Deficits in Short-Latency Tracking Eye Movements after Chemical Lesions in Monkey Cortical Areas MT and MST

Past work has suggested that the medial superior temporal area (MST) is involved in the initiation of three kinds of eye movements at short latency by large-field visual stimuli. These eye movements consist of (1) version elicited by linear motion (the ocular following response), (2) vergence elicited by binocular parallax (the disparity vergence response), and (3) vergence elicited by global motion toward or away from the fovea (the radial-flow vergence response). We investigated this hypothesis by recording the effects of ibotenic acid injections in the superior temporal sulcus (STS) of both hemispheres in five monkeys. After the injections, all three kinds of eye movements were significantly impaired, with the magnitude of the impairments often showing a strong correlation with the extent of the morphological damage in the three subregions of the STS: dorsal MST on the anterior bank, lateral MST and middle temporal area on the posterior bank. However, the extent of the lesions in the three subregions often covaried, rendering it difficult to assess their relative contributions to the various deficits. The effects of the lesions on other aspects of oculomotor behavior that are known to be important for the normal functioning of the three tracking mechanisms (e.g., ocular stability, fixation disparity) were judged to be generally minor and to contribute little to the impairments. We conclude that, insofar as MST sustained significant damage in all injected hemispheres, our findings are consistent with the hypothesis that MST is a primary site for initiating all three visual tracking eye movements at ultra-short latencies.