Comparison of guinea pig electroretinograms measured with bipolar corneal and unipolar intravitreal electrodes

This study considers the precision and accuracy of bipolar corneal electrod es compared with unipolar intravitreal methods in collecting electroretinog raphic (ERG) recordings from a small animal. Flash ERGs were obtained from 9 adult guinea pigs on three occasions. Corneal bipolar (Burian-Allen) elec trodes were used to collect data on the first two occasions whereas unipola r intravitreal electrodes were used on the last. We identified the a-wave, b-wave, oscillatory potentials, P-III and P-II responses. Intensity-respons e functions were fit using a Naka-Rushton relationship with a bootstrap est imating the 95% confidence limits. Discrepancy analysis was applied to dete rmine the coefficient of agreement. We found significantly larger amplitude s with unipolar intravitreal electrodes (ANOVA; a-wave, p<0.002; b-wave, p< 0.001; Oscillatory potentials (OPs), p<0.005) especially at high intensitie s. Implicit times; showed little differences between electrodes for the a-w ave, significantly faster (p<0.03). b-waves at some intensities, and signif icantly slower (p<0.005) OP implicit times across all intensities. The PIII amplitude (log mu V), sensitivity and timing were not significantly differ ent (p>0.05) if expressed in logarithmic units but P-II amplitude (log mu V ) was significantly smaller with corneal electrodes. We suggest that a conv ersion factor (x1.35) should be applied to data collected with bipolar corn eal electrodes to estimate the amplitudes of the modelled parameters accura tely. The corneal electrode gave a precision of +/- 39 mu V which yields a statistical power of 0.90 for a sample size of 7 subjects. We conclude that bipolar corneal electrodes provide smaller electroretinogram amplitudes du e to their location and reduced span of the retinal generators.