused attention. Finally, outlined a model that relies on two parallel pathways (see also BIB019 . The nonselective pathway extracts global information from the image using a large field of view. The selective pathway extracts detailed visual information that supports object recognition using a more focal search. Although these models are nuanced, they all emphasize the importance of rapidly extracting global information to guide search behavior: an ability which is thought to increase with experience. Although much is known about expertise in 2D images, there is far less research on expertise in volumetric images. Of the existing research, several rudimentary findings have been replicated in volumetric images (Table 2 ). For example, experts are more accurate, search faster, locate abnormalities more quickly, and exhibit more fixations in regions of interest (Bertram, Helle, Kaakinen, & Svedstrom, 2013; BIB016  BIB029 . However, many findings based on 2D medical images have not been replicated using volumetric images (Table 2 ). For example, BIB025 found no differences in average fixation duration between experts and novices, which is typically used as an index of increased processing ability with expertise. In addition, the researchers found no group differences in saccadic amplitude, which is a key index of global processing ability. Similarly, BIB029 failed to find any differences in eye-movements between experts and novices in fly-by endoluminal CT colonography, aside from reduced time to first pursuit. However, both of these studies utilized tasks (e.g., enlarged lymph nodes, visceral abnormalities, and colon polyps) that are far removed from the tasks typically used in studies with 2D images. At present, it is unclear if these differences are due to the nature of the task or fundamental differences in how expertise is expressed in volumetric imaging. In fact, there are very few direct eye-tracking comparisons between 2D and volumetric search. In a rare example of this approach, BIB039 found that breast tomosynthesis was associated with longer fixations and less image coverage than traditional mammography. However, saccadic amplitude was equivalent, which suggests an equal ability to rely on parafoveal processing in both modalities. The increase in holistic processing ability with expertise is one of the most important discoveries in the medical image perception literature, but it is almost completely unknown how these abilities might manifest BIB025 Kelehan, et al., 2019 in volumetric images. One possibility is that global impressions are continually formed on the 2D plane as the observer scrolls through depth. For example, the observer might fixate more quickly on abnormalities from the moment they are first visible on the screen (e.g., BIB030 BIB031 . If this is the case, analogs of eyetracking measures associated with global processing in 2D image interpretation should transfer to volumetric images (Table 2) . However, a global representation of the scene could also be formed by rapidly scrolling through the depth of the image prior to utilizing a more focal search pattern. There is support for this proposal in the literature. In real-world tasks, such as making a sandwich, observers conduct an initial scan of the scene, which helps them locate target objects more quickly during the task BIB007 . Moreover, it is possible that global processing ability is expressed differently based on the search strategy of the observer. For scanners, global impressions might be established on the 2D plane with each transition through depth. In contrast, drillers might establish a global impression by scrolling through depth and then returning to layers of depth that were statistically anomalous. In addition to scanners and drillers, other metrics of scrolling behaviors through depth have been proposed in relation to global processing ability (Table 3) : the number of visits per slice, the number of oscillations (scrolling back and forth through less than 25% of depth), the number of half runs (scrolling back and forth through 25-50% of depth), and the number of full runs (scrolling back and forth through > 50% of depth) BIB020 . Radiologists who engage in more full runs are thought to use a more global search process, which should increase with the experience of the observer. However, this proposal has not yet been tested, and these measures have not been widely used outside of this initial exploratory study using cranial CT images. In other realms of medical imaging that might be considered similar to volumetric images, such as virtual microscopy, there is a clear link between expertise and global processing ability BIB026 BIB013 . Although Option 1: Time to first fixation is calculated relative to the moment ROI is first visible. Option 2: Time to first-pursuit, defined as consecutive fixations within 50 pixels of an ROI for at least 100 ms. .