Category-selective areas in human visual cortex exhibit preferences for stimulus depth.

Multiple regions in the human brain are dedicated to accomplish the feat of object recognition; yet our brains must also compute the 2D and 3D locations of the objects we encounter in order to make sense of our visual environments. A number of studies have explored how various object category-selective regions are sensitive to and have preferences for specific 2D spatial locations in addition to processing their preferred-stimulus categories, but there is no survey of how these regions respond to depth information. In a blocked functional MRI experiment, subjects viewed a series of category-specific (i.e., faces, objects, scenes) and unspecific (e.g., random moving dots) stimuli with red/green anaglyph glasses. Critically, these stimuli were presented at different depth planes such that they appeared in front of, behind, or at the same (i.e., middle) depth plane as the fixation point (Experiment 1) or simultaneously in front of and behind fixation (i.e., mixed depth; Experiment 2). Comparisons of mean response magnitudes between back, middle, and front depth planes reveal that face and object regions OFA and LOC exhibit a preference for front depths, and motion area MT+ exhibits a strong linear preference for front, followed by middle, followed by back depth planes. In contrast, scene-selective regions PPA and OPA prefer front and/or back depth planes (relative to middle). Moreover, the occipital place area demonstrates a strong preference for "mixed" depth above and beyond back alone, raising potential implications about its particular role in scene perception. Crucially, the observed depth preferences in nearly all areas were evoked irrespective of the semantic stimulus category being viewed. These results reveal that the object category-selective regions may play a role in processing or incorporating depth information that is orthogonal to their primary processing of object category information.