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At temporal frequencies between approximately 30 and 70 Hz, the flicker electroretinogram (ERG) of the cone system can exhibit an alternation in response amplitude from cycle to cycle that has been termed synchronous period doubling. This phenomenon has been attributed to a nonlinear feedback mechanism at an early retinal locus. The purpose of the present study was to define the effect of stimulus contrast on period doubling in order to better understand the nature of the underlying mechanism. ERGs were recorded from three visually normal subjects in response to sinusoidal flicker ranging from 20 to 100 Hz, using stimulus contrasts of 37.7, 56.5, 75.4, and 94.2%. Period doubling was quantified as: (1) the amplitude of an harmonic component of the ERG waveform that was 1.5 times the stimulus frequency, and (2) the difference between the mean trough-to-peak amplitudes on even and odd cycles of the ERG waveform. Amplitudes were converted to responsivity by dividing by stimulus contrast. By both measures, subjects showed discrete regions of period doubling that were displaced to lower temporal frequencies as stimulus contrast was increased. The temporal frequency shift of period doubling with altered stimulus contrast can be accounted for quantitatively by postulating a neural threshold for the nonlinear feedback signal that is presumed to generate synchronous period doubling.