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Ling Liu, Huan Luo; Two-stage generative process in illusory shape perception: a MEG study. Journal of Vision 2017;17(10):177. doi: 10.1167/17.10.177.
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Grouping local parts into coherent shapes (e.g., illusory shape perception) is a central function in vision and has been suggested to be a generative process such that feedback signals carry predictions (i.e., the illusory shape) and feedforward signals represent prediction errors (i.e., the mismatch between predictions and actual bottom-up inputs). Although recent fMRI studies provide evidence supporting the predictive coding hypothesis in illusory shape perception, the neuronal dynamics and the associated brain regions underlying this generative process remains largely unknown. To address the issue, we recorded magnetoencephalography (MEG) signals while human subjects were presented with Pac-Man figures, the combination of which is either or not able to induce an illusory shape perception ('Kanizsa triangle'), corresponding to grouping and ungrouping conditions respectively. Critically, here we employed a temporal response function technique (TRF) combine with randomly modulated the luminance of each Pac-Man to extract neuronal response specific for each of the three Pac-Man figures. First, the TRF responses for grouping condition showed decreased activities compared to ungrouping condition, consistent with predictive coding account, given that the predictive errors are assumed to be smaller when shape perception is induced. Second, two time periods, one early and one late, associated with different neuronal oscillatory frequency and different brain regions, showed the inhibition effects. Specifically, within 100 ~150 ms, a beta-band (14-20 Hz) decrease was originated in bilateral early visual cortex (V1 and V2) and TPJ regions; within 200-400 ms, a theta-band (4-7 Hz) inhibition was found to arise from right IFG and TPJ regions. We propose that the illusory shape perception consists of two stages: an early one that quickly encodes predictive error in early sensory areas and a late one that performs background inhibition in right parietal and frontal regions after the establishment of illusory shape as foreground.
Meeting abstract presented at VSS 2017
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