Characteristics of period doubling in the human cone flicker electroretinogram.

Electroretinogram (ERG) responses of the cone system to a flickering stimulus can exhibit a cyclic variation in amplitude. This phenomenon of synchronous period doubling has been attributed to a nonlinear feedback mechanism within the retina that alters response gain. The aim of the present study was to investigate intersubject variability in period doubling in the ERG of the human cone system, and to assess the implications of this variability for signal processing within the retina. Period doubling was examined in a group of 12 visually normal subjects, using sinusoidal full-field flicker and harmonic analysis of the ERG waveforms. For all subjects, the ERG responses to 32-Hz flicker (a frequency commonly used clinically) were characterized by a harmonic component at the stimulus frequency and at higher harmonics that were integral multiples of the stimulus frequency, as expected. In addition, six of the subjects showed period doubling at 32 Hz, characterized by harmonic components at integer multiples of a frequency that was half the stimulus frequency (the subharmonic). However, the subharmonic itself did not exceed the noise level. These findings suggest that the subharmonic is generated prior to or at the site that produces the nonlinear higher harmonics of the ERG response, and that a subsequent band-pass filter attenuates this subharmonic. Examination of harmonic components of the subjects' ERG waveforms at other stimulus frequencies, as well as a cycle-by-cycle analysis of the ERG waveforms, suggested that individual differences in period doubling may be due to intersubject variation in the strength of the hypothesized feedback signal and/or the time constant of its decay.