Spatial properties of horizontal cell responses in the turtle retina.

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Abstract
Intracellular recordings were made from horizontal cells in the retina of the turtle Pseudemys scripta elegans. Spatial properties of the responses were determined using brief flashes of monochromatic light. For a light stimulus in the form of a long narrow slit the peak response decayed approximately exponentially with displacement from the centered position. With variation in the area of a centered circular patch, the peak response increased in a graded manner with stimulus area but was not proportional to area. The model of electrical coupling in the horizontal cell layer proposed by Naka and Rushton was applied to the results. For the case of dim illumination a simplification was applicable, and the voltage distribution for circular and slit-shaped patches of light could be expressed in terms of 2 unknowns: the voltage resulting from diffuse illumination and a characteristic length constant. The measured variation of response amplitude was well described by the theory. Measured length constants were distributed from less than 100 .mu.m to greater than 1 mm, and in a given cell the values determined by the slit displacement method and the area variation method were in reasonable agreement. With dim illumination apparently the model provides an accurate description of the voltage spread in the cells. Deviations occurred at higher intensities and possible reasons were discussed. The implications of the model on the measurement of resistance changes during illumination were discussed.