Dan Crevier and Markus Meister
Department of Molecular and Cellular Biology, Harvard University
Periodic flashes of light have long served to probe the temporal properties of the visual system. It is commonly held that the response of visual neurons occurs at the frequency of the flash stimulus, with components at higher harmonics due to non-linear distortions. Here we show that flicker of high contrast and intensity elicits a very different response, with dominant components at half the stimulus frequency and lower subharmonics. In this regime, retinal ganglion cells of the salamander respond only to every other flash of light. Remarkably, ganglion cell activity across the entire retina is locked to the same flashes. The effect depends sharply on contrast and flash frequency. It results from a period-doubling bifurcation in retinal processing, and a simple model of non-linear feedback is shown to reproduce the phenomenon. The critical interactions appear to occur at the synapse between cones and bipolar cells. Analogous period-doubling was observed in the human electroretinogram and in visually evoked scalp potentials. Comparison to the accompanying perceptual effects suggests that period-doubling can explain certain illusory flicker patterns.