An "antagonistic" surround facilitates central responses by retinal ganglion cells.

Ganglion cells in the turtle retina respond to increments of saturating illumination of the receptive field centre by progressively decreasing the number of spikes of the responses, perhaps as a result of membrane inactivation. Intracellular recording indicates that a simultaneous illumination of the receptive field surround greatly facilitates the "suprasaturated" central responses, while the expected centre-surround antagonism is still present between photoresponses below saturation. It is suggested that both pre- and post-synaptic mechanisms provide the ganglion cells with the unique possibility, among other retinal cells, to shift their full dynamic range across more than 3 log units of illumination intensity.