Binocular cues to 3D face structure increase activation in depth-selective visual cortex with negligible effects in face-selective areas

Studies of visual face processing often use flat images as proxies for real faces due to their ease of manipulation and experimental control. Although flat images capture many features of a face, they lack the rich three-dimensional (3D) structural information available when binocularly viewing real faces (e.g., binocular cues to a long nose). We used functional magnetic resonance imaging to investigate the contribution of naturalistic binocular depth information to univariate activation levels and multivariate activation patterns in depth- and face-selective human brain regions. We used two cameras to capture images of real people from the viewpoints of the two eyes. These images were presented with natural viewing geometry (such that the size, distance, and binocular disparities were comparable to a real face at a typical viewing distance). Participants viewed stereopairs under four conditions: accurate binocular disparity (3D), zero binocular disparity (two-dimensional [2D]), reversed binocular disparity (pseudoscopic 3D), and no binocular disparity (monocular 2D). Although 3D faces (both 3D and pseudoscopic 3D) elicited higher activation levels than 2D faces, as well as distinct activation patterns, in depth-selective occipitoparietal regions (V3A, V3B, IPS0, IPS1, hMT+), face-selective occipitotemporal regions (OFA, FFA, pSTS) showed limited sensitivity to internal facial disparities. These results suggest that 2D images are a reasonable proxy for studying the neural basis of face recognition in face-selective regions, although contributions from 3D structural processing within the dorsal visual stream warrant further consideration.