Outer-retina locus of increased flicker sensitivity of the peripheral retina

We tested alternative hypotheses concerning the locus of enhanced flicker sensitivity observed in response to stimuli presented to the peripheral retina. The first hypothesis attributes increased temporal frequency sensitivity to ganglion cell and higher-order neural processing, whereas the second hypothesis states that the locus of these temporal effects is at the cone photoreceptors. To test these alternative hypotheses we measured retinal electrophysiological and psychophysical temporal modulation thresholds. We found that sensitivity for temporal frequencies < 30 Hz did not vary as a function of retinal location for either the focal electroretinogram (ERG) or the psychophysical measure. However, for both measures, sensitivity for temporal frequencies > or = 30 Hz was greater in the peripheral retina than in the central retina. In addition, critical flicker frequency for the central retina was linear as a function of retinal illuminance for both the psychophysical and the electrophysiological measures. For the peripheral retina the slopes of critical flicker frequency versus log illuminance functions were steeper than the central slopes for both threshold measures. Eccentrically measured focal ERG and psychophysical critical flicker frequency values showed a relative saturation, deviating from the linear slope above 3.5 log Td. The findings of similar focal ERG and psychophysical temporal sensitivity changes with eccentricity support an outer retinal origin of this phenomenon.