Reza Rajimehr

This chapter examines recent fMRI studies that have reported certain low-level preferences/biases in the scene-responsive areas of visual cortex. It looks at selectivity to low-level visual features in the scene-responsive areas specifically in the parahippocampal place area (PPA). Evidence shows that multiple lower-level variables affect the fMRI response properties in the PPA. This area exhibits a bias for stimuli in the peripheral visual field and a bias for the upper visual field. It is likely that PPA and other scene-responsive areas display selectivity to other low-level features such as specific line junctions or specific shapes.

Defining the exact mechanisms by which the brain processes visual objects and scenes remains an unresolved challenge. Valuable clues to this process have emerged from the demonstration that clusters of neurons (“modules”) in inferior temporal cortex apparently respond selectively to specific categories of visual stimuli, such as places/scenes. However, the higher-order “category-selective” response could also reflect specific lower-level spatial factors. Here we tested this idea in multiple functional MRI experiments, in humans and macaque monkeys, by systematically manipulating the spatial content of geometrical shapes and natural images. These tests revealed that visual spatial discontinuities (as reflected by an increased response to high spatial frequencies) selectively activate a well-known place-selective region of visual cortex (the “parahippocampal place area”) in humans. In macaques, we demonstrate a homologous cortical area, and show that it also responds selectively to higher spatial frequencies. The parahippocampal place area may use such information for detecting object borders and scene details during spatial perception and navigation. Many reports suggest that different categories of visual stimuli are processed in correspondingly specific “modules” in the visual cortex. For instance, images of faces are processed in one cortical module (the “fusiform face area”), while images of scenes are processed in an adjacent module (the “parahippocampal place area,” or PPA). How does the PPA encode for such high-level, complex visual scenes? In this study, we show that at least part of the PPA response is due to a lower-level variable, reflected as higher spatial frequencies. These are prominent in the edges and details of scenes, but less prominent in faces and other stimuli. When we altered standard images of faces and places so that they only contained low, medium, or high spatial frequencies, we found that the PPA responded strongly to images containing high spatial frequencies. Importantly, using the same stimuli as for the human studies, we also demonstrated a homolog of human PPA in macaque temporal cortex (“mPPA”). As in humans, mPPA responds selectively to higher spatial frequencies. This demonstration of PPA in macaques paves the way for carrying out further electrophysiological and anatomical studies that may help elucidate the neural mechanisms for place selectivity in the human visual cortex.

In humans and other Old World primates, much of visual cortex comprises a set of retinotopic maps, embedded in a cortical sheet with well known, identifiable folding patterns. However, the relationship between these two prominent cortical variables has not been comprehensively studied. Here, we quantitatively tested this relationship using functional and structural magnetic resonance imaging in monkeys and humans. We found that the vertical meridian of the visual field tends to be represented on gyri (convex folds), whereas the horizontal meridian is preferentially represented in sulci (concave folds), throughout visual cortex in both primate species. This relationship suggests that the retinotopic maps may constrain the pattern of cortical folding during development.

Increasing evidence suggests that primate visual cortex has a specialized architecture for processing discrete object categories such as faces. Human fMRI studies have described a localized region in the fusiform gyrus [the fusiform face area (FFA)] that responds selectively to faces. In contrast, in nonhuman primates, electrophysiological and fMRI studies have instead revealed 2 apparently analogous regions of face representation: the posterior temporal face patch (PTFP) and the anterior temporal face patch (ATFP). An earlier study suggested that human FFA is homologous to the PTFP in macaque. However, in humans, no obvious homologue of the macaque ATFP has been demonstrated. Here, we used fMRI to map face-selective sites in both humans and macaques, based on equivalent stimuli in a quantitative topographic comparison. This fMRI evidence suggests that such a face-selective area exists in human anterior inferotemporal cortex, comprising the apparent homologue of the fMRI-defined ATFP in macaques.