Tolerance of Macaque Middle STS Body Patch Neurons to Shape-preserving Stimulus Transformations

Abstract

Functional imaging studies in human and nonhuman primates have demonstrated regions in the brain that show category selectivity for faces or (headless) bodies. Recent fMRI-guided single unit studies of the macaque face category-selective regions have increased our understanding of the response properties of single neurons in these face patches. However, much less is known about the response properties of neurons in the fMRI-defined body category-selective regions ("body patches"). Recently, we reported that the majority of single neurons in one fMRI-defined body patch, the mid-STS body patch, responded more strongly to bodies compared with other objects. Here we assessed the tolerance of these neurons' responses and stimulus preference for shape-preserving image transformations. After mapping the receptive field of the single neurons, we found that their stimulus preference showed a high degree of tolerance for changes in the position and size of the stimulus. However, their response strongly depended on the in-plane orientation of a body. The selectivity of most neurons was, to a large degree, preserved when silhouettes were presented instead of the original textured and shaded images, suggesting that mainly shape-based features are driving these neurons. In a human psychophysical study, we showed that the information present in silhouettes is largely sufficient for body versus nonbody categorization. These data suggest that mid-STS body patch neurons respond predominantly to oriented shape features that are prevalent in images of bodies. Their responses can inform position- and retinal size-invariant body categorization and discrimination based on shape.

Stimulus features coded by single neurons of a macaque body category selective patch

“…In agreement with our previous study (17) that showed similar responses and selectivity of midSTS body patch neurons for the original and silhouette versions of the objects, Bubbles revealed similar fragments for these two image versions. However, Bubbles revealed larger fragments and more frequently whole bodies for the silhouettes compared with the full images.…”

“…We showed before that many midSTS body patch neurons respond to silhouettes of bodies (17). Silhouettes isolate shape contours, removing texture and shading information.…”

“…Here, we test whether this coding is specific to the image coordinates of the fragment (i.e., a specific location within the RF) or whether it generalizes to different retinal positions and sizes of the fragments. Note that a degree of invariance would be expected because midSTS neurons respond in a translation-and size-tolerant manner to whole body images (17). To test for translation tolerance, we reverse-correlated neuron (n = 46) responses with Bubble stimuli presented in two different locations of the neurons' RF.…”

“…Bubbles stimuli. The Bubbles procedure was applied on the achromatic stimuli of the search test (discussed above) or on their black silhouette versions, for which all object pixels were black (17). We first decomposed the original or silhouette image into five SF bands of one octave each, centered at 11.3, 5.65, 2.8, 1.4, and 0.7 cycles per degree (using the matlabPyrTools for MATLAB, www.cns.nyu.edu/lcv/software.php).…”

Binocular depth processing in the ventral visual pathway

“…Another important unresolved question is the degree to which disparity contributes to well-studied inferotemporal representations such as those for faces [77] or body parts [87]. It has been shown that the face patches strongly overlap with regions activated by near (crossed) disparities [5], but the functional consequences of this overlap for face encoding by IT neurons have not yet been studied.…”