Intensity discrimination capacity of retinal ganglion cells of a tree shrew ( Tupaia chinensis)

Abstract

Responses of single retinal on-centre ganglion cells were recorded in order to study the intensity discrimination threshold behaviour. A Poisson distribution fitted pulse number distributions of the spikes in the burst evoked by short duration stimuli presented within the receptive field centre of sustained units. A normal distribution was found for transient units. The dependence of the mean, standard deviation and regularity of the pulse number distribution on stimulus intensity mimicked the behaviour of the statistical parameters of the maintained activity as a function of background intensity. The transient cells displayed statistical characteristics which are found also in psychophysical studies. These cells, therefore, are assumed to be mainly involved in intensity discrimination. The dynamic range of ganglion cells in response to short duration stimuli is relatively narrow, 1 log unit, so the Weber law which is valid over many log units cannot be explained with the single-channel detection model. A multichannel energy summation model yields a threshold curve that closely resembles psychophysically measured curves. The psychophysical dependency of the discrimination threshold on background intensity level as investigated by Thijssen and Vendrik (1971) could be explained from the responses of both types of cells.