EFFECTS OF FOCAL INACTIVATION OF DORSAL OR VENTRAL LAYERS OF THE LATERAL GENICULATE-NUCLEUS ON CATS ABILITY TO SEE AND FIXATE SMALL TARGETS

To reveal contributions of different subdivisions of the lateral genic ulate nucleus (LGN) to visuomotor behavior, segments of either layer A or the C layers were inactivated with microinjections of gamma-aminob utyric acid while cats made saccades to retinally stabilized spots of light placed either in affected regions of visual space or mirror-symm etric locations in the opposite hemifield. Inactivating layer A reduce d the success rate for saccades to targets presented in affected locat ions from 82.4 to 26.8% while having no effect on saccades to the cont rol hemifield. Saccades to affected sites had reduced accuracy and lon ger initiation latency and tended to be hypometric. In contrast, inact ivating C layers did not affect performance. Data from all conditions fell along the same saccade velocity/amplitude function (''main sequen ce''), suggesting that LGN inactivations cause localization deficits, but do not interfere with saccade dynamics. Cerebral cortex is the onl y target of the A layers, so behavioral decrements caused by inactivat ing layer A must be related to changes in cortical activity. Inactivat ing layer A substantially reduces the activity of large subsets of cor ticotectal cells in areas 17 and 18, whereas few corticotectal cells d epend on C layers for visually driven activity. The parallels between these behavioral and electrophysiological data along with the central role of the superior colliculus in saccadic eye movements suggests tha t the corticotectal pathway is involved in both deficits and remaining capacities resulting from blockade of layer A.