Optokinetic Test to Evaluate Visual Acuity of Each Eye Independently

Abstract

Abstract: : Purpose: To compare visual acuity results in rats tested with the optokinetic testing apparatus of Coffey et al., with a device modified to measure vision in each eye separately. Methods: The original Coffey apparatus allows free movement of the animal. Both eyes are exposed to the optokinetic stimulus with the presumption that each eye responds primarily to movement in one direction only. However, contribution of both eyes to the response cannot be ruled out. The modified apparatus consists also of a rotating drum with stripes of 3 different spatial frequencies (0.25, 0.5, and 1 cycles/degree). Half of the rotating drum is evenly illuminated from the outside. The other half of the drum is rotating behind a stationary black wall opposite from where the light comes. The rat sits unrestrained in a transparent tube in the center and is initially prevented from climbing out by an electrical shock. Thus, one eye is unexposed to the rotating stripes. Normal pigmented rats and transgenic pigmented s334ter-3 rats were tested, 2 min. per eye, 1 min. in each direction, and the amount of time (sec) spent head tracking was measured. The modified method was compared with the original Coffey method. Results: In normal rats, when testing with the modified method at a spatial frequency of 0.25 cycles/degree, the duration of head tracking for the left eye was 42.3±3.4 with the drum rotating in the temporal-nasal direction and 11.6±2.6 for rotation in the opposite direction. Values for the right eye were 38.0±2.6 and 12.3±2.8, respectively. Using the Coffey method, the duration of tracking was 33.0±3.4 (clockwise rotation) vs 26.0±3.6 (anti-clockwise). In s334ter-3 rats at the age of 205 days, the tracking duration for the left eye was 11.5±1.8 (temporal-nasal direction) and 1.0±0.7 (nasal-temporal direction) and for the right eye 12.2±1.2 vs 0.5±0.2. When the same rats were tested using the Coffey method, the duration of tracking was 20.2±2.1 vs 16.3±2.9. Conclusions: In both retinal degenerate and normal pigmented rats, some head tracking is observed in both directions of the drum rotation, although the response is larger in the preferred direction (according to the modified method). In degenerate animals, the score tended to be higher with the original method, possibly due to fellow eye stimulation. Contributions from the fellow eye to the optokinetic tracking response can be limited by this testing modification, thereby allowing monocular visual behavioral testing in rats. Supported by: Foundation Fighting Blindness; Foundation for Retinal Research; Fletcher Jones Foundation and Anonymous Sponsor.

Keywords: visual acuity • retinal degenerations: hereditary • animal model