RESPONSES OF MT AND MST NEURONS TO ONE AND 2 MOVING-OBJECTS IN THE RECEPTIVE-FIELD

To test the effects of complex visual motion stimuli on the responses of single neurons in the middle temporal visual area (MT) and the medi al superior temporal area (MST) of the macaque monkey, we compared the response elicited by one object in motion through the receptive field with the response of two simultaneously presented objects moving in d ifferent directions through the receptive field. There was an increase d response to a stimulus moving in a direction other than the best dir ection when it was paired with a stimulus moving in the best direction . This increase was significant for all directions of motion of the no n-best stimulus and the magnitude of the difference increased as the d ifference in the directions of the two stimuli increased. Similarly, t here was a decreased response to a stimulus moving in a non-null direc tion when it was paired with a stimulus moving in the null direction. This decreased response in MT did not reach significance unless the se cond stimulus added to the null direction moved in the best direction, whereas in MST the decrease was significant when the second stimulus direction differed from the null by 90 degrees or more. Further analys is showed that the two-object responses were better predicted by takin g the averaged response of the neuron to the two single-object stimuli than by summation, multiplication, or vector addition of the response s to each of the two single-object stimuli. Neurons in MST showed larg er modulations than did neurons in MT with stimuli moving in both the best direction and in the null direction and the average better predic ted the two-object response in area MST than in area MT. This indicate s that areas MT and MST probably use a similar integrative mechanisms to create their responses to complex moving visual stimuli, but that t his mechanism is further refined in MST. These experiments show that n eurons in both MT and MST integrate the motion of all directions in th eir responses to complex moving stimuli. These results with the motion of objects were in sound agreement with those previously reported wit h the use of random dot patterns for the study of transparent motion i n MT and suggest that these neurons use similar lar computational mech anisms in the processing of object and global motion stimuli.