Parietal cortex mediates perceptual Gestalt grouping independent of stimulus size

Serial processing of proximity groups and similarity groups

Proximity and feature similarity are two important determinants of perceptual grouping in vision. When viewing visual scenes conveying both grouping options simultaneously, people most usually detect proximity groups faster than similarity groups. This article demonstrates that perceptual judgments of grouping orientation guided by either proximity or contrast similarity are indicative of a sequential organization of grouping operations in the visual pathway, which lends a temporal processing advantage to proximity grouping (Experiment 1). Invoking the double-factorial paradigm, latent cognitive architecture for perceptual grouping is also investigated in a task with redundant signals (Experiment 2). Reaction time data from this task is assessed in terms of the race model inequality, workload capacity analysis, and interaction contrasts of means and survivor functions. Again, empirical benchmarks indicate serial processing of proximity groups and similarity groups, with a self-terminating stopping rule for processing. A subset of participants exhibit atypical performance metrics, hinting at possible individual differences in configural visual processing.

Parietal theta burst TMS does not modulate bistable perception

The role of the parietal cortex in perceptual awareness and in resolving perceptual ambiguity is unsettled. Early influential transcranial magnetic stimulation studies have revealed differences in conscious perception following parietal stimulation, fuelling the notion that parietal cortex causally contributes to resolving perceptual ambiguity. However, central to this conclusion is the reliability of the method employed. Several prior studies have revealed opposing effects, such as shortening, lengthening, or no effect on multistable perceptual transitions following parietal stimulation. Here we addressed the reliability of continuous theta-burst stimulation (cTBS) on parietal cortex on the perception of bistable stimuli. We conducted three cTBS experiments that were matched to prior experiments in terms of stimuli, stimulation protocol, and target site, and used a higher number of participants. None of our cTBS experiments replicated prior cTBS results. The only experiment using individual functional localizers led to weak effects, while the two others led to null results. Individual variability of motor cortex cTBS did not predict parietal cTBS effects. In view of recent reports of highly variable cTBS effects over motor cortex, our results suggest that cTBS is particularly unreliable in modulating bistable perception when applied over parietal cortex.

Intrinsic excitability of human right parietal cortex shapes the experienced visual size illusions

Converging evidence has found that the perceived visual size illusions are heritable, raising the possibility that visual size illusions might be predicted by intrinsic brain activity without external stimuli. Here we measured resting-state brain activity and 2 classic visual size illusions (i.e. the Ebbinghaus and the Ponzo illusions) in succession, and conducted spectral dynamic causal modeling analysis among relevant cortical regions. Results revealed that forward connection from right V1 to superior parietal lobule (SPL) was predictive of the Ebbinghaus illusion, and self-connection in the right SPL predicted the Ponzo illusion. Moreover, disruption of intrinsic activity in the right SPL by repetitive transcranial magnetic stimulation (TMS) temporally increased the Ebbinghaus rather than the Ponzo illusion. These findings provide a better mechanistic understanding of visual size illusions by showing the causal and distinct contributions of right parietal cortex to them, and suggest that spontaneous fluctuations in intrinsic brain activity are relevant to individual difference in behavior.

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.

Activity in the Fronto-Parietal and Visual Cortex Is Modulated by Feature-Based Attentional Weighting

In day-to-day dynamic activities where sensory input is abundant, stimulus representations in the visual cortex are modulated based on their attentional priority. Several studies have established the top-down role of a fronto-parietal dorsal attention network in selective attention. In the current study, we aimed to investigate whether activity of subregions of this network and the visual cortex is modulated by feature-based attentional weighting, and if so, whether their timecourses of activity are correlated. To this end, we analyzed fMRI data of 28 healthy subjects, who performed a feature-based go/no-go task. Participants had to attend to one or two colored streams of sinusoidal gratings and respond to each grating in the task-relevant stream(s) except to a single non-target grating. Univariate and multivariate fMRI results indicated that activity in bilateral fronto-parietal (frontal eye fields, intraparietal sulcus and superior parietal lobe) and visual (V1-V4, lateral occipital cortex and fusiform gyrus) regions was modulated by selecting one instead of attending to two gratings. Functional connectivity was not significantly different between fronto-parietal and visual regions when attending to one as opposed to two gratings. Our study demonstrates that activity in subregions of both the fronto-parietal and visual cortex is modified by feature-based attentional weighting.

Interocular Grouping in Perceptual Rivalry Localized with fMRI

Bistable perception refers to a broad class of dynamically alternating visual illusions that result from ambiguous images. These illusions provide a powerful method to study the mechanisms that determine how visual input is integrated over space and time. Binocular rivalry occurs when subjects view different images in each eye, and a similar experience called stimulus rivalry occurs even when the left and right images are exchanged at a fast rate. Many previous studies have identified with fMRI a network of cortical regions that are recruited during binocular rivalry, relative to non-rivalrous control conditions (termed replay) that use physically changing stimuli to mimic rivalry. However, we show here for the first time that additional cortical areas are activated when subjects experience rivalry with interocular grouping. When interocular grouping occurs, activation levels broadly increase, with a slight shift towards right hemisphere lateralization. Moreover, direct comparison of binocular rivalry with and without grouping highlights strong focused activity in the intraparietal sulcus and lateral occipital areas, such as right-sided retinotopic visual areas LO1 and IP2, as well as activity in left-sided visual areas LO1, and IP0-IP2. The equivalent analyses for comparable stimulus (eye-swap) rivalry showed very similar results; the main difference is greater recruitment of the right superior parietal cortex for binocular rivalry, as previously reported. Thus, we found minimal interaction between the novel networks isolated here for interocular grouping, and those previously attributed to stimulus and binocular rivalry. We conclude that spatial integration (i.e,. image grouping/segmentation) is a key function of lateral occipital/intraparietal cortex that acts similarly on competing binocular stimulus representations, regardless of fast monocular changes.

Gaze Parameters in the Analysis of Ambiguous Geometric Shapes

This study explores perceptual organisation and shape perception when viewing a tetragon and an additional element (a dot) that is located at varying positions and distances next to the tetragon. The aim of the study is to determine the factors that can alter the interpretation of object configuration and impact whether the presented tetragon is perceived as a diamond or a square. Methods used in this study are a forced-choice task as a subjective measurement and eye tracking as an objective measurement of perceptual processes. Overall, 31 stimuli were presented to the participants: a tetragon in two different sizes with an additional element (a dot) located inside or outside the object at three different positions at three distances. The results indicate significant changes in shape perception, depending on the location of the additional element. The results are complemented with eye movement analysis indicating that as the distance between the elements increases, there is a higher probability of either of the two shape interpretations and the gaze is less likely to be directed to the area between the stimuli. Furthermore, the subjective perception of shape is codetermined by the shape perception when the tetragon is presented without the additional element.

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.

Investigating holistic face processing within and outside of face-responsive brain regions

It has been shown that human faces are processed holistically (i.e. as indecomposable wholes, rather than by their component parts) and this holistic face processing is linked to brain activity in face-responsive brain regions. Although several brain regions outside of the face-responsive network are also sensitive to relational processing and perceptual grouping, whether these non-face-responsive regions contribute to holistic processing remains unclear. Here, we investigated holistic face processing in the composite face paradigm both within and outside of face-responsive brain regions. We recorded participants' brain activity using fMRI while they performed a composite face task. Behavioural results indicate that participants tend to judge the same top face halves as different when they are aligned with different bottom face halves but not when they are misaligned, demonstrating a composite face effect. Neuroimaging results revealed significant differences in responses to aligned and misaligned faces in the lateral occipital complex (LOC), and trends in the anterior part of the fusiform face area (FFA2) and transverse occipital sulcus (TOS), suggesting that these regions are sensitive to holistic versus part-based face processing. Furthermore, the retrosplenial cortex (RSC) and the parahippocampal place area (PPA) showed a pattern of neural activity consistent with a holistic representation of face identity, which also correlated with the strength of the behavioural composite face effect. These results suggest that neural activity in brain regions both within and outside of the face-responsive network contributes to the composite-face effect.

Topographic signatures of global object perception in human visual cortex

Our visual system readily groups dynamic fragmented input into global objects. How the brain represents global object perception remains however unclear. To address this question, we recorded brain responses using functional magnetic resonance imaging whilst observers viewed a dynamic bistable stimulus that could either be perceived globally (i.e., as a grouped and coherently moving shape) or locally (i.e., as ungrouped and incoherently moving elements). We further estimated population receptive fields and used these to back-project the brain activity measured during stimulus perception into visual space via a searchlight procedure. Global perception resulted in universal suppression of responses in lower visual cortex accompanied by wide-spread enhancement in higher object-sensitive cortex. However, follow-up experiments indicated that higher object-sensitive cortex is suppressed if global perception lacks shape grouping, and that grouping-related suppression can be diffusely confined to stimulated sites and accompanied by background enhancement once stimulus size is reduced. These results speak to a non-generic involvement of higher object-sensitive cortex in perceptual grouping and point to an enhancement-suppression mechanism mediating the perception of figure and ground.

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Lower visual cortex activity to grouped vs ungrouped dynamic stimuli is suppressed.

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When grouping a shape, activity in higher object-sensitive cortex is enhanced.

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Without shape grouping, activity in higher object-sensitive cortex is suppressed.

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Grouping-related suppression can be diffusely confined to stimulated cortical sites.

Encoding of Auditory Temporal Gestalt in the Human Brain

The perception of an acoustic rhythm is invariant to the absolute temporal intervals constituting a sound sequence. It is unknown where in the brain temporal Gestalt, the percept emerging from the relative temporal proximity between acoustic events, is encoded. Two different relative temporal patterns, each induced by three experimental conditions with different absolute temporal patterns as sensory basis, were presented to participants. A linear support vector machine classifier was trained to differentiate activation patterns in functional magnetic resonance imaging data to the two different percepts. Across the sensory constituents the classifier decoded which percept was perceived. A searchlight analysis localized activation patterns specific to the temporal Gestalt bilaterally to the temporoparietal junction, including the planum temporale and supramarginal gyrus, and unilaterally to the right inferior frontal gyrus (pars opercularis). We show that auditory areas not only process absolute temporal intervals, but also integrate them into percepts of Gestalt and that encoding of these percepts persists in high-level associative areas. The findings complement existing knowledge regarding the processing of absolute temporal patterns to the processing of relative temporal patterns relevant to the sequential binding of perceptual elements into Gestalt.

Alternating-color words influence Chinese sentence reading: Evidence from neural connectivity

In order to investigate how language and attention systems are affected by word boundary information during reading, we conducted a functional magnetic resonance imaging (fMRI) experiment in which text-color in naturally unspaced Chinese sentences were systematically manipulated in three experimental conditions, that is, text-color alternation consistent or inconsistent with word boundary (i.e., alternating-color word and non-word conditions), as well as a mono-color baseline condition. Twenty college students (14 females; 23.1 years old) were required to silently read 72 sentences during fMRI scanning. We found that the conditions of word boundary modulated the brain connections between the visual word form area (VWFA) and dorsal attention regions, and between the VWFA and language-related regions. These results suggest that the coordination between the VWFA and dorsal attention regions plays an important role in grouping characters and guiding the saccade according to perceptual grouping based on color, and that the connection between VWFA and MTG could be the neural mechanism of lexical access during Chinese text reading.

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.