Differential activity to shapes under shape-from-motion condition in macaque middle temporal area

To investigate the neural mechanisms of motion-defined shape processing, we recorded single unit activity in the middle temporal area (MT) while monkeys performed a shape discrimination task under the shape-from-motion (SFM) condition, where a motion cue is critical for shape perception. About 40% of MT neurons responded differentially to shapes under the SFM condition. The differential responses to shapes could not be explained by either the heterogeneous structure of the receptive field or the amount of motion signal. On the other hand, under the shape-from-luminance (SFL) condition, where a luminance cue is critical for shape perception, the proportion of neurons showing differential responses to shapes was smaller than that under the SFM condition and the magnitudes of differential responses themselves were weaker. Thus, the requirement for motion processing for shape perception may facilitate a differential response to shapes under the SFM condition. We compared neuronal activities during the shape discrimination task with those during the direction discrimination task under the SFM condition. Differential responses to shapes were observed more frequently during the shape discrimination task than during the direction discrimination task. Thus, the motion-defined shape processing influenced MT activity.