Heads, shoulders, knees and toes: Body representations in human extrastriate visual cortex

Abstract

Human fMRI studies have shown that the extrastriate body area (EBA) responds more strongly when people view body parts than other objects and may play a specific role in body part recognition. However, the average response in EBA is similar for different body parts, suggesting little sensitivity to body part category. Here, we conducted a multi-voxel pattern analysis to investigate whether the spatial pattern of response across EBA contains information about body part category. Specifically, we conducted a category (face, shoulder, elbow, hand, knee, foot) x side of body (left, right) x visual field (left, right, 2° from fixation) event-related fMRI experiment. On each trial, participants fixated while a single body part was presented from an allocentric point of view. We used a split-half correlation technique (Haxby et al., 2001) to compare the pattern of activation across voxels in an independently localized EBA. We found stronger within than between body part correlations in EBA demonstrating the presence of body part category information. Furthermore there was a strong interaction between side of body and visual field, such that there was more information about body parts in their commonly experienced locations. Specifically, in the left visual field, EBA contained information about the category of body parts from the right side of the body, but not the left side. In the right visual field, the converse was true (more information about body parts from the left than the right side), although in general the correlations were weaker. These effects were specific to EBA. In FFA, only faces showed strong within body part correlations and no information about the category of non-face body parts. We conclude that EBA contains information about body part category, but this information is specific to body parts presented in their commonly experienced positions.