Disconnection of parietal and occipital access to the saccadic oculomotor system.

The experiment explored the networks through which signals arising from visual areas of cortex control saccadic eye movements. Electrical stimulation of the inferior parietal and the occipital cortex (here termed the "posterior eye fields") normally evokes saccadic eye movements. We replicated previous reports that these evoked eye movements ceased after large tectal ablations. This initial finding suggested that the "posterior eye fields" depended on a single route of access to the saccade generator, one descending through the superior colliculus (SC). On closer examination, the critical lesion appeared to be one which removed the SC and cut efferents from the frontal eye field (FEF) coursing nearby. Subsequently we confirmed that eye movements evoked from the posterior eye fields ceased after cooling the SC, or cutting its efferents- but only when one of these procedures was combined with FEF ablation. Thus, visual signals from the occipital and inferior parietal cortex have more than one, but perhaps only two routes of access to the oculomotor system. One passes through the superior colliculus, the other through the frontal eye field. Ancillary experiments revealed that inferior parietal and FEF ablations, alone or combined, do not disrupt saccades evoked from the occipital lobe. Striate and prestriate areas can therefore use their own direct input to the SC or to the basal ganglia to drive saccadic eye movements.