The Neural Control of Visually Guided Eye Movements
C. Cortical Mechanisms of Visually Guided Saccadic Eye Movements

Here we show a schematic wiring diagram of the circuits involved in the generation of visually guided saccadic eye movements. As noted earlier, in the retina several distinct classes of ganglion cells originate. Three of these are depicted in the diagram, the midget system, the parasol system and the w system. The w system sends direct connections to the superior colliculus as well as to the LGN. Little is known at this stage about the cortical circuitry of this system.

The midget and parasol systems have both separate and combined components in the projections from V1 to higher centers. The processing stream through MIT to the parietal lobe is dominated by the parasol system whereas the stream through V4 to the temporal and parietal lobes is mixed. The interaction between the midget and parasol systems is further enhanced by the plethora of reciprocal connections among the various cortical areas. The projections to the superior colliculus from V1, however, is driven exclusively by the parasol system.

The generation of visually guided eye movements involves two major systems, the anterior and the posterior. The anterior system consists of direct connections from the cortex to the brainstem the most prominent of which are from the frontal and medial eye fields. The posterior system is comprised of connections from the cortex through the superior colliculus to the brainstem.

Inhibitory circuits play a central role in visually guided saccadic eye-movement generation. Two of several such circuits is shown, one from cortex through the substantial nigra and the other intracortical circuitry in the frontal eye fields. Both excitatory and disinhibitiotory activity is central for the production of accurate and rapid saccadic eye movements.