Difference in visual motion representation between cortical areas MT and MST during ocular following responses.

The middle temporal (MT) and medial superior temporal (MST) areas are successive stations of the visual motion-processing stream and project in parallel to the pontine nucleus, which is closely associated with rapid stabilization of gaze. We recorded the neural activities of MT and MST neurons of monkeys during short-latency ocular following responses (OFRs) elicited by large-field sinusoidal gratings with different spatial frequencies drifting at different temporal frequencies, and examined the dependence on spatiotemporal frequency. The results indicate that most MT/MST neurons were tuned almost separately for spatial and temporal frequencies of motion stimuli. The difference between MT and MST neurons was particularly striking for the optimal spatial frequency (higher for MT and lower for MST). The spatiotemporal frequency dependence of the OFRs could be reproduced by a weighted sum of the population activities of the MT and MST neurons. We conclude that MT and MST neurons work as spatiotemporal frequency sensors that extract motions of finer and coarser visual features and that both areas contribute to generation of OFRs.