Directional selectivity and colour coding in the frog retina

Abstract

The impulse discharge of ganglion cells was recorded with extracellular micro-electrodes in the excised and opened eye of the common frog, Rana temporaria. The responses of different ganglion cell types to a standard moving spot with various spot-background contrasts are described. Information about such stimulus parameters as the size and contrast as a moving object is given by different classes of ganglion cells with preferences for different stimulus features. Of 171 sustained cells with small receptive fields 29 were found directionally selective, i.e. they responded well to movements only in some directions. Experiments with double stimulus fields suggest that this selectivity is due to an amacrine cell-mediated lateral inhibition nonsymmetrically arranged around the centre of the receptive field. The dichromatic colour vision of the frog is based on partly opponent signals from yellow-sensitive cones and blue-sensitive green rods. These opponent inputs make the ganglion cells respond to blue spots moving against a yellow-green background, irrespective of the relative intensities of the two colours. When the green rods are stimulated with blue light the ganglion cells produce long "on"-responses with significantly lower impulse frequencies than the short cone-mediated responses.