Characteristics of Late Negative ERG Responses Elicited By Sawtooth Flicker

BACKGROUND: This study aimed to determine whether the properties of the late negative responses (LNRs) of the electroretinogram (ERG) elicited by sawtooth flicker are consistent with the characteristics of the photopic negative response generated by a light pulse (PhNR(pulse)). METHODS: ERG recordings were obtained from 10 visually normal individuals and from 6 patients with optic atrophy (OA) in response to 8-Hz rapid-on and rapid-off sawtooth flicker and to brief (4 ms) light pulses. All stimuli were either long wavelength (R), middle wavelength (G), or a combination of equal luminances of long and middle wavelengths (Y) presented on a short-wavelength, rod-saturating adapting field. Amplitudes of LNRs were obtained in response to rapid-on (LNR(on)) and rapid-off (LNR(off)) sawtooth flicker and were also derived from the sum of the ERG waveforms to the two sawtooth phases (LNR(add)). RESULTS: For the control subjects, PhNR(pulse) amplitude varied with stimulus wavelength, being largest in response to a long-wavelength pulse, as expected. However, the amplitudes of LNR(on), LNR(off), and LNR(add) were not significantly different for R, Y, and G sawtooth flicker. Despite the absence of a chromatic effect, LNR(off) and LNR(add) amplitudes were significantly smaller in the OA patients than in the controls, similar to the results for the PhNR(pulse), implying an inner retinal origin for the LNR(off) and LNR(add). However, LNR(on) amplitudes did not differ significantly between the OA patients and controls, although there was a significant correlation between the LNR(on) and PhNR(pulse) for R stimuli. CONCLUSION: We conclude that LNR(off) and LNR(add) but not LNR(on) can be useful measures to assess the integrity of the inner retina that can complement the PhNR(pulse).