Abstract
Abstract: :
Purpose: To document and examine an order effect found when sequentially recording from both eyes in a single session. Methods: Two mfERGs were recorded from 103 patches within the central 45 degrees of each eye in 12 normal subjects between the ages of 22–57 years. The first eye was recorded while the other was occluded for about 30 min, then the second eye was recorded after adapting to the room light for about 15 min. First order mfERG amplitudes and implicit times were measured using a template stretching method (Hood & Li, 1997). Scalar products (0–80 ms post–flash epoch) and RMS noise values (120–200 ms epoch) were calculated by the VERIS 4 software. Percentage differences (difference/max value) between the first and second eye and correlations between the two eyes were analyzed across the 103 locations and within 5 concentric rings around the fovea. 5 of the 12 subjects were tested a second time, in a different session, in reversed eye order to examine the repeatability of the order effect. Results: The correlation between eyes for amplitude (R2 = 0.62) and implicit time (R2 = 0.77) were both significant (P < 0.001). Despite the correlation, the first eye’s amplitude was larger than the second in 10 out of the 12 subjects. The mean amplitude, of all 12 subjects, recorded from the first eye (0.20 ± 0.02 µV) was significantly larger (P < 0.01) from that of the second eye (0.17 ± 0.01 µV), on average by 15 ± 4%. The scalar products followed the same behavior as the amplitudes. Analysis of rings showed that the amplitude order effect (∼15%) was consistent at all tested eccentricities. Within the repeated subjects, the amplitude order effect in the first session was ∼12% and was ∼10% in the second session. There was a consistent difference in implicit time between the first eye (28.64 ± 0.23 ms) and second eye (28.37 ± 0.25 ms) recording (P < 0.01), with the first eye being 1 ± 0.2% longer than the second. RMS noise did not differ significantly between first and second eyes (P = 0.42). Conclusions: The order effect in implicit time was statistically significant but the 1% difference suggests that it is not clinically important. However, the amplitude order effect, about 15%, may be important to consider when interpreting sequential mfERG recordings. Since the order effect occurred to the same degree in the foveal and peripheral regions, differences in fixation stability between the sequential recordings do not appear to account for the effect. Fatigue and/or recording quality also do not appear to account for the order effect, since noise levels in the sequential recordings did not differ. Light adaptation may play a role in the order effect.
Keywords: electroretinography: clinical • electroretinography: non-clinical • electrophysiology: clinical