| The response to flickering light has long been studied to understand the temporal processing of the visual system. The retina generally responds to every flash of light, gradually ceasing to respond as the frequency is increased above the critical fusion frequency. In this thesis, I describe a phenomenon where the retina approaches the critical fusion frequency in a much different manner. Under very bright full field stimulus in a certain frequency range, the retina responds to only every other flash of light, with a period double that of the stimulus period. Moreover, the entire retina responds in synchrony.; This phenomenon was observed in the larval tiger salamander retina as well as in humans. In the tiger salamander, the electroretinogram as well as fiber and single cell responses were used to characterize the response. In humans, the electroretinogram and visually evoked potentials were measured to likewise characterize the phenomenon. The effects of various stimulus parameters are discussed, such as frequency, contrast, and mean intensity.; During this bright full field flicker stimulus in humans, there are some interesting visual illusions observed. These illusions are discussed, and possible explanations are presented based on the period-doubling phenomenon.; Pharmacology was used in the salamander retina to elucidate the retinal elements involved in generating the phenomenon. The results suggest that the critical feedback interactions involve cone photoreceptors and bipolar cells. The synchronization potentially results from electrical coupling between bipolar cells. A simple nonlinear feedback model is presented, which is able to capture the qualitative behavior of the phenomenon in the retina. Possible biophysical mechanisms are discussed, in light of the pharmacological studies. |
