This project deals with two aspects of development in the rabbit visual system. First, developmental changes in neuronal and synaptic morphology of laminae I,III and IV of the striate cortex will be examined. Using Golgi and EM techniques, normative data will be taken from adults. Other adults will undergo stereotaxic lesions of the dLGN and the geniculocortical input studied with reduced silver and EM. A series of young animals of various ages will then be studied similarly, with emphasis on changes in populations of intrinsic synapses and distribution of dLGN input. Selected tissues will be studied with combined Golgi-EM or reduced silver-EM techniques, allowing direct correlations between light- and EM observations. Developmental findings will be compared with known data on receptive field ontogenesis. Second, the degree of plasticity in axons innervating various laminae of the superior colliculus will be studied. Neonatal lesions will be placed in one of the visual, auditory or somethetic tracts projecting to SC. After many months a lesion will be placed in another pathway and terminal distribution in SC studied with Nauta and EM. Silver techniques will test for abnormal distribution of axonal input, and EM will examine the extent of actual synaptic contact between axons and abnormal postsynaptic neurons. Other studies will study the size changes in deafferented collicular neurons. Assessments will be made of the capability for axonal plasticity in situations involving intra-modal vs. cross-modal interaction, deep vs. superficial layer, cortical vs. subcortical axonal systems, and limitations imposed by physical distance. These experiments will systematically examine the potential for axonal plasticity in a mammalian system, and try to determine some limits on that potential.