INFORMATION TRANSFER AND CONTRAST SENSITIVITY IN THE LGN

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This proposal aims at a better understanding of the functional organization of the central visual pathways of mammals, and attempts to relate visual functions to neuronal populations in the primate. I will focus on two main problems: 1) What are the dynamic properties of the contrast gain control we recently discovered in the Lateral Geniculate Nucleus (LGN)? and 2) What is the neural substrate for the psychophysically determined luminance and color contrast sensitivities? The first question stems from the observations that the LGN is a dynamic filter of the incoming retinal information, and that its filtering (attenuation) of retinal input increases with stimulus contrast. This observation lead to a set of new experiments, in which the contrast of a pattern is modulated in time. I will record in anesthized and paralyzed cats and monkeys the responses of single LGN cells together with their retinal input (recorded as synaptic (S) potentials), and measure the time course with which the LGN adjusts its transmission ratio (LGN response/retinal response) following a step increase in contrast. The importance of temporal and spatial stimulus parameters, as well as that of retinal eccentricity and binocular interactions, will all be explored. The second question is stimulated by the observation that in the monkey LGN the parvocellular neuons (and their drives, the P cells) are much less sensitive to luminance contrast than the magnocellular neurons (or the retinal M cells), and by recent psychophysical work (Mullen, 1985) which compared the contrast sensitivity for color and luminance patterns in humans. I will establish which neuronal population could account for these results by measruing the responses of single P and M cells in the monkey LGN to both luminance and chromatic patterns. I will also measure the sensitivities of the receptor field center and periphery to luminance and color modulations, to uncover the reason for the unbounded increase in response to color stimuli with stimulus size, noted in the P cells, and the source of the difference in contrast gain between M and P cells. The results will shed light on the function of the M cells and P cells in primate vision.