THALAMIC CONTROL OF CORTICAL VISUAL PROCESSING

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The lamination pattern of the lateral geniculate nucleus (LGN) anatomically defines several parallel channels of afferent information to the visual cortex. The long-term objectives of this project are to determine the contributions of each geniculate subdivision to cortical physiology and visuomotor behavior, to find out to what extent and under what circumstances the transmission of information through the geniculate layers is modulated by non- retinal inputs, and to obtain a better understanding of the intracortical mechanisms that geniculate inputs feed. The first specific aim of this proposal is to determine the effects of reversibly inactivating the main portion of the LGN (A and C layers), on activity in area 17 and on visuomotor behavior in awake cats. Inactivation will be accomplished by injecting small amounts of lidocaine hydrochloride into the LGN while the animal is attempting to fixate small visual targets. Of particular interest is the extent to which activity in layer 4 and in the supragranular layers is dissociated by this procedure, and the interaction of spatially-directed attention with this dissociation. The cortical and oculomotor effects of geniculate inactivation when the target of a saccade appears in pseudorandom locations will be compared with those when the target appears in an anticipated location. The second specific aim is to test the hypothesis that the gains of different geniculate layers are dynamically and independently controlled by central feedback. Single cells in retinotopically corresponding parts of geniculate layers A and C will simultaneously recorded while animals are performing the same visuomotor tasks used for Specific Aim 1. We will also determine the extent to which central modulation of geniculate excitability is restricted retinotopically to attended regions of the visual field. The third specific aim is to test the hypothesis that the medial interlaminar nucleus (MIN) has a special role in dim-light vision. Contrast-sensitivity and contrast-response functions of MIN and will be compared with those of other geniculate cells over a range of illumination levels in the anesthetized cat.