A variety of anatomical and physiological criteria have shown that the extrastriate visual cortex of the macaque monkey is subdivided into many distinct areas. There is evidence to suggest that there is functional specialization among these areas. Previous studies have shown that the middle temporal visual area (MT) contains a high proportion of cells which are selective for the direction of movement of visual stimuli, and yet relatively non-selective for stimulus color or form. The experiments reported here examined the response properties in MT in greater detail, and demonstrated the inputs and outputs of the area by means of anatomical tracers.
A computer-driven stimulator was used to examine quantitatively the responses of 163 single units from five anesthetized and paralyzed Macaca fascicularis. The experiments included tests of selectivity for stimulus direction, speed, orientation and disparity. Cells were also tested with stimuli which simulated trajectories with components of motion toward or away from the animal. The results show that in addition to direction selectivity, many cells in MT are sharply tuned for stimulus speed and disparity. This suggests that neurons in MT are well adapted for the analysis of motion in three-dimensional space.
Horseradish peroxidase and 3H-proline were injected into MT in three animals to demonstrate its anatomical inputs and outputs. Connections were seen with a large number of subcortical and cortical areas. In addition, connections with MT provide evidence contributing to the identification of two previously unrecognized cortical areas, which we have designated the medial superior temporal area (MST) and the ventral intraparietal area (VIP). The cortical layers in which projections originate and terminate are shown to provide objective anatomical criteria for assigning most cortical visual areas to a hierarchical order.
