The neurons of the primary visual cortex are highly selective for specific features of the visual image such as orientation, size, depth, and direction of motion. The analysis of these features requires the integration of visual information not only from different regions of the visual field, but in the case of motion selectivity, from different moments in time. How this integration is accomplished by the intricate connections of the cortical neurocircuitry is the subject of this proposal. The mechanisms of integration will be studied by recording intracellularly from cortical neurons in the intact animal and analyzing the synaptic potentials evoked by visual stimulation. These potentials give a direct indication of what type of visual information a neuron receives, from which presynaptic neurons it receives that information, and by what mechanisms the neuron processes the information. Three series of experiments are proposed for the next 5 years of the project. 1. The contribution of synaptic input from the lateral geniculate nucleus (LGN) to the spatial selectivity of cortical neurons will be investigated. The region of cortex containing an intracellularly- recorded neuron will be cooled. The cooling will inactivate surrounding cortical neurons, leaving the neurons of the LGN as the only remaining functional synaptic input. The visual responses of the synaptic potentials arising from the LGN can then be measured quantitatively. This experiment could resolve a long-standing question about the origin of spatial selectivity in cortical neurons. 2. Current models of motion sensitivity in cortical neurons rely on multiple synaptic inputs, each of which responds to visual stimulation with a different time delay. The source of these delays is not known, but in one model, the delays are generated in a class of neurons in the LGN called lagged cells. This model will be tested by selectively inactivating lagged cells by the application of pharmacological agents to the LGN while recording from motion selective neurons in the visual cortex. 3. A little understood feature of the visual cortex is contrast normalization. Contrast normalization is critical to the function of cortical neurons in that it enables cells to maintain their selectivity for stimulus features in the face of changing stimulus contrast. Without it, the way in which cells encoded a visual stimulus would change with changing contrast. Intracellular recording will be used to examine the cellular mechanisms underlying contrast adaptation.
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