Equally invisible but neurally unequal: Cortical responses to invisible objects differ as a function of presentation method

摘要

Visual stimuli presented without any corresponding visual experience may still lead to neural processing (Moutoussis & Zeki, 2002). Understanding the stimulus-processing capabilities of the brain under these circumstances is a key goal of psychological research. It is unknown whether unconscious processing differs depending on the method used to prevent stimuli from entering awareness. We asked whether two methods for rendering stimuli invisible (chromatic flicker fusion and continuous flash suppression) had different effects on object processing within the human brain. Using functional magnetic resonance imaging (fMRI) we presented subjects (N=15, 9 males) with face and tool stimuli that were either fully visible or rendered invisible using one or the other method. Whole-brain decoding and multivariate searchlight decoding showed that both category and, surprisingly, subcategory-level information was present when stimuli were invisible as well as visible. However, the cortical and subcortical response pattern differed as a function of stimulation method. Visible category and subcategory decoding primarily relied on dorsal cortical (frontoparietal) and subcortical (superior colliculus, thalamus) attentional regions and ventral cortical (occipitotemporal) perceptual regions. Chromatic flicker fusion permitted decoding within cortical and subcortical areas, including prefrontal and striatal regions, that were largely nonoverlapping with regions that permitted decoding stimulus category and subcategory during continuous flash suppression, which primarily relied on parietal and occipitotemporal regions. In conclusion, category- and subcategory-level information is available even when stimuli do not enter awareness, and the availability of this information across the whole brain depends critically on the method used to render stimuli invisible. These results have implications for studying the neural correlates of consciousness, suggesting that some fine-grained subcategory-level information is available in the absence of awareness. Furthermore, they indicate that the suppression method used to study object processing in the absence of awareness should be tailored to the specific mechanisms or brain regions being studied.