.) This work is intended to increase our knowledge of the functional architectures of the visual cortex. Anatomical pathways have been derived, but little is yet known about how cortical receptive fields acquire their distinct behavior. The primary hypothesis of this proposal is that inhibition plays a fundamental role in shaping cortical receptive fields. The inhibition is believed to operate at two levels: a local component that has direct involvement in shaping the spatial selectivity of the receptive field; and a global component, which integrates responses strength across large numbers of cells and uses this information to set the operating point of large number of cells. These two types of inhibition may be non-exclusive (i.e., cooperative). Recent description of oscillation in cortical responses that is phase-locked over great distances across cortex suggests a basis for a global control mechanism. The proposed program will test this hypothesis with two general experimental approaches. Extracellular action potentials will be recorded under stimulating conditions designed to reveal more about the spatial distribution, spatial selectivity and temporal dynamics of inhibition. All data will be analyzed for the presence and characteristics of response oscillation. The applicants will also analyze similarly responses from dLGN and retina; preliminary results show oscillation in the dLGN, and it may also be detectable in the retina. Recording of action potentials from two or more cells will address the question of whether there is a common origin to the inhibition (or oscillation) that may feed arrays of cells. Inhibition must be mediated by a neurotransmitter. GABA is present in substantial concentrations in the visual cortex, but it does not control certain aspects of the response that must derive from inhibition. The characteristics of other identified neurotransmitters found in the visual cortex will be studied by iontophoretic injection of these transmitters while simultaneously recording. Glycine is a likely inhibitory agent that has previously only been studied superficially. Iontophorectic injection will also be used to block the corticothalamic feedback. The function of this important pathway has only been poorly characterized thus far.
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