Quantification of the Optokinetic Reflex Allows to Discriminate Visual Function of Wild-Type Mice From Rod-Deficient Mutant

Abstract

Purpose:: Evaluation of the optokinetic reflex (OR) is a simple manner to evaluate visual function in rodents. As retinal dystrophies can affect differently rods and cones, it would be advantageous to be able to monitor independantly the functional response of the different photoreceptor (PR) subtypes. Our goal is to establish a simple protocol to allow this discrimination by quantification of the OR. In this study, we evaluate visual acuity at different luminances, and we validate our system by the use of mice displaying different visual abilities.

Methods:: The experimental setup consists in a platform surounded by a vertical rotating drum (9 rotations per min). The wall of the drum is covered with black and white vertical stripes at various spatial frequencies. In a first step, the animals were pretrained to the task. Then each test session lasted 4 minutes, the rotation direction being changed at mid-session. Animals were tested with spatial frequencies increasing from 0,025 to 0,5 cycles per degree (cpd), and with different illumination conditions. Each animal was videotaped during the session, and the head movements were scored manually. A movement was counted when the animal followed the drum rotation at the same angular speed and in the same direction.

Results:: We first tested wild-type mice (C57Bl6) in photopic conditions (150 lux). We observed an OR of 13 ± 2 mov./min. for spatial frequencies ranging from 0,025 to 0,2 cpd. From 0,3 to 0,5 cpd, the OR dropped and gave values similar to control settings (white and black decors): 2 ± 1 mov./min. We next tested the OR under scotopic conditions (dim light < 0,01 lux), and observed the same response and visual acuity as in photopic conditions. We are currently evaluating the visual acuity in rod-deficient mice. Preliminary experiments indicate that this strain behaves similarly to C57Bl6 in photopic conditions, but has no OR in scotopic ones.

Conclusions:: These results suggest that the recruitment of rods and cones for visual function is highly correlated with the intensity of illumination. This is consistent with previous studies in mutants showing that visual acuity is governed by both rod and cone systems. This study further demonstrates that rod and cone functional responses can be studied independantly by analysis of the OR. This is of prime importance for the evaluation of retinal dystrophies in which PR subtypes degenerate at different rates.

Keywords: photoreceptors: visual performance • visual acuity