STRUCTURE-FUNCTION RELATIONSHIPS IN VISUAL CORTEX

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The long range goal of this research program is to understand the structure/function relationships that underlie information processing in the lateral geniculate nucleus (LGN) and visual cortex. Emphasis is on the role of the LGN in temporally transforming afferent signals derived from the retina, and on the contribution these modified signals make to the generation of receptive field structure and visual response properties of neurons in cortical areas 17 and 18. The impetus for the experiments is the finding that the A-laminae of the LGN contain two groups of relay neurons, termed lagged and nonlagged cells, which differ markedly in the timing of their responses to visual stimuli. We have evidence that these timing differences are used in visual cortex to set up the spatiotemporal structure of receptive fields and to create neurons that are sensitive to the direction of stimulus movement. The experiments proposed will reveal the timing properties of neurons in other divisions of the geniculate complex and their potential contribution to the response properties of cortical cells, and determine whether response timing in the LGN and cortex can be modified by abnormal visual stimulation during development. Specific aims are as follows: (1) To characterize the response timing of relay neurons in the geniculate C-laminae and the medial interlaminar nucleus (MIN) in order to complete our ongoing description of geniculate inputs to cortical areas 17 and 18. (2) To examine relationships between the morphological and physiological properties of cells examined in Aim 1, as a basis for elucidating structural mechanisms underlying information processing in the LGN. (3) To reveal the termination zones in cortex of different geniculate cell types, and to search for a sublaminar distribution of afferent types and/or response timing as a basis for interpreting the origin of laminar differences in response timing among cortical cells. (4) To characterize the spatiotemporal structure and direction selectivity of simple cell receptive fields in area 18 and to compare their response timing with that of the geniculate afferents. (5) To investigate the impact of raising animals in a stroboscopically illuminated environment on the response timing of cells in the LGN and (6 and 7) on the spatiotemporal structure of receptive fields in areas 17 and 18. Strobe rearing selectively abolishes direction selectivity in cortex. Parallel changes in response timing between LGN and cortex in these animals will provide further evidence for the role of the LGN in contributing to direction selectivity in cortex. These studies will also provide the first detailed evidence on mechanisms underlying the loss of direction selectivity in strobe-reared animals, and hence contribute to our overall understanding of the substrates for direction selectivity.