Elizabeth N. Johnson - US grants

DESCRIPTION Most objects in the normal visual environment vary in luminance and chromaticity, and are rarely differentiated solely by either brightness or color. Psychophysical studies using the simultaneously presentation of color-varying and luminance-varying stimuli indicate that there is a wide range of non-linear color-luminance interactions, but the nature and extent of chromatic and luminance interactions in early visual cortex remains unclear. The proposed experiments aim to determine whether these interactions occur in single neurons in the primary visual cortex (V1) of anesthetized, paralyzed macaque monkeys. The monkeys will be presented with drifting gratings containing a super-position of equiluminant color and luminance. The contrasts of the chromatic and the luminance components are varied independently, allowing for a complete parametric analysis of the effects of a broad range of chromatic and luminance contrasts. Spatial features, such as waveform, spatial frequency, and relative phase will be varied. This range of stimuli closely resembles the stimuli used in the psychophysical studies. It will therefore bee possible to determine whether responses as early as V1 show the same strong non-linear effects observed psychophysically. Control experiments in the lateral geniculate nucleus will verify the range of interactions that originate in V1. Histological reconstruction of each electrode penetration will allow cells to be classified according to their anatomical organization to determine trends among cells linked to the processing of color and luminance within the known V1 network hierarchy. Discerning the mechanisms involved in normal early cortical processing of color and luminance will enable us to correlate neurophysiological responses to perception. In addition, it will allow for a better understanding of the chromatic and spatial vision deficits facing many patients with cortical damage, aiding the development of better diagnostic tests for such visual deficits.

DESCRIPTION Most objects in the normal visual environment vary in luminance and chromaticity, and are rarely differentiated solely by either brightness or color. Psychophysical studies using the simultaneously presentation of color-varying and luminance-varying stimuli indicate that there is a wide range of non-linear color-luminance interactions, but the nature and extent of chromatic and luminance interactions in early visual cortex remains unclear. The proposed experiments aim to determine whether these interactions occur in single neurons in the primary visual cortex (V1) of anesthetized, paralyzed macaque monkeys. The monkeys will be presented with drifting gratings containing a super-position of equiluminant color and luminance. The contrasts of the chromatic and the luminance components are varied independently, allowing for a complete parametric analysis of the effects of a broad range of chromatic and luminance contrasts. Spatial features, such as waveform, spatial frequency, and relative phase will be varied. This range of stimuli closely resembles the stimuli used in the psychophysical studies. It will therefore bee possible to determine whether responses as early as V1 show the same strong non-linear effects observed psychophysically. Control experiments in the lateral geniculate nucleus will verify the range of interactions that originate in V1. Histological reconstruction of each electrode penetration will allow cells to be classified according to their anatomical organization to determine trends among cells linked to the processing of color and luminance within the known V1 network hierarchy. Discerning the mechanisms involved in normal early cortical processing of color and luminance will enable us to correlate neurophysiological responses to perception. In addition, it will allow for a better understanding of the chromatic and spatial vision deficits facing many patients with cortical damage, aiding the development of better diagnostic tests for such visual deficits.

DESCRIPTION (provided by applicant): Deciphering the spectral coding of neurons provides an important key to understanding the mechanisms of color vision. Although there is a growing body of research on spectral coding in the retina, much less is known about chromatic inputs once they reach central targets. My proposed project focuses on the cortical input of short-wavelength-sensitive cones (scones) by exploring the functional organization and dynamics of s-cone inputs in the primary visual codex (V1) of the tree shrew. In particular, the proposed experiments aim to 1) determine whether stimuli that selectively isolate the s-cones are capable of activating V1 circuits and whether there is a systematic organization of that activity that is different from cortical activation with stimuli that excite the ml-cones, 2) examine the spatial and temporal dynamics of the cortical population response to s-cone mediated signals, 3) compare the laminar distribution of s-cone and ml-cone driven inputs and their degree of convergence onto single V1 neurons. These experiments will explore how activity in the s-cone pathway is represented in V1, providing new insights into the cortical mechanisms that are responsible for color processing.