Expanding the stimulus domain: Co-occurrence of motion and body-category selectivity in the macaque ventral STS

The primate Superior Temporal Sulcus (STS) plays a pivotal role in the recognition of bodies and their actions, which is essential for survival and social interaction with conspecifics. Here, we show that, surprisingly, a sizable proportion of macaque middle ventral STS units are selective for static bodies and random dot motion. They show a faithful representation of random dot motion direction, with motion directions differing by 180 degrees being represented distinctly, although responding more strongly to complex optic flow patterns. This aligns with an fMRI experiment in which we show that the mid-STS body patch, defined by a greater activation to static bodies compared to faces and objects, is also more strongly activated by moving random dot patterns compared to static ones, especially when including complex optic flow patterns. More anterior ventral STS body-selective units demonstrate a less pronounced random dot motion selectivity and this is mainly for complex optic flow patterns. Moreover, middle STS units, but rarely those of the anterior STS, respond selectively to dynamic dot patterns in which body parts are visible solely through motion, and their preference correlates with those for videos of acting monkeys. Overall, these findings highlight an association between body and motion processing in the macaque ventral STS, which might result from the co-occurrence of body features and motion during the observation of bodily actions.