Modulation of neuronal activity with cue-invariant shape discrimination in the primate superior temporal sulcus

Image Segmentation Based on Relative Motion and Relative Disparity Cues in Topographically Organized Areas of Human Visual Cortex

The borders between objects and their backgrounds create discontinuities in image feature maps that can be used to recover object shape. Here we used functional magnetic resonance imaging to identify cortical areas that encode two of the most important image segmentation cues: relative motion and relative disparity. Relative motion and disparity cues were isolated by defining a central 2-degree disk using random-dot kinematograms and stereograms, respectively. For motion, the disk elicited retinotopically organized activations starting in V1 and extending through V2 and V3. In the surrounding region, we observed phase-inverted activations indicative of suppression, extending out to at least 6 degrees of retinal eccentricity. For disparity, disk activations were only found in V3, while suppression was observed in all early visual areas. Outside of early visual cortex, several areas were sensitive to both types of cues, most notably LO1, LO2 and V3B, making them additional candidate areas for motion- and disparity-cue combination. Adding an orthogonal task at fixation did not diminish these effects, and in fact led to small but measurable disk activations in V1 and V2 for disparity. The overall pattern of extra-striate activations is consistent with recent three-stream models of cortical organization.

Neuronal correlates of motion-defined shape perception in primate dorsal and ventral streams

Human and non-human primates can readily perceive the shape of objects using visual motion. Classically, shape, and motion are considered to be separately processed via ventral and dorsal cortical pathways, respectively. However, many lines of anatomical and physiological evidence have indicated that these two pathways are likely to be interconnected at some stage. For motion-defined shape perception, these two pathways should interact with each other because the ventral pathway must utilize motion, which the dorsal pathway processes, to extract shape signal. However, it is unknown how interactions between cortical pathways are involved in neural mechanisms underlying motion-defined shape perception. We review evidence from psychophysical, lesion, neuroimaging and physiological research on motion-defined shape perception and then discuss the effects of behavioral demands on neural activity in ventral and dorsal cortical areas. Further, we discuss functions of two candidate sets of levels: early and higher-order cortical areas. The extrastriate area V4 and middle temporal (MT) area, which are reciprocally connected, at the early level are plausible areas for extracting the shape and/or constituent parts of shape from motion cues because neural dynamics are different from those during luminance-defined shape perception. On the other hand, among other higher-order visual areas, the anterior superior temporal sulcus likely contributes to the processing of cue-invariant shape recognition rather than cue-dependent shape processing. We suggest that sharing information about motion and shape between the early visual areas in the dorsal and ventral pathways is dependent on visual cues and behavioral requirements, indicating the interplay between the pathways.

Temporal property of single-cell activity in response to motion-defined shapes in monkey dorsal and ventral cortical areas

In the primate brain, shape and motion are considered to be separately processed in the ventral and dorsal visual cortical areas, respectively. However, to achieve shape perception with a motion cue, shape and motion cannot be processed exclusively in separate cortical areas. Interactions between ventral and dorsal cortical areas are required, and yet, the neural mechanisms underlying motion-defined shape perception remain unclear. Here, we assessed the temporal properties of single-unit activity recorded from V4, the middle temporal area, and the anterior superior temporal sulcus while monkeys discriminated shapes defined by motion and luminance cues. Visual response latencies of V4 neurons were shorter in the luminance-cue condition than in the motion-cue condition. Meanwhile, the timings of initiation of shape selectivity were not different between cue conditions, indicating a difference in processing time. Middle temporal neurons were less shape modulated in the luminance-cue condition than in the motion-cue condition. Temporal properties of neural activities in the lower bank of anterior superior temporal sulcus were similar between cue conditions. These results suggest that an interaction of the ventral cortex with the dorsal cortex is required for shape discrimination with different visual cues.