Effect of Experimentally Induced Anisometropia on Binocular Vision Studied by Pattern Reversal Visual Evoked Potentials

Optimal binocular vision is associated with depth perception and enhancement of certain other aspects of vision. Binocular interaction can, however, be compromised when one of the monocular inputs is altered in conditions like anisometropia. We aimed to evaluate the effects of interocular refractive differences, electrophysiologically, by recording transient pattern reversal visual evoked potentials in normal and induced anisometropic conditions. Transient PRVEPs (pattern reversal visual evoked potentials) were recorded in 50 adults in the age group of 18-25 years. N75-P100 amplitude and P100 latency were studied in normal monocular, normal binocular conditions and after inducing simple myopic anisometropia (0.5 to 5 dioptres). Mean binocular amplitudes were compared in normal and anisometropic states using paired t-test. Binocular summation ratio was calculated for normal refractive states and anisometropia. Mean binocular N75-P100 amplitude decreased significantly (p<0.0001) in anisometropia compared to that in normal refraction, while P100 latency change was not statistically significant. Anisometropia of more than 2 dioptres reduced the binocular amplitude highly significantly (p<0.0001). Binocular summation ratio was 1.18±0.2 in normal refraction, while in anisometropic binocular conditions, it was 0.94±0.1. Anisometropia interferes with normal binocular interaction. Transient PRVEPs seem to be reliable electrophysiological measure to assess binocular interactions objectively. The study also emphasizes the importance of using a binocular approach in the treatment of anisometropic amblyopia.