Roles of the lateral suprasylvian cortex in convergence eye movement in cats

Ocular convergence and lens accomodation were evoked by microstimulation in the lateral suprasylvian area (LS cortex) in the parieto-occipital cortex in the cat. Electrolytic lesions in LS cortex reduced the amplitude and velocity of ocular convergence. Neurons in LS cortex discharged in relation to ocular convergence and/or lens accommodation. These results support the hypothesis that the LS cortex plays an important role in controlling ocular convergence. The LS cortex receives visual inputs from cortical visual areas 17, 18 and 19, and in addition from the superior colliculus through the LP nucleus of the thalamus. Electrophysiological recordings have revealed that these visual inputs, which include cues about 3-dimensional target motion, are integrated in the LS cortex. The integrated output from LS cortex may provide the brainstem motor centers with the neural signals that facilitate eye movements, especially when the target is moving at high speeds. Outputs from the LS cortex travel directly to brainstem structures including the superior colliculus and pretectum. Evidence from monkey suggests that information may also travel to the mesencephalic reticular formation, where neurons have been recorded that are related to ocular convergence, lens accomodation or both. Although comparable data is lacking in the cat, it is suggested that the efferent circuit from the LS cortex to the motor nuclei in the brainstem includes both the superior colliculus and the mesencephalic reticular formation. It is also suggested that this pathway is rather short, given that the mean latency of the early component of evoked disjunctive eye movements was approximately 60 ms.