Improving reproducibility of VEP recording in rats: electrodes, stimulus source and peak analysis.

The aims of this study were to evaluate and improve the reproducibility of visual evoked potential (VEP) measurement in rats and to develop a mini-Ganzfeld stimulator for rat VEP recording. VEPs of Sprague-Dawley rats were recorded on one randomly selected eye on three separate days within a week, and the recordings were repeated three times on the first day to evaluate the intrasession repeatability and intersession reproducibility. The VEPs were recorded with subdermal needle and implanted skull screw electrodes, respectively, to evaluate the effect of electrode configuration on VEP reproducibility. We also designed a mini-Ganzfeld stimulator for rats, which provided better eye isolation than the conventional visual stimuli such as flash strobes and large Ganzfeld systems. The VEP responses from mini-Ganzfeld were compared with PS33-PLUS photic strobe and single light-emitting diode (LED). The latencies of P1, N1, P2, N2, and P3 and the amplitude of each component were measured and analysed. Intrasession and intersession within-subject standard deviations (Sw), coefficient of variation, repeatability (R95) and intraclass correlation coefficient (ICC) were calculated. The VEPs recorded using the implanted skull electrodes showed significantly larger amplitude and higher reproducibility compared to the needle electrodes (P<0.05). The mini-Ganzfeld stimulator showed superior repeatability and reproducibility in VEP recording. The intra/intersession ICCs of latency were 0.85/0.70 for mini-Ganzfeld, 0.72/0.62 for PS33-PLUS and only 0.59/0.42 for single LED. The latencies of the early peaks (N1 and P2) demonstrated better reproducibility than the later waves. The mean intrasession and intersession ICCs were 0.96 and 0.86 for the early peaks. Using a combination of skull screw electrodes, mini-Ganzfeld stimulator and early peak analysis, we achieved a high reproducibility in the rat VEP measurement. The latencies of the early peaks of rat VEPs were more consistent, which may be due to their generation in the primary visual cortex via the retino-geniculate fibres.