Resting-state neural correlates of visual Gestalt experience.

Subjective perceptual experience is influenced not only by bottom-up sensory information and experience-based top-down processes, but also by an individual's current brain state. Specifically, a previous study found increased prestimulus insula and intraparietal sulcus (IPS) activity before participants perceived an illusory Gestalt (global) compared with the non-illusory (local) interpretation of a bistable stimulus. That study provided only a snapshot of the brain state that favors the illusory interpretation. In the current study, we tested whether areas that differentiate between the illusory and non-illusory perception, immediately before stimulus onset, are also associated with an individual's general tendency to perceive it, which remains stable over time. We examined individual differences in task-free functional connectivity of insula and IPS and related them to differences in the individuals' duration of the two stimulus interpretations. We found stronger connectivity of the IPS with areas of the default mode and visual networks to be associated with shorter local perceptual phases, i.e. a faster switch to an illusory percept, and an opposite effect for insula connectivity with the early visual cortex. Our findings suggest an important role of IPS and insula interactions with nodes of key intrinsic networks in forming a perceptual tendency toward illusory Gestalt perception.

Fast and functionally specific cortical thickness changes induced by visual stimulation.

Structural characteristics of the human brain serve as important markers of brain development, aging, disease progression, and neural plasticity. They are considered stable properties, changing slowly over time. Multiple recent studies reported that structural brain changes measured with magnetic resonance imaging (MRI) may occur much faster than previously thought, within hours or even minutes. The mechanisms behind such fast changes remain unclear, with hemodynamics as one possible explanation. Here we investigated the functional specificity of cortical thickness changes induced by a flickering checkerboard and compared them to blood oxygenation level-dependent (BOLD) functional MRI activity. We found that checkerboard stimulation led to a significant thickness increase, which was driven by an expansion at the gray-white matter boundary, functionally specific to V1, confined to the retinotopic representation of the checkerboard stimulus, and amounted to 1.3% or 0.022 mm. Although functional specificity and the effect size of these changes were comparable to those of the BOLD signal in V1, thickness effects were substantially weaker in V3. Furthermore, a comparison of predicted and measured thickness changes for different stimulus timings suggested a slow increase of thickness over time, speaking against a hemodynamic explanation. Altogether, our findings suggest that visual stimulation can induce structural gray matter enlargement measurable with MRI.

Dissociation between Attention-Dependent and Spatially Specific Illusory Shape Responses within the Topographic Areas of the Posterior Parietal Cortex.

The human visual system consists of multiple topographic maps that extend from the early visual cortex (EVC) along the dorsal and ventral processing streams. Responses to illusory shapes within these maps have been demonstrated in the ventral stream areas, in particular the lateral occipital complex (LOC). Recently, the intraparietal sulcus (IPS) of the dorsal stream has been linked to the processing of illusory shapes defined by motion. It remains unclear whether the topographically organized parietal areas also respond to stationary illusory shapes, which would suggest their generic role in representing illusory content. In the current study we measured brain responses using fMRI while 30 human participants (12 male) observed flickering inducers around the fixation task. The inducers either formed an illusory diamond in the center, a triangle in the left or right hemifield, or were inverted such that no illusory figure was formed. We compared responses of parietal regions IPS0-IPS5 and SPL1 to each illusory figure with the nonillusory condition. To determine the role of attentional modulation on illusory shape responses we manipulated the difficulty of the fixation task. Our results show that all IPS areas responded to illusory shapes. The more posterior areas IPS0-IPS3 additionally displayed a preference toward contralateral shapes, while the more anterior areas IPS4 and IPS5 showed response attenuation with increased task difficulty. We suggest that the IPS can represent illusory content generated not only by moving, but also by stationary stimuli, and that there is a functional dissociation between attention-dependent anterior and spatially specific posterior topographic maps.SIGNIFICANCE STATEMENT The traditional view of the ventral visual pathway being solely responsible for representation of objects has recently been challenged by demonstrating illusory shape representation within the dorsal visual pathway with moving bistable stimuli. Our results provide evidence for the dorsal stream contribution to representing not only moving, but also stationary illusory shapes. Our results also show a functional subdivision along the topographic maps, with spatially specific shape responses in the more posterior, and attention-dependent responses in the more anterior areas. These findings have implications for our understanding of the relationship between attention and grouping in healthy individuals and neuropsychological patients. Furthermore, IPS areas should be considered in theoretical accounts and models of how subjective content is generated in the brain.

Increased insula activity precedes the formation of subjective illusory Gestalt.

The constructive nature of human perception sometimes leads us to perceiving rather complex impressions from simple sensory input: for example, recognizing animal contours in cloud formations or seeing living creatures in shadows of objects. A special type of bistable stimuli gives us a rare opportunity to study the neural mechanisms behind this process. Such stimuli can be visually interpreted either as simple or as more complex illusory content on the basis of the same sensory input. Previous studies demonstrated increased activity in the superior parietal cortex during the perception of an illusory Gestalt impression compared to a simpler interpretation. Here, we examined the role of slow fluctuations of resting-state fMRI activity in shaping the subsequent illusory interpretation by investigating activity related to the illusory Gestalt not only during, but also prior to its perception. We presented 31 participants with a bistable motion stimulus, which can be perceived either as four moving dot pairs (local) or two moving illusory squares (global). fMRI was used to measure brain activity in a slow event-related design. We observed stronger IPS and putamen responses to the stimulus when participants perceived the global interpretation compared to the local, confirming the findings of previous studies. Most importantly, we also observed that the global stimulus interpretation was preceded by an increased activity of the bilateral dorsal insula, which is known to process saliency and gate information for conscious access. Our data suggest an important role of the dorsal insula in shaping complex illusory interpretations of the sensory input.

Improving in vivo human cerebral cortical surface reconstruction using data-driven super-resolution.

Accurate and automated reconstruction of the in vivo human cerebral cortical surface from anatomical magnetic resonance (MR) images facilitates the quantitative analysis of cortical structure. Anatomical MR images with sub-millimeter isotropic spatial resolution improve the accuracy of cortical surface and thickness estimation compared to the standard 1-millimeter isotropic resolution. Nonetheless, sub-millimeter resolution acquisitions require averaging multiple repetitions to achieve sufficient signal-to-noise ratio and are therefore long and potentially vulnerable to subject motion. We address this challenge by synthesizing sub-millimeter resolution images from standard 1-millimeter isotropic resolution images using a data-driven supervised machine learning-based super-resolution approach achieved via a deep convolutional neural network. We systematically characterize our approach using a large-scale simulated dataset and demonstrate its efficacy in empirical data. The super-resolution data provide improved cortical surfaces similar to those obtained from native sub-millimeter resolution data. The whole-brain mean absolute discrepancy in cortical surface positioning and thickness estimation is below 100 µm at the single-subject level and below 50 µm at the group level for the simulated data, and below 200 µm at the single-subject level and below 100 µm at the group level for the empirical data, making the accuracy of cortical surfaces derived from super-resolution sufficient for most applications.

Improved cortical surface reconstruction using sub-millimeter resolution MPRAGE by image denoising.

Automatic cerebral cortical surface reconstruction is a useful tool for cortical anatomy quantification, analysis and visualization. Recently, the Human Connectome Project and several studies have shown the advantages of using T1-weighted magnetic resonance (MR) images with sub-millimeter isotropic spatial resolution instead of the standard 1-mm isotropic resolution for improved accuracy of cortical surface positioning and thickness estimation. Nonetheless, sub-millimeter resolution images are noisy by nature and require averaging multiple repetitions to increase the signal-to-noise ratio for precisely delineating the cortical boundary. The prolonged acquisition time and potential motion artifacts pose significant barriers to the wide adoption of cortical surface reconstruction at sub-millimeter resolution for a broad range of neuroscientific and clinical applications. We address this challenge by evaluating the cortical surface reconstruction resulting from denoised single-repetition sub-millimeter T1-weighted images. We systematically characterized the effects of image denoising on empirical data acquired at 0.6 mm isotropic resolution using three classical denoising methods, including denoising convolutional neural network (DnCNN), block-matching and 4-dimensional filtering (BM4D) and adaptive optimized non-local means (AONLM). The denoised single-repetition images were found to be highly similar to 6-repetition averaged images, with a low whole-brain averaged mean absolute difference of ~0.016, high whole-brain averaged peak signal-to-noise ratio of ~33.5 dB and structural similarity index of ~0.92, and minimal gray matter-white matter contrast loss (2% to 9%). The whole-brain mean absolute discrepancies in gray matter-white matter surface placement, gray matter-cerebrospinal fluid surface placement and cortical thickness estimation were lower than 165 µm, 155 µm and 145 µm-sufficiently accurate for most applications. These discrepancies were approximately one third to half of those from 1-mm isotropic resolution data. The denoising performance was equivalent to averaging ~2.5 repetitions of the data in terms of image similarity, and 1.6-2.2 repetitions in terms of the cortical surface placement accuracy. The scan-rescan variability of the cortical surface positioning and thickness estimation was lower than 170 µm. Our unique dataset and systematic characterization support the use of denoising methods for improved cortical surface reconstruction at sub-millimeter resolution.

Eye-selective fMRI activity in human primary visual cortex: Comparison between 3 ​T and 9.4 ​T, and effects across cortical depth.

The primary visual cortex of humans contains patches of neurons responding preferentially to stimulation of one eye (the ocular dominance columns). Multiple previous studies attempted to detect their activity using fMRI. The majority of these fMRI studies used magnetic field strengths of 4 â€‹T and higher. However, there have been reports of reliable eye-selective activations at 3 â€‹T as well. In this study we investigated the possibility of detecting eye-selective V1 activity using high-resolution GE-EPI fMRI at 3 â€‹T and sub-millimeter resolution fMRI at ultrahigh 9.4 â€‹T magnetic field strengths with acquisition parameters optimized for each field strength. High-resolution fMRI at 9.4 â€‹T also allowed us to examine the eye-selectivity responses across the cortical depth, which are expected to be strongest in the middle layers. We observed a substantial increase in the percentage of eye-selective voxels, as well as a doubling in run-to-run consistency of eye preference at ultrahigh field compared to 3 â€‹T. We also found that across cortical depth, eye selectivity increased towards the superficial layers, and that signal contrast increased while noise remained nearly constant towards the surface. The depth-resolved results are consistent with a distortion of spatial specificity of the GE-EPI signal by ascending venules and large draining veins on the cortical surface. The effects of larger vessels cause increasing signal amplitude, but also displacement of the maximum BOLD signal relative to neural activity. In summary, our results show that increase in spatial resolution, reduced partial volume effects, and improved sensitivity at 9.4 â€‹T allow for better detection of eye-selective signals related to ocular dominance columns. However, although ultrahigh field yields higher sensitivity to the ocular dominance signal, GE-EPI still suffers from specificity issues, with a prominent signal contribution at shallow depths from larger cortical vessels.

A Generic Mechanism for Perceptual Organization in the Parietal Cortex.

Our visual system's ability to group visual elements into meaningful entities and to separate them from others is referred to as scene segmentation. Visual motion often provides a powerful cue for this process as parallax or coherence can inform the visual system about scene or object structure. Here we tested the hypothesis that scene segmentation by motion cues relies on a common neural substrate in the parietal cortex. We used fMRI and a set of three entirely distinct motion stimuli to examine scene segmentation in the human brain. The stimuli covered a wide range of high-level processes, including perceptual grouping, transparent motion, and depth perception. All stimuli were perceptually bistable such that percepts alternated every few seconds while the physical stimulation remained constant. The perceptual states were asymmetric, in that one reflected the default (nonsegmented) interpretation, and the other the non-default (segmented) interpretation. We confirmed behaviorally that upon stimulus presentation, the default percept was always perceived first, before perceptual alternations ensued. Imaging results showed that across all stimulus classes perceptual scene-segmentation was associated with an increase of activity in the posterior parietal cortex together with a decrease of neural signal in the early visual cortex. This pattern of activation is compatible with predictive coding models of visual perception, and suggests that parietal cortex hosts a generic mechanism for scene segmentation.SIGNIFICANCE STATEMENT Making sense of cluttered visual scenes is crucial for everyday perception. An important cue to scene segmentation is visual motion: slight movements of scene elements give away which elements belong to the foreground or background or to the same object. We used three distinct stimuli that engage visual scene segmentation mechanisms based on motion. They involved perceptual grouping, transparent motion, and depth perception. Brain activity associated with all three mechanisms converged in the same parietal region with concurrent deactivation of early visual areas. The results suggest that posterior parietal cortex is a hub involved in structuring visual scenes based on different motion cues, and that feedback modulates early cortical processing in accord with predictive coding theory.

Analysis strategies for high-resolution UHF-fMRI data.

Functional MRI (fMRI) benefits from both increased sensitivity and specificity with increasing magnetic field strength, making it a key application for Ultra-High Field (UHF) MRI scanners. Most UHF-fMRI studies utilize the dramatic increases in sensitivity and specificity to acquire high-resolution data reaching sub-millimeter scales, which enable new classes of experiments to probe the functional organization of the human brain. This review article surveys advanced data analysis strategies developed for high-resolution fMRI at UHF. These include strategies designed to mitigate distortion and artifacts associated with higher fields in ways that attempt to preserve spatial resolution of the fMRI data, as well as recently introduced analysis techniques that are enabled by these extremely high-resolution data. Particular focus is placed on anatomically-informed analyses, including cortical surface-based analysis, which are powerful techniques that can guide each step of the analysis from preprocessing to statistical analysis to interpretation and visualization. New intracortical analysis techniques for laminar and columnar fMRI are also reviewed and discussed. Prospects for single-subject individualized analyses are also presented and discussed. Altogether, there are both specific challenges and opportunities presented by UHF-fMRI, and the use of proper analysis strategies can help these valuable data reach their full potential.

Advantages of cortical surface reconstruction using submillimeter 7 T MEMPRAGE.

Recent advances in MR technology have enabled increased spatial resolution for routine functional and anatomical imaging, which has created demand for software tools that are able to process these data. The availability of high-resolution data also raises the question of whether higher resolution leads to substantial gains in accuracy of quantitative morphometric neuroimaging procedures, in particular the cortical surface reconstruction and cortical thickness estimation. In this study we adapted the FreeSurfer cortical surface reconstruction pipeline to process structural data at native submillimeter resolution. We then quantified the differences in surface placement between meshes generated from (0.75 mm)3 isotropic resolution data acquired in 39 volunteers and the same data downsampled to the conventional 1 mm3 voxel size. We find that when processed at native resolution, cortex is estimated to be thinner in most areas, but thicker around the Cingulate and the Calcarine sulci as well as in the posterior bank of the Central sulcus. Thickness differences are driven by two kinds of effects. First, the gray-white surface is found closer to the white matter, especially in cortical areas with high myelin content, and thus low contrast, such as the Calcarine and the Central sulci, causing local increases in thickness estimates. Second, the gray-CSF surface is placed more interiorly, especially in the deep sulci, contributing to local decreases in thickness estimates. We suggest that both effects are due to reduced partial volume effects at higher spatial resolution. Submillimeter voxel sizes can therefore provide improved accuracy for measuring cortical thickness.

Binocular rivalry transitions predict inattention symptom severity in adult ADHD.

Attention deficit and hyperactivity disorder (ADHD) is a prevalent childhood disorder that is often maintained throughout the development and persists into adulthood. Established etiology models suggest that deficient inhibition underlies the core ADHD symptoms. While experimental evidence for impaired motor inhibition is overwhelming, little is known about the sensory inhibition processes, their changes throughout the development, and the relationship to ADHD symptoms. Here, we used the well-established binocular rivalry (BR) paradigm to investigate for the very first time the inhibitory processes related to visual perception in adults with ADHD. In BR, perception alternates between two dichoptically presented images throughout the viewing period, with shorter dominant percept durations and longer transition periods indicating poorer suppression/inhibition. Healthy controls (N = 28) and patients with ADHD (N = 32) were presented with two dissimilar images (orthogonal gratings) separately to each eye through a mirror stereoscope and asked to report their perceptual experiences. There were no differences between groups in any of the BR markers. However, an association between transition durations and symptom severity emerged in the ADHD group. Importantly, an exploratory multiple regression analysis revealed that inattention symptoms were the sole predictor for the duration of transition periods. The lack of impairments to sensory inhibition in adult, but not pediatric ADHD may reflect compensatory changes associated with development, while a correlation between inhibition and inattention symptoms may reveal an invariant core of the disorder.

Scene segmentation in early visual cortex during suppression of ventral stream regions.

A growing body of literature suggests that feedback modulation of early visual processing is ubiquitous and central to cortical computation. In particular stimuli with high-level content that invariably activate ventral object responsive regions have been shown to suppress early visual cortex. This suppression was typically interpreted in the framework of predictive coding and feedback from ventral regions. Here we examined early visual modulation during perception of a bistable Gestalt illusion that has previously been shown to be mediated by dorsal parietal cortex rather than by ventral regions that were not activated. The bistable dynamic stimulus consisted of moving dots that could either be perceived as corners of a large moving cube (global Gestalt) or as distributed sets of locally moving elements. We found that perceptual binding of local moving elements into an illusory Gestalt led to spatially segregated differential modulations in both, V1 and V2: representations of illusory lines and foreground were enhanced, while inducers and background were suppressed. Furthermore, correlation analyses suggest that distinct mechanisms govern fore- and background modulation. Our results demonstrate that motion-induced Gestalt perception differentially modulates early visual cortex in the absence of ventral stream activation.

Parietal cortex mediates perceptual Gestalt grouping independent of stimulus size.

The integration of local moving elements into a unified gestalt percept has previously been linked to the posterior parietal cortex. There are two possible interpretations for the lack of involvement of other occipital regions. The first is that parietal cortex is indeed uniquely functionally specialized to perform grouping. Another possibility is that other visual regions can perform grouping as well, but that the large spatial separation of the local elements used previously exceeded their neurons' receptive field (RF) sizes, preventing their involvement. In this study we distinguished between these two alternatives. We measured whole-brain activity using fMRI in response to a bistable motion illusion that induced mutually exclusive percepts of either an illusory global Gestalt or of local elements. The stimulus was presented in two sizes, a large version known to activate IPS only, and a version sufficiently small to fit into the RFs of mid-level dorsal regions such as V5/MT. We found that none of the separately localized motion regions apart from parietal cortex showed a preference for global Gestalt perception, even for the smaller version of the stimulus. This outcome suggests that grouping-by-motion is mediated by a specialized size-invariant mechanism with parietal cortex as its anatomical substrate.

Gestalt perception is associated with reduced parietal beta oscillations.

The ability to perceive composite objects as a whole is fundamental for visual perception in a complex and cluttered natural environment. This ability may be mediated by increased communication between neural representations of distinct object elements, and has been linked to increased synchronization of oscillatory brain activity in the gamma band. Previous studies of perceptual grouping either guided attention between local and global aspects of a given stimulus or manipulated its physical properties to achieve grouped and ungrouped perceptual conditions. In contrast to those studies, we fully matched the physical properties underlying global and local percepts using a bistable stimulus that causes the viewer to perceive either local motion of multiple elements or global motion of two illusory shapes without any external change. To test the synchronization hypothesis we recorded brain activity with EEG, while human participants viewed the stimulus and reported changes in their perception. In contrast to previous findings we show that power of the beta-band was lower during perception of global Gestalt than during that of local elements. Source localization places these differences in the posterior parietal cortex, overlapping with a site previously associated with both attention and Gestalt perception. These findings reveal a role of parietal beta-band activity in internally, rather than externally or attention-driven processes of Gestalt perception. They also add to the growing evidence for shared neural substrates of attention and Gestalt perception, both being linked to parietal cortex.

Parietal cortex mediates conscious perception of illusory gestalt.

Grouping local elements into a holistic percept, also known as spatial binding, is crucial for meaningful perception. Previous studies have shown that neurons in early visual areas V1 and V2 can signal complex grouping-related information, such as illusory contours or object-border ownerships. However, relatively little is known about higher-level processes contributing to these signals and mediating global Gestalt perception. We used a novel bistable motion illusion that induced alternating and mutually exclusive vivid conscious experiences of either dynamic illusory contours forming a global Gestalt or moving ungrouped local elements while the visual stimulation remained the same. fMRI in healthy human volunteers revealed that activity fluctuations in two sites of the parietal cortex, the superior parietal lobe and the anterior intraparietal sulcus (aIPS), correlated specifically with the perception of the grouped illusory Gestalt as opposed to perception of ungrouped local elements. We then disturbed activity at these two sites in the same participants using transcranial magnetic stimulation (TMS). TMS over aIPS led to a selective shortening of the duration of the global Gestalt percept, with no effect on that of local elements. The results suggest that aIPS activity is directly involved in the process of spatial binding during effortless viewing in the healthy brain. Conscious perception of global Gestalt is therefore associated with aIPS function, similar to attention and perceptual selection.

High-resolution dataset of manual claustrum segmentation.

The claustrum has a unique thin sheet-like structure that makes it hard to identify in typical anatomical MRI scans. Attempts have been made to identify the claustrum in anatomical images with either automatic segmentation techniques or using atlas-based approaches. However, the resulting labels fail to include the ventral claustrum portion, which consists of fragmented grey matter referred to as "puddles". The current dataset is a high-resolution label of the whole claustrum manually defined using an ultra-high resolution postmortem MRI image of one individual. Manual labelling was performed by four independent research trainees. Two trainees labelled the left claustrum and another two trainees labelled the right claustrum. For every hemisphere we created a union of the two labels and assessed the label correspondence using dice coefficients. We provide size measurements of the labels in MNI space by calculating the oriented bounding box size. These data are the first manual claustrum segmentation labels that include both the dorsal and ventral claustrum regions at such a high resolution in standard space. The label can be used to approximate the claustrum location in typical in vivo MRI scans of healthy individuals.

No common factor for illusory percepts, but a link between pareidolia and delusion tendency: A test of predictive coding theory.

Predictive coding theory is an influential view of perception and cognition. It proposes that subjective experience of the sensory information results from a comparison between the sensory input and the top-down prediction about this input, the latter being critical for shaping the final perceptual outcome. The theory is able to explain a wide range of phenomena extending from sensory experiences such as visual illusions to complex pathological states such as hallucinations and psychosis. In the current study we aimed at testing the proposed connection between different phenomena explained by the predictive coding theory by measuring the manifestation of top-down predictions at progressing levels of complexity, starting from bistable visual illusions (alternating subjective experience of the same sensory input) and pareidolias (alternative meaningful interpretation of the sensory input) to self-reports of hallucinations and delusional ideations in everyday life. Examining the correlation structure of these measures in 82 adult healthy subjects revealed a positive association between pareidolia proneness and a tendency for delusional ideations, yet without any relationship to bistable illusions. These results show that only a subset of the phenomena that are explained by the predictive coding theory can be attributed to one common underlying factor. Our findings thus support the hierarchical view of predictive processing with independent top-down effects at the sensory and cognitive levels.

Zooming-in on higher-level vision: High-resolution fMRI for understanding visual perception and awareness.

One of the central questions in visual neuroscience is how the sparse retinal signals leaving our eyes are transformed into a rich subjective visual experience of the world. Invasive physiology studies, which offers the highest spatial resolution, have revealed many facts about the processing of simple visual features like contrast, color, and orientation, focusing on the early visual areas. At the same time, standard human fMRI studies with comparably coarser spatial resolution have revealed more complex, functionally specialized, and category-selective responses in higher visual areas. Although the visual system is the best understood among the sensory modalities, these two areas of research remain largely segregated. High-resolution fMRI opens up a possibility for linking them. On the one hand, it allows studying how the higher-level visual functions affect the fine-scale activity in early visual areas. On the other hand, it allows discovering the fine-scale functional organization of higher visual areas and exploring their functional connectivity with visual areas lower in the hierarchy. In this review, I will discuss examples of successful work undertaken in these directions using high-resolution fMRI and discuss where this method could be applied in the future to advance our understanding of the complexity of higher-level visual processing.

Perceptual effects of stimulating V5/hMT+ during binocular rivalry are state specific.

Binocular rivalry occurs when two distinct visual stimuli are presented separately to each eye, causing perceptual ambiguity. The conscious state of the observer then alternates between the perceptual dominance of one of the stimuli while the other is suppressed, and vice versa. These vivid changes in perception during constant visual stimulation allow the study of brain processes involved in conscious visual experience. There is abundant electrophysiological as well as fMRI evidence that neural activity in stimulus-selective areas of the temporal lobe correlates with perceptual changes during rivalry [1­3]. Yet, almost nothing is known about the causal contribution of these areas to dominance and suppression of their preferred stimulus. We induced binocular rivalry in human observers using moving dots presented to one eye and a static face to the other eye, and applied transcranial magnetic stimulation (TMS) over the motion area V5/hMT+. We show that disrupting activity in V5/hMT+ during rivalry extends periods of motion suppression, with no effect on periods of motion dominance, revealing a state-specific contribution of V5/hMT+ to the competition for awareness in rivalry.

Disrupting parietal function prolongs dominance durations in binocular rivalry.

Human brain imaging studies of bistable perceptual phenomena revealed that frontal and parietal areas are activated during perceptual switches between the two conflicting percepts. However, these studies do not provide information about causality, i.e., whether activity reports a consequence or a cause of the perceptual change. Here we used functional magnetic resonance imaging to individually localize four parietal regions involved in perceptual switches during binocular rivalry in 15 subjects and subsequently disturbed their neural processing and that of a control site using 2 Hz repetitive transcranial magnetic stimulation (TMS) during binocular rivalry. We found that TMS over one of the sites, the right intraparietal sulcus (IPS), prolonged the periods of stable percepts. Additionally, the more lateralized the blood oxygen level-dependent signal was in IPS, the more lateralized the TMS effects were. Lateralization varied considerably across subjects, with a right-hemispheric bias. Control replay experiments rule out nonspecific effects of TMS on task performance, reaction times, or eye blinks. Our results thus demonstrate a causal, destabilizing, and individually lateralized effect of normal IPS function on perceptual continuity in rivalry. This is in accord with a role of IPS in perceptual selection, relating its role in rivalrous perception to that in attention.