Categorical representation of visually suppressed objects in visual cortex

摘要

Functional imaging has been used to understand how visual objects of different categories are represented in the brain. However, the relationship between the fMRI-derived object representation and consciously experiencing a particular object remains poorly understood. Recent studies investigating BOLD responses to visually suppressed objects suggest that even in the absence of awareness, specific cortical regions differentially represent different objects categories. However, drawing strong conclusions about categorical representation within specific brain regions is difficult since these studies have focused only on pairs of object categories and constrained analyses to very restricted and/or loosely defined regions of cortex Here, we extend this past work using fMRI combined with continuous binocular switch suppression to simultaneously investigate representations of visually suppressed objects across occipital, parietal, and temporal cortex and include faces, houses, tools and scrambled objects in our analyses. Univariate and multivariate (MVPA) analyses were conducted (N=8) within functionally defined ROIs including retinotopic areas: V1, V2, V3, V4 and V3A/B, V7, IPS1-5, SPL1 and object category areas: OFA, FFA, PPA, LOC and EBA. In the invisible conditions, univariate analyses found no differences in BOLD signal across object category in any ROI. In contrast, the MVPA yielded above-chance performance classifying the four image categories within the FFA and PPA as well as IPS2, IPS3, IPS4 and IPS5. However, secondary pair-wise MVPA revealed that this performance was largely mediated by differentiating between intact and scrambled images in all but the FFA in which faces could be dissociated from houses. However, within the FFA the MVPA could not accurately classify faces versus tools or faces versus scrambled pictures. Although MVPA analyses could classify at above-chance levels visually suppressed faces, tools, houses and scrambled objects within several areas of ventral and dorsal visual cortex, we find no evidence that the underlying representations are specifically categorical in nature.