A Generic Mechanism for Perceptual Organization in the Parietal Cortex

Abstract

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. 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.

Topographic Signatures of Global Object Perception in Human Visual Cortex

“…This explanation seems unlikely because an inverse V1-LOC relationship has also been established for non-ambiguous shape-like stimuli vs unstructured displays (Murray et al, 2002). Moreover, at least broadly in line with our results, recent studies (Grassi et al, 2016, 2018) found no evidence for the involvement of the LOC when a dynamic, bistable global-local stimulus constantly triggered shape-based interpretations (i.e., moving disks forming large squares or small circles).…”

“…These conditions echoed the global-local aspects of the diamond stimulus without varying in shape information. We hypothesized that if the between-area response amplitude mechanism between lower visual cortex and VLOC/LOC we and others observed (Fang et al, 2008; Grassi et al, 2018; Murray et al, 2002) indeed mediates global object perception per se, we should be able to conceptually replicate this relationship.…”

“…However, it is unclear whether the inverse relationship between the VLOC/LOC and lower visual cortex we and others quantified (Fang et al, 2008; Grassi et al, 2018; Murray et al, 2002) can be regarded as a generic perceptual grouping mechanism operating irrespective of shape perception. Recent evidence suggests, for instance, that activity in the LOC also decreases for intact vs scattered objects with abolished inter-part relations (Margalit et al, 2017) as it is the case during the no-diamond percept.…”

“…When participants experienced the global compared to the local percept, a striking pattern of results was observed: a reduction of activity in V1 (and also V2) accompanied by an increase of activity in the lateral-occipital complex (LOC) – a brain region known to respond more strongly to images of intact objects and shapes than a scrambled version thereof (e.g., Grill-Spector et al, 1998; Malach et al, 1995). Notably, this response pattern has recently been replicated (Grassi et al, 2018).…”

Manipulating patterns of dynamic deformation elicits the impression of cloth with varying stiffness

“…Previous work suggests that visual motion often provides powerful cues for scene segmentation for both human and artificial systems (Wagemans et al, 2012;Shi & Malik, 1998). More recently, an fMRI study finds that when showing a class of bistable moving stimuli, the perceptual scene-segmentation was associated with an increased activity in the posterior parietal cortex (PPC) together with a decreased neural signal in the early visual cortex (Grassi, Zaretskaya, & Bartels, 2018). This suggests that PPC is a hub involved in structuring visual scenes based on different motion cues.…”