Response to motion in extrastriate area MSTl: center-surround interactions.

The medial superior temporal area of the macaque monkey extrastriate visual cortex can be divided into a dorsal medial (MSTd) and a lateral ventral (MSTl) region. The functions of the two regions may not be identical: MSTd may process optic flow information that results from the movement of the observer, whereas MSTl may be related more closely to processing visual motion related specifically to the motion of objects. If MSTl were related to such object motion, one would expect to see mechanisms for the segregation of objects from their surround. We investigated one of these mechanisms in MSTl neurons: the effect of stimuli falling in the region surrounding the receptive field center on the response to stimuli falling in the field center. We found the effects of the surround stimulation to be modulatory with little response to the surround stimulus itself but a clear effect on the response to the stimulus falling on the receptive field center. The response to motion in the center in the direction preferred for the neuron usually increased when the surround motion was in the opposite direction to that in the center and decreased when surround motion was in the same direction as that in the center. Fifty-seven percent of the neurons showed a ratio of response for center motion with a surround moving in the opposite direction to that in the center for center motion alone that was >1. The response to motion in the center also increased when the surround stimulus was stationary, and this increase was sometimes larger than that with a moving surround. Nearly 70% of the neurons showed a ratio of response to center motion with a stationary surround to center motion alone that was >1. This is in contrast to the minimal effect of stationary surrounds in middle temporal area neurons. When the stimulus presentation was reversed so that the stimulus in the center was stationary and the surround moved, some MSTl neurons responded when the direction of motion in the surround was in the direction opposite to the preferred direction of motion in the center of the receptive field. Stimulation of the surround thus had a profound effect on the response of MSTl neurons, and this pronounced effect of the surround is consistent with a role in the segmentation of objects using motion.