On the Nature of the Gated Colour Opponency in the On-Units of the Frog Retina: Electrophysiological Study and Model

Ganglion cells of the ON-type in the frog retina produce colour-dependent responses differing in temporal patterns (short bursts to excitation of red-sensitive cones as opposed to prolonged discharges if blue-sensitive ‘green rods’ are excited). Their gated colour opponency (Kicliter et al, 1981 Brain Research 210 103 – 113; Maximov et al, 1985 Vision Research 25 1037 – 1049) becomes apparent from the OFF-responses in conditions when the test stimuli are superimposed on a background of another colour. So, when blue glass is introduced in the light beam (decreasing the excitation mainly of red-sensitive cones), an OFF-response is observed, much like the response to the onset of blue light. It has been suggested that opponency in ON-cells is asymmetric, ie that the red signal reaches the blue channel with reversed sign, but not vice versa.

A single-unit-recording study revealed the dependence of ON-cell responses both on the colour of stimuli presented in the centre of the receptive field and on the steady illumination of its surround. Surround illumination was found to favour OFF-responses in ON-units. In some cases even the cessation of blue light elicited an OFF-response with a discharge pattern resembling that of the onset of red light. In these cases an ON-response to yellow glass could also be obtained. These observations prove some degree of symmetry in the opponency of the red and blue channels. It is suggested that feedback from horizontal cells onto photoreceptor terminals is involved in the gated colour opponency. A circuit model that reproduces the observed phenomena is presented.