The highly specific receptive field properties of neurons in the mammalian visual cortex attest to the complexity of the perceptual task performed there. How this task is accomplished by the intricate synaptic interconnections among cortical neurons is the subject of this proposal. Two separate methods are being employed. First, electrophysiological circuit-tracing techniques, initially developed in the spinal cord, are used to map out the synaptic connections that exist among cortical neurons. Second, the contribution of these synaptic connections to neuronal receptive field properties are explored by examining the visual responses of intracellularly recorded synaptic potentials. The emerging picture of how cortical neurons interact relates not only to the contribution of the cortex to visual perception. Recent results show that reverberating excitatory pathways may underlie the generation of cortical seizures under pathological conditions. Three specific experiments are proposed for the next 5 years of the project. 1. In previous grant periods the synaptic basis for orientation selectivity was investigated in the cat cortex. In this period, a study of the basis of direction selectivity is proposed. A fundamental difference in these two properties lies in the dependence of direction selectivity on the spatial and temporal properties of a neuron's synaptic inputs, while orientation selectivity is dependent only on the spatial organization of the receptive field. Recent results indicate that in direction selective cells the latency of the response to flashing stimuli may vary with position within the receptive field. Experiments are proposed to investigate the synaptic mechanisms underlying these variations. 2. Two types of inhibition could in theory occur in the mammalian nervous system: hyperpolarizing inhibition and shunting inhibition. Which of the two types predominates is of considerable theoretical importance to understanding neuronal function. The computational capabilities of the brain depend critically on whether synaptic inputs interact linearly, as hyperpolarizing inhibition does, or nonlinearly, as shunting inhibition does. Experiments are proposed to determine which type of inhibition predominates in the visual cortex. 3. That neurons of the M and P pathways in primate visual system possess distinctive receptive field properties implies that the receptive field of neurons in different layers of striate cortex and in different areas of extrastriate cortex will depend heavily on whether these areas receive their predominant input from the M or P pathways. To trace these parallel pathways within striate cortex and beyond, experiments are proposed based on the difference between the thresholds to electrical stimulation of the axons of M and P retinal ganglion cells. The thresholds of responses evoked in cortical neurons by electrical stimulation of the optic nerves indicate whether a neuron, whose receptive field properties and location have been determined, receives input from the M pathway.