Motion perception without explicit activity in areas MT and MST.

It is widely accepted that middle temporal (MT) and middle superior temporal (MST) cortical areas in the brain of rhesus monkeys are essential for processing visual motion. We asked whether this assumption holds true if the moving stimulus consists of a second-order motion stimulus. In addition, we asked whether neurons in area MT and MST code for moving sound sources. To answer these questions, we trained three rhesus monkeys on a direction-discrimination task. Our monkeys were able to correctly report the direction of all motion stimuli used in this study. Firing rates of directionally selective neurons from area MT (n = 38) and MST (n = 68) were recorded during task performance. These neurons coded only for the stimulus movement if the motion stimulus was separated from the background by luminance or flicker (Fourier and drift-balanced motion). If these segregation cues were absent (in the case of theta motion and of the moving sound source), firing rates did not code for the stimulus' direction. Therefore we conclude that although areas MT and MST are undoubtedly involved in processing a moving stimulus, they are not the final cortical stages responsible for perceiving it.