The main goal of the proposed research is to study the functional organization of visual cortex in the rat. First, we plan to identify the morphology of specific neuron populations which project to known extrinsic targets. We will continue to use a new modified technique (Fran et al. '80) of adding lysolecithin to horesradish peroxidase (HRP) and then ionto- phoretically injecting this mixture via a micropipette (tip sixe 10 Mum) with either ster- eotaxid and/or electrophysiologcal identification of appropriate brain regions. The detailed morphological classes of homogenously backfilled neurons of a particular pathway will be analyzed. In related experiments, collateral axonal projections will be studied by injecting suspected targets with two different fluorescent dyes. The potential results from these studies will set the stage for our functional analysis of neurons via intracellular recording and morphological identification. Second, we will continue the quantitative analysis of single neurons in terms of their receptive field properties by extracellular recording and their afferent and efferent connections determined by electrically stimulating known sources of inputs to the visual cortex such as the thalamus and contralateral cortex. Then the neuron will be peneatrated by the microlectrode, rechecked to ensure the identity of the neuron and injected with HRP. The HRP-filling reveals a detailed morphology including the dendritic and axonal branching patterns. An attempt will be made to correlate the dendritic and/or axonal arborization patterns to response properties such as orientation and direction selectivities, receptive field size, affarent and efferent connections and conduction velocity. In conclusion, we expect that these approaches will provide valuable insights and establish relationships between the neuronal morphology and physiological properties as well as known extrinsic projections.