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What is the temporal resolution of categorical perception?

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Abstract What levels of neural processing correspond with subjective perception? Numerous studies addressed this question but few if any compared the temporal dynamics of neural activity to that of subjective perception. Here we examined the neural response to sustained, full contrast, supraliminal images, and looked for neural responses which track the presence of the stimulus with high fidelity for its entire duration. Intracranial EEG was recorded from the surface of the cortex in patients (> 1000 iEEG electrodes), while they viewed images presented for varying durations of 300-1500ms. The fidelity of sustained high-frequency broadband activity gradually decreased across the ventral stream: single-trial stimulus duration could be decoded with an accuracy of at least 150ms from early visual cortex but unreliably in anterior, category-selective regions. This result presents an apparent dissociation between neural activity and perception, as the patients could clearly see and categorize the images while they were displayed. Next, fifteen healthy participants were tested using slow event-related fMRI with images lasting 9 or 15 seconds. In this case, we found robust sustained BOLD responses across the entire ventral stream hierarchy. The two sets of results can be reconciled by taking into account the different temporal resolutions of BOLD signal and iEEG. Sporadically-bursting neural activity, which is not informative about the presence of the stimulus on a sub-second timescale, can still be informative about the stimulus presence with a coarser resolution reflected in the hemodynamic response. The combined findings raise new questions about which temporal resolution is relevant for subjective perception.

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