A Generic Mechanism for Perceptual Organization in the Parietal Cortex - PubMed (original) (raw)
A Generic Mechanism for Perceptual Organization in the Parietal Cortex
Pablo R Grassi et al. J Neurosci. 2018.
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.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.
Keywords: bistable perception; fMRI; parietal cortex; pattern motion; perceptual organization; predictive coding.
Copyright © 2018 the authors 0270-6474/18/387158-12$15.00/0.
Figures
Figure 1.
Illustration of the asymmetric bistable stimuli used. A, Plaid stimulus. This stimulus could be perceived either as a single plaid moving upward (default percept) or as two depth-separated transparent gratings sliding over each other in opposite directions (alternative percept). B, Occluded diamond stimulus and its two interpretations: local vertical motion of separate elements (default percept) and global horizontal motion of a bound shape (alternative percept). C, Illusory Gestalt display used previously (Zaretskaya et al., 2013): four pairs of dots moving circularly in phase could be perceived either as local motion of separate pairs of dots (default percept) or as of two illusory squares sliding over each other in planar motion (alternative percept).
Figure 2.
First percept bias and mean percept durations during continuous viewing. A, Bar plots show the probability for a given percept to be perceived first upon stimulus presentation, for all three bistable stimuli (mean ± SEM). For all stimuli, one of the two percepts had an overwhelming predominance of being initially perceived. We defined the first percept as the default percept. The right bar plot shows the first percept bias for the Gestalt illusion, which had not been analyzed by Zaretskaya et al. (2013). We statistically tested the first percept bias against chance (p = 0.5) by means of a one-sample t test. B, The bar graphs depict the average perceptual durations for all percepts during continuous viewing (mean ± SEM). Durations for the illusory Gestalt display were published by Zaretskaya et al. (2013) and are included for completeness. ***p < 0.001, **p < 0.01, *p < 0.05, uncorrected.
Figure 3.
ROI analysis results. Responses of the posterior parietal cortex ROI and early visual cortex ROIs for the plaid (A) and diamond (B) displays (mean ± SEM). The PPC showed differential modulation favoring the alternative percept in both stimuli. Early visual areas V1 and V2 were consistently deactivated during perception of the alternative percept in both stimuli. Area V3 revealed no percept-driven modulation. The baseline corresponds to the mean fixation-only response. C, Group parietal ROI together with three individual examples of the early visual ROIs in MNI space. ***p < 0.001; **p < 0.01; *p < 0.05, Holms-Bonferroni corrected; +p < 0.05, uncorrected.
Figure 4.
ROI analysis of the ventral shape selective and dorsal motion selective areas for the plaid (A) and diamond (B) displays (mean ± SEM). Responses of dorsal and ventral areas to perceptual changes during viewing of the Gestalt illusion display can be found in our previous work Grassi et al. (2016, 2017). None of the regions was consistently up- or down-modulated for a given percept type across all three bistable stimulus classes. ***p < 0.001; **p < 0.01; *p < 0.05, Holms-Bonferroni corrected; +p < 0.05, uncorrected.
Figure 5.
Whole-brain results showing the main contrast alternative > default for all stimulus displays. A, Whole-brain results for the plaid, diamond, and illusory Gestalt displays. Illusory Gestalt results are reproduced here for comparison from the work by Zaretskaya et al., 2013. B, First control analysis: whole-brain results for the plaid and diamond displays testing subjects who reported not to have guided their perception to any particular interpretation of the stimuli (diamond: n = 10, plaid: n = 13). C, Second control analysis: axial section through PPC for the two participants (S4 and S10) who reported more difficulty in perceiving the default percept for the diamond display, plus an additional two participants (S1 and S12) who reported the same for the plaid display (Table 1) (from the 3 mm (FWHM) smoothed data; thresholded at p < 0.001, uncorrected). PPC activity was hence driven by percept type, not by difficulty. Results of A and B are projected onto average inflated cortical hemispheres from the FreeSurfer software (Fischl et al., 1999). For better visualization we thresholded the results at the liberal value of t = 2. EVC, early visual cortex; LOC, lateral occipital complex; MT+, motion-selective cortex; pF, posterior fusiform gyrus; PPC, posterior parietal cortex; preC, precuneus; LH, left hemisphere; RH, right hemisphere.
Figure 6.
Conjunction analysis. The figure shows overlapping voxels in a glass brain (A) and cross-sections (B, C) from the contrast alternative > default that reached a threshold of t > 3 and t < −3 (p < 0.005) in each of the three display types. Across all three stimuli (plaid, diamond, and illusory Gestalt), we found consistent PPC activation (B) and EVC deactivation (C) during perception of the non-default interpretation. LH, Left hemisphere; RH, right hemisphere.
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