Although pattern onset stimulation from a gray background displays less temporal contrast than reversal stimulation,
16 we were able to produce consistent onset PERGs in our cohort. These waveforms were akin to those obtained from clinical reversal stimulation when stimulation occurred from a gray background (
Fig. 4B, first part of figure). The standard ISCEV PERG waveform is identified by three main components: N35, P50, and N95. When patterns appeared from a black background (light increment), a pronounced negativity with a timing matching the N35 could be observed immediately before the main positive peak (P50). Upon light decrement stimulation (pattern appearing from a white background), this negativity was not evident in any of the subjects, but a second prominent positive peak emerged at approximately 250 ms post stimulus, which most likely represents the stimulus offset response (
Fig. 4B). Further, when averaging light increment and decrement responses together, the resulting waveform is almost identical to the one obtained from clinical pattern onset stimulation to a gray background (
Fig. 4B, purple trace). An overview of all traces is given in
Figure 5.
The amplitudes of the N35, P50, and N95 components were statistically different between the light increment, decrement, as well as gray background conditions with the light increment conditions giving consistently the biggest and best-defined responses (
Fig. 6A). Responses elicited from light decrement stimulation were consistently smallest for these components, with the responses elicited to the gray background condition being of intermediate amplitude. This picture was reversed when looking at the offset response, where the light decrement condition provoked the biggest responses and the light increment condition the smallest (1-way ANOVA with repeated measures and post hoc Bonferroni means comparison: P50: increment > decrement
P = 0.001, increment > gray
P = 0.0055; N95: increment > decrement
P < 0.001, increment > gray
P < 0.001; offset: decrement > increment
P = 0.0108, offset decrement > gray
P = 0.0418). When times to peak were compared across conditions and components, no statistically significant difference was found (
Fig. 6B).
Upon closer component analysis of the onset PERG waveforms obtained in our study group, we found that onset response (P50) amplitudes to 60′ checks across all conditions were constantly bigger than offset response amplitudes. Similar results were previously reported by Arden and Vaegan and others.
17–19 This difference was not found to be significant when patterns arose from gray or white backgrounds (light decrement); however, in response to a light increment, onset amplitudes were significantly higher than the corresponding offset response (
P < 0.001,
Fig. 7).