Frontiers in Psychology (Aug 2011)
Intercepting the first pass: rapid categorization is suppressed for unseen stimuli
Abstract
The operations and processes that the human brain employs to achieve fast visual categorizationremain a matter of debate. A first issue concerns the timing and place of rapid visual categorizationand to what extent it can be performed with an early feed-forward pass of information throughthe visual system. A second issue involves the categorization of stimuli that do not reach visualawareness. There is disagreement over the degree to which these stimuli activate the same earlymechanisms as stimuli that are consciously perceived. We employed continuous flash suppression,EEG recordings and machine learning techniques to study visual categorization of seen and unseenstimuli. Our classifiers were able to predict from the EEG recordings the category of stimuli onseen trials but not on unseen trials. Rapid categorization of conscious images could be detectedaround 100 ms on the occipital electrodes, consistent with a fast, feed-forward mechanism of targetdetection. For the invisible stimuli, however, continuous flash suppression eliminated all traces ofearly processing. Our results support the idea of a fast mechanism of categorization and suggestthat this early categorization process plays an important role in later, more subtle categorizationsand perceptual processes.
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