Visual fields in the Limulus eye

Abstract

The dynamic response of single photoreceptors and their inhibitory interaction are well described by the (time dependent) Hartline Ratliff equations. To compute how a pattern of illumination in space is mapped into the pattern of optic nerve activity one must also know how the ommatidial optics sample visual space and the spatial dependence of inhibition conversion onto an ommatidum. Taking advantage of the remarkable linearity of the response of this retina to incremental stimuli, the authors measured modulation to the spike rate of an ommatidum which looked at a TV like display where the intensity of a thin bar was sinusoidally modulated around the same mean intensity as the background. Responses measured at different bar positions map the visual field which has an excitatory center and inhibitory surround. The central field is gaussian, about 7° wide at half height (range 4° (extreme light adaptation) to 10°). In a horozontal plane the directions of the optic axes of adjacent ommatidia diverge uniformly by about 4.5°, and the inhibitory field is broad and flat. In a vertical plane, divergence is not uniform, being over 7° above, and less than 2.5° below, the center, and the inhibitory fields are generally asymmetrical. The authors compared the measured inhibitory fields with solutions of a Hartline Ratliff model that take explicit account of the orientation of optical axes. From these computations they conclude, in agreement with previous work, that the map of retinal connectivity is center symmetric with elliptical contours, but it is flatter and broader than that deduced from measurement of single unit interactions.