Where is your shoulder? Neural correlates of localizing others' body parts

Neuropsychological studies, based on pointing to body parts paradigms, suggest that left posterior parietal lobe is involved in the visual processing of other persons' bodies. In addition, some patients have been found with mild deficit when dealing with abstract human representations but marked impairment with realistically represented bodies, suggesting that this processing could be modulated by the abstraction level of the body to be analyzed. These issues were examined in the present fMRI experiment, designed to evaluate the effects of visually processing human bodies of different abstraction levels on brain activity. The human specificity of the studied processes was assessed using whole-body representations of humans and of dogs, while the effects of the abstraction level of the representation were assessed using drawings, photographs, and videos. To assess the effect of species and stimulus complexity on BOLD signal, we performed a two-way ANOVA with factors species (human versus animal) and stimulus complexity (drawings, photographs and videos). When pointing to body parts irrespective of the stimulus complexity, we observed a positive effect of humans upon animals in the left angular gyrus (BA 39), as suggested by lesion studies. This effect was also present in midline cortical structures including mesial prefrontal, anterior cingulate and precuneal regions. When pointing to body parts irrespective of the species to be processed, we observed a positive effect of videos upon photographs and drawings in the right superior parietal lobule (BA 7), and bilaterally in the superior temporal sulcus, the supramarginal gyrus (BA 40) and the lateral extrastriate visual cortex (including the "extrastriate body area"). Taken together, these data suggest that, in comparison with other mammalians, the visual processing of other humans' bodies is associated with left angular gyrus activity, but also with midline structures commonly implicated in self-reference. They also suggest a role of the lateral extrastriate cortex in the processing of dynamic and biologically relevant body representations.