Abstract
Purpose: :
At certain frequencies of a flickering stimulus, the ERG response of the cone system is characterized by an alternation in waveform shape from cycle to cycle, termed period doubling. The purpose of this study was to characterize the effect of stimulus wavelength on period doubling in order to better understand the underlying mechanism.
Methods: :
Five visually normal subjects participated in the study. Stimuli consisted of long-wavelength (640 nm [R]), middle-wavelength (512 nm [G]), and a combination of long and middle wavelength (equal luminance of 640 and 512 nm [Y]) sinusoidally modulated flicker presented at frequencies from 25 to 100 Hz in 0.3 log unit steps, with a contrast of 100% and a mean luminance of 200 cd/m2. The flicker was presented for one second on a rod-saturating background of 12.3 cd/m2. Stimuli were generated by LED arrays in a ColorDome stimulator, and ERG responses were acquired using a DTL electrode and an E2 console. Fourier analysis was used to obtain the response amplitude at the fundamental frequency (F) and at a frequency of 3F/2, which was used as an estimate of the magnitude of period doubling.
Results: :
Period doubling was observed at stimulus frequencies between approximately 30 and 70 Hz. The shape of the frequency response function for period doubling was dependent on the chromatic characteristics of the stimulus. For the Y stimulus, the frequency response function typically showed two peaks: a lower frequency maximum at approximately 33 Hz and a higher frequency maximum at approximately 50 Hz, with a minimum near 40 Hz. The magnitude of period doubling was significantly smaller for the R stimulus than for the Y stimulus within the lower frequency region (p < 0.05), but period doubling was significantly smaller for the G stimulus than for the Y stimulus within the higher frequency region (p < 0.05). In addition, the amplitude of period doubling for the R stimulus was influenced by the wavelength of the pre-stimulus adapting field, such that period doubling within the higher frequency region was smallest when the R stimulus was preceded by a G adapting field.
Conclusions: :
Equiluminant middle- and long-wavelength stimuli can yield quite different magnitudes of period doubling, indicating that the nonlinear feedback mechanism that is presumed to generate period doubling is influenced by chromatic signals in a frequency-dependent manner. Our results are consistent with previous evidence that chromatic adaptation can affect the characteristics of the high-frequency, luminance-driven cone flicker ERG.
Keywords: electroretinography: non-clinical