Excitation in the goldfish retina: evidence for a non‐linear intensity code

Abstract

Experiments were done on isolated photopic goldfish retinas. They were stimulated by brief flashes of red light, and the spike activity of single ganglion cells was monitored by micro-electrodes. Red-ON-units [units excited by onset of red light] were used exclusively. The spatial integration of intensity was investigated using concentric disks of various diameters. Under these conditions, Ricco''s relation (1877) was obtained. Two small spots of light were positioned on 2 equisensi-tive sites in the receptive field; the (equal) intensities of both were varied in unison, and the responses recorded. An identical response was evoked by simultaneous illumination of both sites with an intensity, L or by illumination of a single one of the sites with an intensity, KI. K [where K = empirical constants] always exceeded by 2 (average of 4) and constant in any one experiment. The analysis of these results employed the assumption that an hypothetical quantity, the excitation, intervenes between the stimulus (light intensity) and the response (spike train). The excitation is a function of intensity, and it determines the response. The excitation from 2 spots is assumed to be twice that from one. It was inferred that the excitation (E) was a power function of the intensity (I): E = CIn, in which C and n are constants. The exponent, n, was always less than unity. Two other experiments tested the predictive value of this inference. It accurately predicted the responses to a single spot anywhere in the field, and to 2 unequal intensities simultaneously illuminating 2 equisensitive sites.