Results show that with either method, the PERG amplitude was largest in younger controls and was relatively smaller in older controls and in similarly aged patients with glaucoma and NAION, in agreement with previous reports (e.g., Refs.
16 and
28). However, the magnitude of the PERGx signal was systematically higher (1.39×) than that of the PERGLA signal across groups. With either method, the magnitude of the noise was independent from age of controls or presence of disease. With the PERGx method, however, the magnitude of the noise was lower (0.37×) than that of the PERGLA. A consequence of the larger amplitude and lower noise of the PERGx compared to PERGLA was that the SNR was remarkably higher (4.2×) in the former. Statistical analysis of amplitude and noise in individual eyes of NAION patients showed that in three of five eyes the amplitude was not significantly different from noise with the PERGLA method while it was well above the noise level (
P < 0.0001) with the PERGx method in five of five eyes. The expanded dynamic range of the PERGx method thus offers the opportunity to record robust responses in advanced stages of optic nerve diseases. This represents an important expansion of the PERG technique with skin electrodes
16 and of the PERG technique in general, which so far has been considered useful primarily for monitoring initial stages of disease due to its limited dynamic range.
15,36,37 Likely explanations for the larger amplitude of the PERGx method, compared to PERGLA, are that the pattern stimulus was slightly larger in the former (square with 25° side versus circle with 25° diameter) and had higher luminance (800 vs. 50 cd/m
2).
30 In addition, with the PERGx method, the LED display generated synchronous contrast-reversals over the entire pattern stimulus, whereas with the CRT display contrast-reversal occurred in a sweeping manner.
17 Asynchrony in activation of contrast-driven PERG generators can affect the strength of their response, as well as smear it in time and delaying the time-to-peak. A likely explanation for the lower noise of the PERGx method compared to PERGLA is that the number of averaged epochs was higher in the former (1024 epochs versus 600 epochs). Results also show that the PERGx latency was on average shorter than that of PERGLA by approximately 16 ms. The delay of the PERGLA response compared to PERGx may be due to temporal differences in the onset of pattern reversal
8 between LED and CRT displays, sequentially delayed retinal outputs in response to raster-generated patterns
38 compared to instantaneous retinal output in response to LED-generated patterns, and responses of longer latency for CRT display with relatively lower luminance than LED display.
30