As shown in Fig.
1, the PhNR amplitude can be measured from baseline to the minimum point in the trough (BT). It also can be measured from the peak of the
b-wave to the maximum amplitude in trough (PT). Alternatively, PhNR amplitude can be measured at a fixed time, for example, at 65–75 ms after the flash in the trough of the response (not shown). Using a fixed time could be helpful when responses in diseased eyes are small and the trough is difficult to locate. Note that the PT measurement is largely dominated by the
b-wave amplitude, and a change in
b-wave amplitude reflecting a change in bipolar cell function must be considered when interpreting a change in PhNR amplitude. When measuring the PhNR, it may also be necessary to take account of the
i-wave, or
i-waves, positive deflection(s) of Off pathway origin [
11] in the falling limb of the
b-wave, and/or later in the trough (Fig.
1). For responses to the suggested narrowband stimuli, such as those used for responses in Fig.
1, the maximum trough amplitude generally occurs after the initial
i-wave. Given the slow nature of the response, and the variety of amplitude criteria that have been used, peak time of the PhNR is generally not reported. The PhNR is moderately affected by age, so, for the particular measure(s) chosen, appropriate age-matched normative data should be used [
3,
22]. Comparisons of longitudinal findings in patients to normal test-retest repeatability of PhNR amplitudes are also important, as the test–retest variability of PhNR amplitudes can be greater than that of
a- and
b-waves [
21‐
24].