SYNCHRONOUS PERIOD-DOUBLING IN FLICKER VISION OF SALAMANDER AND MAN

Periodic flashes of light have long served to probe the temporal prope rties of the visual system. Here we show that during rapid flicker of high contrast and intensity the eye reports to the brain only every ot her flash of light. In this regime, retinal ganglion cells of the sala mander fire spikes on alternating flashes. Neurons across the entire r etina are locked to the same hashes. The effect depends sharply on con trast and flash frequency. It results from a period-doubling bifurcati on in retinal processing, and a simple model of nonlinear feedback rep roduces the phenomenon. Pharmacological studies indicate that the crit ical feedback interactions require only cone photoreceptors and bipola r cells. Analogous period-doubling is observed in the human visual sys tem. Under bright full-field flicker, the electroretinogram (ERG) show s a regime of period-doubling between 30 and 70 Hz. In visual evoked p otentials from the occiput, the subharmonic component is even stronger . By analyzing the accompanying perceptual effects, we find that retin al period-doubling begins in the periphery of the visual field, and th at it is the cause of a long mysterious illusory flicker pattern.