Refining Flash Visual Evoked Potential Analysis in Rats: A Novel Approach Using Bilateral Epidural Electrodes

Visual evoked potentials (VEPs) are electrical signals generated at the visual cortex following visual stimulation. Flash VEPs (fVEPs) are produced by global retinal stimulation and are considered an objective measure of the integrity of the entire visual pathway. However, fVEP measurements are highly sensitive to external variables, making relative comparisons of the fVEP waveforms between the two eyes in the same individual challenging. We used the rodent non-arteritic anterior ischemic optic neuropathy (rNAION) model to induce unilateral ischemic optic neuropathy. The severity of optic disc edema was measured with spectral-domain optical coherence tomography, and visual acuity was measured using a virtual optokinetic system. We developed a procedure utilizing implanted bilateral epidural electrodes and derived a mathematical formula to accurately estimate functional differences between the optic nerves. Immunohistology was performed to quantify retinal ganglion cell (RGC) survival using stereology. Compared to subcutaneous methods, the new approach significantly improves the signal-to-noise ratio and is more repeatable when comparing the two eyes. The derived formula accounts for asymmetry in the afferent inputs to the visual cortex. Visual function calculated using the formula correlates strongly with other recognized metrics of visual function, including RGC survival and visual acuity. We have developed a repeatable and accurate method to calculate the relative visual function of diseased optic nerves compared with a contralateral control eye. Our novel method improves fVEP measurement sensitivity and accuracy in rodent preclinical trials, reducing the number of animals needed to achieve statistical significance.