Dynamic Properties of Visual Cortical Cirucits

University Of Southern California, Los Angeles CA

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Abstract

[unreadable] DESCRIPTION (provided by applicant): The main route from the retina to the visual cortex is through the lateral geniculate nucleus of the thalamus. While retinal input drives thalamic relay cells to fire and outlines their receptive fields, as yet undefined suppressive mechanisms in the thalamus play critical roles in shaping the signals that ultimately reach cortex. Here we explore directly the means by which two separate inhibitory circuits in the thalamus, one intrinsic to the lateral geniculate and the other originating in the adjacent perigeniculate nucleus, influence spatial and temporal integration within the relay cell's receptive field. Our main approach is whole-cell recording in vivo, which allows us to measure synaptic inhibition directly as well as to identify the neurons whose responses we record as X or Y relay cells or interneurons. (1) The receptive fields of thalamic relay cells inherit their center-surround structure from the retina, whose output is purely excitatory. In the retinal center and surround, stimuli of the reverse contrast evoke intracellular responses of the opposite sign -"push-pull". Is the excitatory (push) structure of the thalamic receptive field routinely matched by inhibition (pull) provided by local interneurons? How do these excitatory and inhibitory inputs interact to shape the information transmitted to cortex? (2) Stimuli presented beyond the classical receptive field, that is, the region bounded by retinal input, have a suppressive effect. Spatially diffuse suppression from this "extra surround" is thought to originate in the perigeniculate nucleus. It will be possible to study the influence of the perigeniculate on relay cells selectively because most neurons there are binocular, thus, permitting stimulation via the non-dominant eye. (3) Diversity in response timing in thalamus is far greater than in retina--a feature central to models of cortical direction selectivity. Current theory holds that the novel delays in excitatory responses result from same-sign inhibition. We will ask whether or not the temporal envelopes of inhibition provided by thalamic interneurons are capable of producing the observed delays in excitation. Knowledge of how the brain operates normally provides a standard against which to judge changes that result from various disorders, as well as a model system on which to test drugs developed to treat illness. From this perspective, the visual thalamus is an obvious site to study; its function and anatomy are better resolved than any other thalamic region. A deeper understanding of local synaptic mechanisms provides insight into processes that go awry during disease. For example, the work proposed here bears directly on a central theme in research on amblyopia, the examination of how abnormal visual experience leads to changes in central processing. [unreadable] [unreadable]