Stimulus chromatic properties affect period doubling in the human cone flicker ERG

Introduction: Period doubling in the full-field cone flicker electroretinogram (ERG) refers to an alternation in waveform amplitude and/or shape from cycle to cycle, presumably owing to the operation of a nonlinear gain control mechanism. This study examined the influence of stimulus chromatic properties on the characteristics of period doubling in order to better understand the underlying mechanism. Methods: ERGs were acquired from 5 visually normal subjects in response to sinusoidally modulated flicker presented at frequencies from 25 to 100 Hz. The test stimuli and the pre-test-stimulus adaptation were either long wavelength (R), middle wavelength (G), or an equal combination of long and middle wavelengths (Y), all equated for photopic luminance. Fourier analysis was used to obtain the response amplitude at the stimulus frequency F and at a harmonic frequency of 3F/2, which was used as the index of period doubling. Results: The frequency-response function for 3F/2 typically showed two peaks, occurring at approximately 33.3 and 50 Hz. However, the magnitude of period doubling within these frequency regions was dependent on the chromatic properties of both the test stimulus and the pre-stimulus adaptation. Period doubling was generally smallest when an R test was used, even though the stimuli were luminance-equated and the amplitude of F did not differ between the various conditions. Discussion: The pattern of results indicates that the mechanism that generates period doubling is influenced by chromatic signals from both the test stimulus and the pre-stimulus adaptation, even though the high stimulus frequencies presumably favor the achromatic luminance system.