We propose experiments to examine the anatomical substrate and physiological consequences of a dramatic rerouting of retinal projections to the auditory thalamus and cortex in ferrets. The organization of the visual pathway in normal ferrets in very similar to that in cats. We have successfully induced, with appropriate surgery in neonatal ferrets, retinal projections into the medial geniculate nucleus (MGN), the principal auditory thalamic nucleus. The MGN with retinal input retains its projections to auditory cortex. We now propose to study visual responses from, and the extrinsic and intrinsic connectivity of, the MGN and primary auditory cortex in operated ferrets reared to adulthood. Specific electrophysiological experiments include mapping the projection of the visual field onto the MGN and primary auditory cortex, studying the types of retinal input and receptive fields of visual cells in the MGN, and the receptive field properties of visual cells in primary auditory cortex. Anatomical experiments include defining in detail the projection of the two eyes to the auditory thalamus and cortex, the cells of origin of retinal projections to the MGN, and the subcortical and cortical connectivity of auditory thalamus and cortex in operated animals. We shall initiate ultrastructural studies of the synaptic organization of MGN cells with retinal input. We shall compare these features with corresponding features of the lateral geniculate nucleus (LGN) and primary visual cortex, or the MGN and primary auditory cortex, of normal animals. The overall goal of our experiments is thus to address a fundamental issue in development: whether structure and function in a target is determined by intrinsic properties of the target, or by its inputs during development.