Neural decoding dissociates perceptual grouping between proximity and similarity in visual perception

Abstract Previous research on perceptual grouping primarily focused on the dynamics of single grouping principle in light of the Gestalt psychology. Yet, there has been comparatively little emphasis on the dissociation across two or more grouping principles. To tackle this issue, the current study aims at investigating how , when , and where the processing of two grouping principles (proximity and similarity) are established in the human brain by using a dimotif lattice paradigm and adjusting the strength of one grouping principle. Specifically, we measured the modulated strength of the other grouping principle, thus forming six visual stimuli. The current psychophysical results showed that similarity grouping effect was enhanced with reduced proximity effect when the grouping cues of proximity and similarity were presented simultaneously. Meanwhile, electrophysiological (EEG) response patterns were able to decode the specific pattern out of the six visual stimuli involving both principles in each trail by using time-resolved multivariate pattern analysis (MVPA). The onsets of the dissociation between the two grouping principles coincided within three time windows: the earliest proximity-defined local visual element arrangement in the middle occipital cortex, the middle-stage processing for feature selection modulating low-level visual cortex in the inferior occipital cortex and fusiform cortex, and the higher-level cognitive integration to make decisions for specific grouping preference in the parietal areas. In addition, brain responses were highly correlated with behavioral grouping. The results therefore provide direct evidence for a link between human perceptual space of grouping decision-making and neural space of these brain response patterns. Significance statement How does our brain aggregate meaningless local elements to meaningful global patterns? Here, we used visual stimuli that can be perceived as a whole orientation from one of the two grouping cues (proximity and similarity) when they were presented simultaneously. Behavioral responses showed a trade-off between proximity and similarity. Further, time-resolved MVPA and source estimation results showed three stages for the processing of two grouping cues, involving an early stage in the low-level visual cortex, then the middle stage in the lateral visual cortex, and finally the late stage in the parietal areas. This was among the first attempt employing multivariate analysis to decode the dissociative processing of both proximity and similarity principles within one single study.