Enabling global processing in simultanagnosia by psychophysical biasing of visual pathways

The Impact of Spatial Frequency on the Perception of Crowd Emotion: An fMRI Study

Recognizing the emotions of faces in a crowd is crucial for understanding overall behavior and intention as well as for smooth and friendly social interactions. However, it is unclear whether the spatial frequency of faces affects the discrimination of crowd emotion. Although high- and low-spatial-frequency information for individual faces is processed by distinct neural channels, there is a lack of evidence on how this applies to crowd faces. Here, we used functional magnetic resonance imaging (fMRI) to investigate neural representations of crowd faces at different spatial frequencies. Thirty-three participants were asked to compare whether a test face was happy or more fearful than a crowd face that varied in high, low, and broad spatial frequencies. Our findings revealed that fearful faces with low spatial frequencies were easier to recognize in terms of accuracy (78.9%) and response time (927 ms). Brain regions, such as the fusiform gyrus, located in the ventral visual stream, were preferentially activated in high spatial frequency crowds, which, however, were the most difficult to recognize behaviorally (68.9%). Finally, the right inferior frontal gyrus was found to be better activated in the broad spatial frequency crowds. Our study suggests that people are more sensitive to fearful crowd faces with low spatial frequency and that high spatial frequency does not promote crowd face recognition.

Form Properties of Moving Targets Bias Smooth Pursuit Target Selection in Monkeys

During natural viewing, we often recognize multiple objects, detect their motion, and select one object as the target to track. It remains to be determined how such behavior is guided by the integration of visual form and motion perception. To address this, we studied how monkeys made a choice to track moving targets with different forms by smooth pursuit eye movements in a two-target task. We found that pursuit responses were biased toward the motion direction of a target with a hole. By computing the relative weighting, we found that the target with a hole exhibited a larger weight for vector computation. The global hole feature dominated other form properties. This dominance failed to account for changes in pursuit responses to a target with different forms moving singly. These findings suggest that the integration of visual form and motion perception can reshape the competition in sensorimotor networks to guide behavioral selection.

Temporal asymmetries and interactions between dorsal and ventral visual pathways during object recognition

Despite their anatomical and functional distinctions, there is growing evidence that the dorsal and ventral visual pathways interact to support object recognition. However, the exact nature of these interactions remains poorly understood. Is the presence of identity-relevant object information in the dorsal pathway simply a byproduct of ventral input? Or, might the dorsal pathway be a source of input to the ventral pathway for object recognition? In the current study, we used high-density EEG-a technique with high temporal precision and spatial resolution sufficient to distinguish parietal and temporal lobes-to characterise the dynamics of dorsal and ventral pathways during object viewing. Using multivariate analyses, we found that category decoding in the dorsal pathway preceded that in the ventral pathway. Importantly, the dorsal pathway predicted the multivariate responses of the ventral pathway in a time-dependent manner, rather than the other way around. Together, these findings suggest that the dorsal pathway is a critical source of input to the ventral pathway for object recognition.

A generative adversarial model of intrusive imagery in the human brain

The mechanisms underlying the subjective experiences of mental disorders remain poorly understood. This is partly due to long-standing over-emphasis on behavioral and physiological symptoms and a de-emphasis of the patient's subjective experiences when searching for treatments. Here, we provide a new perspective on the subjective experience of mental disorders based on findings in neuroscience and artificial intelligence (AI). Specifically, we propose the subjective experience that occurs in visual imagination depends on mechanisms similar to generative adversarial networks that have recently been developed in AI. The basic idea is that a generator network fabricates a prediction of the world, and a discriminator network determines whether it is likely real or not. Given that similar adversarial interactions occur in the two major visual pathways of perception in people, we explored whether we could leverage this AI-inspired approach to better understand the intrusive imagery experiences of patients suffering from mental illnesses such as post-traumatic stress disorder (PTSD) and acute stress disorder. In our model, a nonconscious visual pathway generates predictions of the environment that influence the parallel but interacting conscious pathway. We propose that in some patients, an imbalance in these adversarial interactions leads to an overrepresentation of disturbing content relative to current reality, and results in debilitating flashbacks. By situating the subjective experience of intrusive visual imagery in the adversarial interaction of these visual pathways, we propose testable hypotheses on novel mechanisms and clinical applications for controlling and possibly preventing symptoms resulting from intrusive imagery.

Does the brain's ventral visual pathway compute object shape?

A rich behavioral literature has shown that human object recognition is supported by a representation of shape that is tolerant to variations in an object's appearance. Such 'global' shape representations are achieved by describing objects via the spatial arrangement of their local features, or structure, rather than by the appearance of the features themselves. However, accumulating evidence suggests that the ventral visual pathway - the primary substrate underlying object recognition - may not represent global shape. Instead, ventral representations may be better described as a basis set of local image features. We suggest that this evidence forces a reevaluation of the role of the ventral pathway in object perception and posits a broader network for shape perception that encompasses contributions from the dorsal pathway.

The influence of magnocellular and parvocellular visual information on global processing in White and Asian populations

Humans have the remarkable ability to efficiently group elements of a scene together to form a global whole. However, cross-cultural comparisons show that East Asian individuals process scenes more globally than White individuals. This experiment presents new insights into global processing, revealing the relative contributions of two types of visual cells in mediating global and local visual processing in these two groups. Participants completed the Navon hierarchical letters task under divided-attention conditions, indicating whether a target letter “H” was present in the stimuli. Stimuli were either ‘unbiased’, displayed as black letters on a grey screen, or biased to predominantly process low spatial frequency information using psychophysical thresholds that converted unbiased stimuli into achromatic magnocellular-biased stimuli and red-green isoluminant parvocellular-biased stimuli. White participants processed stimuli more globally than Asian participants when low spatial frequency information was conveyed via the parvocellular pathway, while Asian participants showed a global processing advantage when low spatial frequency information was conveyed via the magnocellular pathway, and to a lesser extent through the parvocellular pathway. These findings suggest that the means by which a global processing bias is achieved depends on the subcortical pathway through which visual information is transmitted, and provides a deeper understanding of the relationship between global/local processing, subcortical pathways and spatial frequencies.

The Dorsal Visual Pathway Represents Object-Centered Spatial Relations for Object Recognition

Although there is mounting evidence that input from the dorsal visual pathway is crucial for object processes in the ventral pathway, the specific functional contributions of dorsal cortex to these processes remain poorly understood. Here, we hypothesized that dorsal cortex computes the spatial relations among an object's parts, a process crucial for forming global shape percepts, and transmits this information to the ventral pathway to support object categorization. Using fMRI with human participants (females and males), we discovered regions in the intraparietal sulcus (IPS) that were selectively involved in computing object-centered part relations. These regions exhibited task-dependent functional and effective connectivity with ventral cortex, and were distinct from other dorsal regions, such as those representing allocentric relations, 3D shape, and tools. In a subsequent experiment, we found that the multivariate response of posterior (p)IPS, defined on the basis of part-relations, could be used to decode object category at levels comparable to ventral object regions. Moreover, mediation and multivariate effective connectivity analyses further suggested that IPS may account for representations of part relations in the ventral pathway. Together, our results highlight specific contributions of the dorsal visual pathway to object recognition. We suggest that dorsal cortex is a crucial source of input to the ventral pathway and may support the ability to categorize objects on the basis of global shape.SIGNIFICANCE STATEMENT Humans categorize novel objects rapidly and effortlessly. Such categorization is achieved by representing an object's global shape structure, that is, the relations among object parts. Yet, despite their importance, it is unclear how part relations are represented neurally. Here, we hypothesized that object-centered part relations may be computed by the dorsal visual pathway, which is typically implicated in visuospatial processing. Using fMRI, we identified regions selective for the part relations in dorsal cortex. We found that these regions can support object categorization, and even mediate representations of part relations in the ventral pathway, the region typically thought to support object categorization. Together, these findings shed light on the broader network of brain regions that support object categorization.

The Birthday Party Test (BPT): A new picture description test to support the assessment of simultanagnosia in patients with acquired brain injury

There is broad consensus on the utility of complex pictures in the assessment of simultanagnosia in patients with acquired brain injury (ABI). To overcome various shortcomings of current instruments, we have developed the Birthday Party Test (BPT); a picture description test that contains a neutral scene, a balanced representation of events, and provides clear instructions and a scoring-aid. We have applied the BPT in a large group of patients with ABI (n = 502) and in an age-matched healthy control group (n = 194). Our results show that performance on the BPT was associated with a range of descriptive, neuropsychological and clinical characteristics and that poor test performance appeared to be more common in patients with etiologies that have an increased risk of bilateral damage. Furthermore, we assume a high correspondence between test performance on the BPT and the assessor's clinical judgment of likely having simultanagnosia in preliminary analyses. This study shows the potential usefulness of the BPT to support diagnostic decision making in simultanagnosia. The BPT is made freely available to facilitate its broad application in the clinical assessment of patients with visual impairment and to enable a further evaluation of its utility and validity in future studies.

Brain structural and functional anomalies associated with simultanagnosia in patients with posterior cortical atrophy

Simultanagnosia is a common symptom of posterior cortical atrophy, and its association with brain structural and functional changes remains unclear. In our study, 18 posterior cortical atrophy patients with simultanagnosia, 29 patients with Alzheimer’s disease and 20 cognitively normal controls were recruited and subjected to full neuropsychological evaluation, including simultanagnosia tests, and structural and resting-state functional MRI. The gray matter volume was assessed by voxel-based morphometry, while the intrinsic functional connectivity was evaluated using the reduced gray matter volume regions of interest as the seed. In contrast to the patients with Alzheimer’s disease, those with posterior cortical atrophy showed the following: (1) markedly lower simultanagnosia test scores, (2) an altered regional gray matter volume of the left middle occipital gyrus and ventral occipital areas, and (3) lowered intrinsic functional connectivity with the left middle occipital gyrus, left lingual gyrus and right middle occipital gyrus separately. Additionally, the gray matter volume of the left middle occipital gyrus and left inferior occipital gyrus were each correlated with simultanagnosia in posterior cortical atrophy patients. The intrinsic functional connectivity of the left middle occipital gyrus with the right superior occipital gyrus and that of the right middle occipital gyrus with the left superior parietal gyrus were also correlated with simultanagnosia in posterior cortical atrophy patients. In summary, this study indicated that simultanagnosia is associated with gray matter reductions and decreased functional connectivity in the left middle occipital gyrus and the left inferior occipital gyrus in patients with posterior cortical atrophy.

Differential neurodynamics and connectivity in the dorsal and ventral visual pathways during perception of emotional crowds and individuals: a MEG study

Reading the prevailing emotion of groups of people ("crowd emotion") is critical to understanding their overall intention and disposition. It alerts us to potential dangers, such as angry mobs or panicked crowds, giving us time to escape. A critical aspect of processing crowd emotion is that it must occur rapidly, because delays often are costly. Although knowing the timing of neural events is crucial for understanding how the brain guides behaviors using coherent signals from a glimpse of multiple faces, this information is currently lacking in the literature on face ensemble coding. Therefore, we used magnetoencephalography to examine the neurodynamics in the dorsal and ventral visual streams and the periamygdaloid cortex to compare perception of groups of faces versus individual faces. Forty-six participants compared two groups of four faces or two individual faces with varying emotional expressions and chose which group or individual they would avoid. We found that the dorsal stream was activated as early as 68 msec after the onset of stimuli containing groups of faces. In contrast, the ventral stream was activated later and predominantly for individual face stimuli. The latencies of the dorsal stream activation peaks correlated with participants' response times for facial crowds. We also found enhanced connectivity earlier between the periamygdaloid cortex and the dorsal stream regions for crowd emotion perception. Our findings suggest that ensemble coding of facial crowds proceeds rapidly and in parallel by engaging the dorsal stream to mediate adaptive social behaviors, via a distinct route from single face perception.

Visuoperceptive Impairments in Severe Alcohol Use Disorder: A Critical Review of Behavioral Studies

The present literature review is aimed at offering a comprehensive and critical view of behavioral data collected during the past seventy years concerning visuoperception in severe alcohol use disorders (AUD). To pave the way for a renewal of research and clinical approaches in this very little understood field, this paper (1) provides a critical review of previous behavioral studies exploring visuoperceptive processing in severe AUD, (2) identifies the alcohol-related parameters and demographic factors that influence the deficits, and (3) addresses the limitations of this literature and their implications for current clinical strategies. By doing so, this review highlights the presence of visuoperceptive deficits but also shows how the lack of in-depth studies exploring the visual system in this clinical population results in the current absence of integration of these deficits in the dominant models of vision. Given the predominance of vision in everyday life, we stress the need to better delineate the extent, the specificity, and the actual implications of the deficits for severe AUD.

Superior Parietal Lobule: A Role in Relative Localization of Multiple Different Elements

Simultanagnosia is an impairment in processing multiple visual elements simultaneously consecutive to bilateral posterior parietal damage, and neuroimaging data have specifically implicated the superior parietal lobule (SPL) in multiple element processing. We previously reported that a patient with focal and bilateral lesions of the SPL performed slower than controls in visual search but only for stimuli consisting of separable lines. Here, we further explored this patient's visual processing of plain object (colored disk) versus object consisting of separable lines (letter), presented in isolation (single object) versus in triplets. Identification of objects was normal in isolation but dropped to chance level when surrounded by distracters, irrespective of eccentricity and spacing. We speculate that this poor performance reflects a deficit in processing objects' relative locations within the triplet (for colored disks), aggravated by a deficit in processing the relative location of each separable line (for letters). Confirming this, performance improved when the patient just had to detect the presence of a specific colored disk within the triplets (visual search instruction), while the inability to identify the middle letter was alleviated when the distracters were identical letters that could be grouped, thereby reducing the number of ways individual lines could be bound.

Resting EEG in alpha band predicts individual differences in visual size perception

Human visual size perception results from an interaction of external sensory information and internal state. The cognitive mechanisms involved in the processing of context-dependent visual size perception have been found to be innate in nature to some extent, suggesting that visual size perception might correlate with human intrinsic brain activity. Here we recorded human resting alpha activity (8-12 Hz), which is an inverse indicator of sustained alertness. Moreover, we measured an object's perceived size in a two-alternative forced-choice manner and the Ebbinghaus illusion magnitude which is a classic illustration of context-dependent visual size perception. The results showed that alpha activity along the ventral visual pathway, including left V1, right LOC and bilateral inferior temporal gyrus, negatively correlated with an object's perceived size. Moreover, alpha activity in the left superior temporal gyrus positively correlated with size discrimination threshold and size illusion magnitude. The findings provide clear evidence that human visual size perception scales as a function of intrinsic alertness, with higher alertness linking to larger perceived size of objects and better performance in size discrimination and size illusion tasks, and suggest that individual variation in resting-state brain activity provides a neural explanation for individual variation in cognitive performance of normal participants.

Convergence Along the Visual Hierarchy Is Altered in Posterior Cortical Atrophy

Purpose

Posterior cortical atrophy (PCA) is a rare neurodegenerative syndrome manifesting with visuospatial processing impairment. We recently suggested that abnormal population receptive field properties are associated with the symptoms of PCA patients. Specifically, simultanagnosia, the inability to perceive multiple items simultaneously, can be explained by smaller peripheral population receptive fields, and foveal crowding, in which nearby distractors interfere with object perception, may result from larger foveal population receptive fields. These effects occurred predominantly in V1, even though atrophy mainly involves high-order areas. In this study, we used connective field modeling to better understand these inter-area interactions.

Methods

We used functional magnetic resonance imaging to scan six PCA patients and eight controls while they viewed drifting bar stimuli. Resting-state data were also collected. Connective field modeling was applied for both conditions: once when the source was V1 and the targets were extrastriate areas and once for the opposite direction. The difference between the two was defined as convergence magnitude.

Results

With stimulus, the convergence magnitude of the controls increased along the visual pathway, suggesting that spatial integration from V1 becomes larger up the visual hierarchy. No such slope was found in the PCA patients. The difference between the groups originated mainly from the dorsal pathway. Without stimulus, the convergence magnitude was negative, slightly more so for the PCA patients, with no slope, suggesting constant divergence along the visual hierarchy.

Conclusions

Atrophy in one part of the visual system can affect other areas within the network through complex intervisual area interactions, resulting in modulation of population receptive field properties and an ensemble of visuocognitive function impairments.

Progressive dysexecutive syndrome due to Alzheimer's disease: a description of 55 cases and comparison to other phenotypes

We report a group of patients presenting with a progressive dementia syndrome characterized by predominant dysfunction in core executive functions, relatively young age of onset and positive biomarkers for Alzheimer's pathophysiology. Atypical frontal, dysexecutive/behavioural variants and early-onset variants of Alzheimer's disease have been previously reported, but no diagnostic criteria exist for a progressive dysexecutive syndrome. In this retrospective review, we report on 55 participants diagnosed with a clinically defined progressive dysexecutive syndrome with 18F-fluorodeoxyglucose-positron emission tomography and Alzheimer's disease biomarkers available. Sixty-two per cent of participants were female with a mean of 15.2 years of education. The mean age of reported symptom onset was 53.8 years while the mean age at diagnosis was 57.2 years. Participants and informants commonly referred to initial cognitive symptoms as 'memory problems' but upon further inquiry described problems with core executive functions of working memory, cognitive flexibility and cognitive inhibitory control. Multi-domain cognitive impairment was evident in neuropsychological testing with executive dysfunction most consistently affected. The frontal and parietal regions which overlap with working memory networks consistently demonstrated hypometabolism on positron emission tomography. Genetic testing for autosomal dominant genes was negative in all eight participants tested and at least one APOE ε4 allele was present in 14/26 participants tested. EEG was abnormal in 14/17 cases with 13 described as diffuse slowing. Furthermore, CSF or neuroimaging biomarkers were consistent with Alzheimer's disease pathophysiology, although CSF p-tau was normal in 24% of cases. Fifteen of the executive predominate participants enrolled in research neuroimaging protocols and were compared to amnestic (n = 110), visual (n = 18) and language (n = 7) predominate clinical phenotypes of Alzheimer's disease. This revealed a consistent pattern of hypometabolism in parieto-frontal brain regions supporting executive functions with relative sparing of the medial temporal lobe (versus amnestic phenotype), occipital (versus visual phenotype) and left temporal (versus language phenotype). We propose that this progressive dysexecutive syndrome should be recognized as a distinct clinical phenotype disambiguated from behavioural presentations and not linked specifically to the frontal lobe or a particular anatomic substrate without further study. This clinical presentation can be due to Alzheimer's disease but is likely not specific for any single aetiology. Diagnostic criteria are proposed to facilitate additional research into this understudied clinical presentation.

Fast saccadic and manual responses to faces presented to the koniocellular visual pathway

The parallel pathways of the human visual system differ in their tuning to luminance, color, and spatial frequency. These attunements recently have been shown to propagate to differential processing of higher-order stimuli, facial threat cues, in the magnocellular (M) and parvocellular (P) pathways, with greater sensitivity to clear and ambiguous threat, respectively. The role of the third, koniocellular (K) pathway in facial threat processing, however, remains unknown. To address this gap in knowledge, we briefly presented peripheral face stimuli psychophysically biased towards M, P, or K pathways. Observers were instructed to report via a key-press whether the face was angry or neutral while their eye movements and manual responses were recorded. We found that short-latency saccades were made more frequently to faces presented in the K channel than to P or M channels. Saccade latencies were not significantly modulated by expressive and identity cues. In contrast, manual response latencies and accuracy were modulated by both pathway biasing and by interactions of facial expression with facial masculinity, such that angry male faces elicited the fastest, and angry female faces, the least accurate, responses. We conclude that face stimuli can evoke fast saccadic and manual responses when projected to the K pathway.

Magnocellular and parvocellular pathway contributions to facial threat cue processing

Human faces evolved to signal emotions, with their meaning contextualized by eye gaze. For instance, a fearful expression paired with averted gaze clearly signals both presence of threat and its probable location. Conversely, direct gaze paired with facial fear leaves the source of the fear-evoking threat ambiguous. Given that visual perception occurs in parallel streams with different processing emphases, our goal was to test a recently developed hypothesis that clear and ambiguous threat cues would differentially engage the magnocellular (M) and parvocellular (P) pathways, respectively. We employed two-tone face images to characterize the neurodynamics evoked by stimuli that were biased toward M or P pathways. Human observers (N = 57) had to identify the expression of fearful or neutral faces with direct or averted gaze while their magnetoencephalogram was recorded. Phase locking between the amygdaloid complex, orbitofrontal cortex (OFC) and fusiform gyrus increased early (0–300 ms) for M-biased clear threat cues (averted-gaze fear) in the β-band (13–30 Hz) while P-biased ambiguous threat cues (direct-gaze fear) evoked increased θ (4–8 Hz) phase locking in connections with OFC of the right hemisphere. We show that M and P pathways are relatively more sensitive toward clear and ambiguous threat processing, respectively, and characterize the neurodynamics underlying emotional face processing in the M and P pathways.

Growing up with Bilateral parieto-occipital injury: over ten years of clinical observation and neuropsychological follow-up

This paper presents a follow-up of a child with Balint's syndrome over more than a decade. The patient experienced traumatic brain injury before age 12, resulting in bilateral occipito-parietal infarctions and a clinical presentation of Balint's syndrome. Neuropsychological assessments at three time points showed average verbal abilities alongside persistent difficulties in visual orientation, mirrored in the patient's daily life. Her outstanding compensatory abilities in the face of these impairments are discussed with respect to the recruitment of the ventral visual stream and the role of top-down processing. This profile may help to determine interventions for younger patients with similar lesions.

Differential magnocellular versus parvocellular pathway contributions to the combinatorial processing of facial threat

Recently, speed of presentation of facially expressive stimuli was found to influence the processing of compound threat cues (e.g., anger/fear/gaze). For instance, greater amygdala responses were found to clear (e.g., direct gaze anger/averted gaze fear) versus ambiguous (averted gaze anger/direct gaze fear) combinations of threat cues when rapidly presented (33 and 300ms), but greater to ambiguous versus clear threat cues when presented for more sustained durations (1, 1.5, and 2s). A working hypothesis was put forth (Adams et al., 2012) that these effects were due to differential magnocellular versus parvocellular pathways contributions to the rapid versus sustained processing of threat, respectively. To test this possibility directly here, we restricted visual stream processing in the fMRI environment using facially expressive stimuli specifically designed to bias visual input exclusively to the magnocellular versus parvocellular pathways. We found that for magnocellular-biased stimuli, activations were predominantly greater to clear versus ambiguous threat-gaze pairs (on par with that previously found for rapid presentations of threat cues), whereas activations to ambiguous versus clear threat-gaze pairs were greater for parvocellular-biased stimuli (on par with that previously found for sustained presentations). We couch these findings in an adaptive dual process account of threat perception and highlight implications for other dual process models within psychology.

More than Action: The Dorsal Pathway Contributes to the Perception of 3-D Structure

An evolving view in cognitive neuroscience is that the dorsal visual pathway not only plays a key role in visuomotor behavior but that it also contributes functionally to the recognition of objects. To characterize the nature of the object representations derived by the dorsal pathway, we assessed perceptual performance in the context of the continuous flash suppression paradigm, which suppresses object processing in the ventral pathway while sparing computation in the dorsal pathway. In a series of experiments, prime stimuli, which were rendered imperceptible by the continuous flash suppression, still contributed to perceptual decisions related to the subsequent perceptible target stimuli. However, the contribution of the prime to perception was contingent on the prime's structural coherence, in that a perceptual advantage was observed only for targets primed by objects with legitimate 3-D structure. Finally, we obtained additional evidence to demonstrate that the processing of the suppressed objects was contingent on the magnocellular, rather than the parvocellular, system, further linking the processing of the suppressed stimuli to the dorsal pathway. Together, these results provide novel evidence that the dorsal pathway does not only support visuomotor control but, rather, that it also derives the structural description of 3-D objects and contributes to shape perception.

Sex-related differences in behavioral and amygdalar responses to compound facial threat cues

During face perception, we integrate facial expression and eye gaze to take advantage of their shared signals. For example, fear with averted gaze provides a congruent avoidance cue, signaling both threat presence and its location, whereas fear with direct gaze sends an incongruent cue, leaving threat location ambiguous. It has been proposed that the processing of different combinations of threat cues is mediated by dual processing routes: reflexive processing via magnocellular (M) pathway and reflective processing via parvocellular (P) pathway. Because growing evidence has identified a variety of sex differences in emotional perception, here we also investigated how M and P processing of fear and eye gaze might be modulated by observer's sex, focusing on the amygdala, a structure important to threat perception and affective appraisal. We adjusted luminance and color of face stimuli to selectively engage M or P processing and asked observers to identify emotion of the face. Female observers showed more accurate behavioral responses to faces with averted gaze and greater left amygdala reactivity both to fearful and neutral faces. Conversely, males showed greater right amygdala activation only for M-biased averted-gaze fear faces. In addition to functional reactivity differences, females had proportionately greater bilateral amygdala volumes, which positively correlated with behavioral accuracy for M-biased fear. Conversely, in males only the right amygdala volume was positively correlated with accuracy for M-biased fear faces. Our findings suggest that M and P processing of facial threat cues is modulated by functional and structural differences in the amygdalae associated with observer's sex.

Line-Drawn Scenes Provide Sufficient Information for Discrimination of Threat and Mere Negativity

Previous work using color photographic scenes has shown that human observers are keenly sensitive to different types of threatening and negative stimuli and reliably classify them by the presence, and spatial and temporal directions of threat. To test whether such distinctions can be extracted from impoverished visual information, we used 500 line drawings made by hand-tracing the original set of photographic scenes. Sixty participants rated the scenes on spatial and temporal dimensions of threat. Based on these ratings, trend analysis revealed five scene categories that were comparable to those identified for the matching color photographic scenes. Another 61 participants were randomly assigned to rate the valence or arousal evoked by the line drawings. The line drawings perceived to be the most negative were also perceived to be the most arousing, replicating the finding for color photographic scenes. We demonstrate here that humans are very sensitive to the spatial and temporal directions of threat even when they must extract this information from simple line drawings, and rate the line drawings very similarly to matched color photographs. The set of 500 hand-traced line-drawing scenes has been made freely available to the research community: http://www.kveragalab.org/threat.html.

Observer's anxiety facilitates magnocellular processing of clear facial threat cues, but impairs parvocellular processing of ambiguous facial threat cues

Facial expression and eye gaze provide a shared signal about threats. While a fear expression with averted gaze clearly points to the source of threat, direct-gaze fear renders the source of threat ambiguous. Separable routes have been proposed to mediate these processes, with preferential attunement of the magnocellular (M) pathway to clear threat, and of the parvocellular (P) pathway to threat ambiguity. Here we investigated how observers’ trait anxiety modulates M- and P-pathway processing of clear and ambiguous threat cues. We scanned subjects (N = 108) widely ranging in trait anxiety while they viewed fearful or neutral faces with averted or directed gaze, with the luminance and color of face stimuli calibrated to selectively engage M- or P-pathways. Higher anxiety facilitated processing of clear threat projected to M-pathway, but impaired perception of ambiguous threat projected to P-pathway. Increased right amygdala reactivity was associated with higher anxiety for M-biased averted-gaze fear, while increased left amygdala reactivity was associated with higher anxiety for P-biased, direct-gaze fear. This lateralization was more pronounced with higher anxiety. Our findings suggest that trait anxiety differentially affects perception of clear (averted-gaze fear) and ambiguous (direct-gaze fear) facial threat cues via selective engagement of M and P pathways and lateralized amygdala reactivity.

Differential hemispheric and visual stream contributions to ensemble coding of crowd emotion

In crowds, where scrutinizing individual facial expressions is inefficient, humans can make snap judgments about the prevailing mood by reading "crowd emotion". We investigated how the brain accomplishes this feat in a set of behavioral and fMRI studies. Participants were asked to either avoid or approach one of two crowds of faces presented in the left and right visual hemifields. Perception of crowd emotion was improved when crowd stimuli contained goal-congruent cues and was highly lateralized to the right hemisphere. The dorsal visual stream was preferentially activated in crowd emotion processing, with activity in the intraparietal sulcus and superior frontal gyrus predicting perceptual accuracy for crowd emotion perception, whereas activity in the fusiform cortex in the ventral stream predicted better perception of individual facial expressions. Our findings thus reveal significant behavioral differences and differential involvement of the hemispheres and the major visual streams in reading crowd versus individual face expressions.

Topological Relations Between Objects Are Categorically Coded

How do individuals compare images-for example, two graphs or diagrams-to identify differences between them? We argue that categorical relations between objects play a critical role. These relations divide continuous space into discrete categories, such as "above" and "below," or "containing" and "overlapping," which are remembered and compared more easily than precise metric values. These relations should lead to categorical perception, such that viewers find it easier to notice a change that crosses a category boundary (one object is now above, rather than below, another, or now contains, rather than overlaps with, another) than a change of equal magnitude that does not cross a boundary. We tested the influence of a set of topological categorical relations from the cognitive-modeling literature. In a visual same/different comparison task, viewers more accurately noticed changes that crossed relational category boundaries, compared with changes that did not cross these boundaries. The results highlight the potential of systematic exploration of the boundaries of between-object relational categories.

Simultanagnosia and object individuation

Simultanagnosic patients have difficulty in perceiving multiple objects when presented simultaneously. In this review article, I discuss how neuropsychological research on simultanagnosia has been inspirational for two interconnected lines of research related to the core mechanisms by which the visual system processes cluttered scenes. First, I review previous studies on enumeration tasks indicating that, despite their inability to identify multiple objects, simultanagnosic patients can enumerate up to 2-3 elements as efficiently as healthy individuals (the so-called "subitizing" phenomenon). This intriguing observation is one of the first results to support the existence of an "object individuation" mechanism that can spatially tag a limited set of objects simultaneously, and resonates with recent research on the brain dynamics of enumeration in healthy individuals. Second, I further develop the implications of the dissociation between object identification and object enumeration in simultanagnosia specifically for the distinction between object identification and individuation. The latter distinction has been the subject of recent neuroimaging research that has provided fine-grained information on the spatial as well as temporal aspects of object individuation and recognition. The lessons learned from neuropsychological research on exact enumeration in simultanagnosia can be generalized to the normal functioning of the human mind, and have provided insightful clues for cognitive neuroscience.

Functional Plasticity in the Absence of Structural Change

This work presents a case of a young woman with apraxia and a severe body scheme disorder, 10 years after a childhood frontal and occipitoparietal brain injury. Despite specific limitations, she is independent in performing all activities of daily living. A battery of tests was administered to evaluate praxis and body representations. Specifically, the Hand Laterality Test was used to compare RS's dynamic body representation to that of healthy controls (N = 14). Results demonstrated RS's severe praxis impairment, and the Hand Laterality Test revealed deficits in accuracy and latency of motor imagery, suggesting a significant impairment in dynamic body representation. However, semantic and structural body representations were intact. These results, coupled with frequent use of verbalizations as a strategy, suggest a possible ventral compensatory mechanism (top-down processing) for dorsal stream deficits, which may explain RS's remarkable recovery of activities of daily living. The link between praxis and dynamic body representation is discussed.

Looking but Not Seeing

Posterior cortical atrophy (PCA) is the canonical “visual dementia,” with affected individuals experiencing a progressive disintegration of their visual world owing to dysfunction and atrophy at the back of the brain. The syndrome, which also affects literacy, numeracy, and gesture, is typically caused by Alzheimer’s disease, but is distinguished from more common amnestic presentations by virtue of relatively preserved episodic memory and insight. Although problems with object and space perception are the most widely reported and investigated symptoms, these higher-order perceptual difficulties are often underpinned by an array of changes in more basic visual and oculomotor processes. Here we review recent studies providing insights into these more elementary aspects of vision in PCA, including fixation stability, saccade generation, point localization, excessive crowding, and factors affecting the effective field of vision. We argue that a more detailed appreciation of these fundamental changes in the early visual system not only will improve the characterization and understanding of this rare clinico-radiological syndrome but will also guide the design of visual aids and strategies aimed at maintaining everyday abilities in individuals with PCA.

Here, there and everywhere: higher visual function and the dorsal visual stream

The dorsal visual stream, often referred to as the 'where' stream, represents the pathway taken by visual information from the primary visual cortex to the posterior parietal lobe and onwards. It partners the ventral or 'what' stream, the subject of a previous review and largely a temporal-based system. Here, we consider the dorsal stream disorders of perception (simultanagnosia, akinetopsia) along with their consequences on action (eg, optic ataxia and oculomotor apraxia, along with Balint's syndrome). The role of the dorsal stream in blindsight and hemispatial neglect is also considered.

Functional neural substrates of posterior cortical atrophy patients

Posterior cortical atrophy (PCA) is a neurodegenerative syndrome in which the most pronounced pathologic involvement is in the occipito-parietal visual regions. Herein, we aimed to better define the cortical reflection of this unique syndrome using a thorough battery of behavioral and functional MRI (fMRI) tests. Eight PCA patients underwent extensive testing to map their visual deficits. Assessments included visual functions associated with lower and higher components of the cortical hierarchy, as well as dorsal- and ventral-related cortical functions. fMRI was performed on five patients to examine the neuronal substrate of their visual functions. The PCA patient cohort exhibited stereopsis, saccadic eye movements and higher dorsal stream-related functional impairments, including simultant perception, image orientation, figure-from-ground segregation, closure and spatial orientation. In accordance with the behavioral findings, fMRI revealed intact activation in the ventral visual regions of face and object perception while more dorsal aspects of perception, including motion and gestalt perception, revealed impaired patterns of activity. In most of the patients, there was a lack of activity in the word form area, which is known to be linked to reading disorders. Finally, there was evidence of reduced cortical representation of the peripheral visual field, corresponding to the behaviorally assessed peripheral visual deficit. The findings are discussed in the context of networks extending from parietal regions, which mediate navigationally related processing, visually guided actions, eye movement control and working memory, suggesting that damage to these networks might explain the wide range of deficits in PCA patients.

Abnormalities of fixation, saccade and pursuit in posterior cortical atrophy

The clinico-neuroradiological syndrome posterior cortical atrophy is the cardinal 'visual dementia' and most common atypical Alzheimer's disease phenotype, offering insights into mechanisms underlying clinical heterogeneity, pathological propagation and basic visual phenomena (e.g. visual crowding). Given the extensive attention paid to patients' (higher order) perceptual function, it is surprising that there have been no systematic analyses of basic oculomotor function in this population. Here 20 patients with posterior cortical atrophy, 17 patients with typical Alzheimer's disease and 22 healthy controls completed tests of fixation, saccade (including fixation/target gap and overlap conditions) and smooth pursuit eye movements using an infrared pupil-tracking system. Participants underwent detailed neuropsychological and neurological examinations, with a proportion also undertaking brain imaging and analysis of molecular pathology. In contrast to informal clinical evaluations of oculomotor dysfunction frequency (previous studies: 38%, current clinical examination: 33%), detailed eyetracking investigations revealed eye movement abnormalities in 80% of patients with posterior cortical atrophy (compared to 17% typical Alzheimer's disease, 5% controls). The greatest differences between posterior cortical atrophy and typical Alzheimer's disease were seen in saccadic performance. Patients with posterior cortical atrophy made significantly shorter saccades especially for distant targets. They also exhibited a significant exacerbation of the normal gap/overlap effect, consistent with 'sticky fixation'. Time to reach saccadic targets was significantly associated with parietal and occipital cortical thickness measures. On fixation stability tasks, patients with typical Alzheimer's disease showed more square wave jerks whose frequency was associated with lower cerebellar grey matter volume, while patients with posterior cortical atrophy showed large saccadic intrusions whose frequency correlated significantly with generalized reductions in cortical thickness. Patients with both posterior cortical atrophy and typical Alzheimer's disease showed lower gain in smooth pursuit compared to controls. The current study establishes that eye movement abnormalities are near-ubiquitous in posterior cortical atrophy, and highlights multiple aspects of saccadic performance which distinguish posterior cortical atrophy from typical Alzheimer's disease. We suggest the posterior cortical atrophy oculomotor profile (e.g. exacerbation of the saccadic gap/overlap effect, preserved saccadic velocity) reflects weak input from degraded occipito-parietal spatial representations of stimulus location into a superior collicular spatial map for eye movement regulation. This may indicate greater impairment of identification of oculomotor targets rather than generation of oculomotor movements. The results highlight the critical role of spatial attention and object identification but also precise stimulus localization in explaining the complex real world perception deficits observed in posterior cortical atrophy and many other patients with dementia-related visual impairment.

The Attentional Fields of Visual Search in Simultanagnosia and Healthy Individuals: How Object and Space Attention Interact

Simultanagnosia is a deficit in which patients are unable to perceive multiple objects simultaneously. To date, it remains disputed whether this deficit results from disrupted object or space perception. We asked both healthy participants as well as a patient with simultanagnosia to perform different visual search tasks of variable difficulty. We also modulated the number of objects (target and distracters) presented. For healthy participants, we found that each visual search task was performed with a specific "attentional field" depending on the difficulty of visual object processing but not on the number of objects falling within this "working space." This was demonstrated by measuring the cost in reaction times using different gaze-contingent visible window sizes. We found that bilateral damage to the superior parietal lobule impairs the spatial integration of separable features (within-object processing), shrinking the attentional field in which a target can be detected, but causing no deficit in processing multiple objects per se.

Abnormal Center–Periphery Gradient in Spatial Attention in Simultanagnosia

Patients suffering from simultanagnosia cannot perceive more than one object at a time. The underlying mechanism is incompletely understood. One hypothesis is that simultanagnosia reflects “tunnel vision,” a constricted attention window around gaze, which precludes the grouping of individual objects. Although this idea has a long history in neuropsychology, the question whether the patients indeed have an abnormal attention gradient around the gaze has so far not been addressed. Here we tested this hypothesis in two simultanagnosia patients with bilateral parieto-occipital lesions and two control groups, with and without brain damage. We assessed the participants' ability to discriminate letters presented briefly at fixation with and without a peripheral distractor or in the visual periphery, with or without a foveal distractor. A constricted span of attention around gaze would predict an increased susceptibility to foveated versus peripheral distractors. Contrary to this prediction and unlike both control groups, the patients' ability to discriminate the target decreased more in the presence of peripheral compared with foveated distractors. Thus, the attentional spotlight in simultanagnosia does not fall on foveated objects as previously assumed, but rather abnormally highlights the periphery. Furthermore, we found the same center–periphery gradient in the patients' ability to recognize multiple objects. They detected multiple, but not single objects more accurately in the periphery than at fixation. These results suggest that an abnormal allocation of attention around the gaze can disrupt the grouping of individual objects into an integrated visual scene.

The temporo-parietal junction contributes to global gestalt perception-evidence from studies in chess experts

In a recent neuroimaging study the comparison of intact vs. disturbed perception of global gestalt indicated a significant role of the temporo-parietal junction (TPJ) in the intact perception of global gestalt (Huberle and Karnath, 2012). This location corresponded well with the areas known to be damaged or impaired in patients with simultanagnosia after stroke or due to neurodegenerative diseases. It was concluded that the TPJ plays an important role in the integration of individual items to a holistic percept. Thus, increased BOLD signals should be found in this region whenever a task calls for the integration of multiple visual items. Behavioral experiments in chess experts suggested that their superior skills in comparison to chess novices are partly based on fast holistic processing of chess positions with multiple pieces. We thus analyzed BOLD data from four fMRI studies that compared chess experts with chess novices during the presentation of complex chess-related visual stimuli (Bilalić et al., 2010, 2011a,b, 2012). Three regions of interests were defined by significant TPJ clusters in the abovementioned study of global gestalt perception (Huberle and Karnath, 2012) and BOLD signal amplitudes in these regions were compared between chess experts and novices. These cross-paradigm ROI analyses revealed higher signals at the TPJ in chess experts in comparison to novices during presentations of complex chess positions. This difference was consistent across the different tasks in five independent experiments. Our results confirm the assumption that the TPJ region identified in previous work on global gestalt perception plays an important role in the processing of complex visual stimulus configurations.

The role of size constancy for the integration of local elements into a global shape

Visual perception depends on the visual context and is likely to be influenced by size constancy, which predicts a size and distance invariant perception of objects. However, size constancy can also result in optical illusions that allow the manipulation of the perceived size. We thus asked whether the integration of local elements into a global object can be influenced by manipulations of the visual context and size constancy? A set of stimuli was applied in healthy individuals that took advantage of the “Kanizsa” illusion, in which three circles with open wedges oriented toward a center point are placed to form an illusionary perception of a triangle. In addition, a 3D-perspective view was implemented in which the global target (“Kanizsa” triangle) was placed in combination with several distractor circles either in a close or a distant position. Subjects were engaged in a global recognition task on the location of the “Kanizsa” triangle. Global recognition of “Kanizsa” triangles improved with a decreasing length of the illusory contour. Interestingly, recognition of “Kanizsa” triangles decreased when they were perceived as if they were located further away. We conclude that the integration of local elements into a global object is dependent on the visual context and dominated by size constancy.

Schemas reveal spatial relations to a patient with simultanagnosia

Maps, graphs, and diagrams use simplified graphic forms, like lines and blobs, to represent basic spatial relations, like boundaries and enclosures. A schema is an iconic representation where perceptual detail has been abstracted away from reality in order to provide a more flexible structure for cognition. Unlike truly symbolic representations of spatial relations (i.e., prepositions) a schema preserves some analog spatial qualities of the relation it stands in for. We tested the efficacy of schemas in facilitating the perception and comprehension of spatial relations in a patient with bilateral occipitoparietal damage and resulting simultanagnosia. Patient E.E. performed six matching tasks involving WORDS (in, on, above, below), photographic PICTURES of objects, and/or SCHEMAS depicting the same spatial relations. E.E. was instructed to match a single spatial relation to a corresponding image from an array of four choices. On the two tasks that did not include matching to or from schemas, E.E. performed at chance levels. On tasks with schemas, performance was significantly better, indicating that schematic representations make spatial relations visible in a manner that symbols and complex images do not. The results provide general insight as to how schemas facilitate spatial reasoning when used in graphic depictions, and how such theoretically intermediate representational structures could serve to link perceptual and verbal representations of spatial relations in the brain.

Insights from neuropsychology: pinpointing the role of the posterior parietal cortex in episodic and working memory

The role of posterior parietal cortex (PPC) in various forms of memory is a current topic of interest in the broader field of cognitive neuroscience. This large cortical region has been linked with a wide range of mnemonic functions affecting each stage of memory processing: encoding, maintenance, and retrieval. Yet, the precise role of the PPC in memory remains mysterious and controversial. Progress in understanding PPC function will require researchers to incorporate findings in a convergent manner from multiple experimental techniques rather than emphasizing a particular type of data. To facilitate this process, here, we review findings from the human neuropsychological research and examine the consequences to memory following PPC damage. Recent patient-based research findings have investigated two typically disconnected fields: working memory (WM) and episodic memory. The findings from patient participants with unilateral and bilateral PPC lesions performing diverse experimental paradigms are summarized. These findings are then related to findings from other techniques including neurostimulation (TMS and tDCS) and the influential and more abundant functional neuroimaging literature. We then review the strengths and weaknesses of hypotheses proposed to account for PPC function in these forms of memory. Finally, we address what missing evidence is needed to clarify the role(s) of the PPC in memory.