PhNR amplitude was measured from the baseline to the PhNR trough. In both control subjects and MS patients, the largest PhNR amplitudes on average occurred at 65 and 70 ms after the flash (
Fig. 2). Given this result, we elected to measure PhNR amplitude at 65 ms, a time also used in a previous study from this laboratory.
26 In
Figure 3A, the PhNR amplitude is plotted as a function of the flash strength in the control, no-ON, ON>6, and ON<6 eyes. PhNR amplitude increased as flash strength increased and was at its maximum at the flash strength of 1.42 cd · s/m
2 for all groups. These stimulus response relations (for responses to all seven flashes tested) were significantly different (
F = 33.43;
P < 0.0001) across the four groups. Compared to the control subjects, amplitudes were lower in the ON>6 (
P < 0.0001), ON<6 (
P < 0.0001), and no-ON (
P < 0.0001) eyes. The ON>6 eyes were not significantly different from the ON<6 eyes (
P > 0.05), but the no-ON eyes had larger PhNR amplitudes than the ON>6 (
P < 0.001) and ON<6 (
P < 0.0001) eyes.
Figure 3B shows plots of the PhNR amplitude in response to a flash strength of 1.42 cd · s/m
2 in control, no-ON, ON>6, and ON<6 eyes. Responses to this stimulus will be used for comparisons with structural measures below. The differences between groups were similar to those for the whole stimulus response curves. Compared to control eyes (29.8 ± 6.5 μV [mean ± SD]), PhNR amplitude was significantly reduced in the no-ON (23.8 ± 9.3 μV;
P < 0.001), ON>6 (17.3 ± 7.6 μV;
P < 0.001), and ON<6 (16.0 ± 6.5 μV;
P < 0.0001) eyes. The no-ON eyes had larger PhNR amplitudes than the ON>6 (
P < 0.001) and ON<6 (
P < 0.001) eyes. PhNR amplitudes for the ON>6 versus the ON<6 eyes were not significantly different (
P > 0.05).