Objective: The guinea pig becomes an important model for studies on myopia, but little is known about its visual performance. In this study, grating acuity was measured using a custom-built automated device to track optomotor responses.
Methods: To record head nystagmus, guinea pigs were individually placed in the center of a rotating drum of 130 cm diameter. The drum was covered inside with square wave gratings of adjustable fundamental spatial frequencies and contrast. The turning movements of the head were tracked using custom-written video software that detected two little white spots painted on a small black piece of cardboard that was attached to the guinea pig's head. Angular head speed was determined from the positions of the two white spots with respect to each other over time, and the ratio of angular head speed to drum speed was determined (the "gain"). In 11 guinea pigs of the same age, but with different refractive states (+9.7 to -15.0 D), responses to spatial frequencies of 0.6 and 2.4 cyc/deg were tested. Furthermore, 17 guinea pigs were tested which had similar refractive states but were different in age (1 to 3 months old). Finally, the effects of different grating contrasts were studied (25%, 50% and 100% contrast respectively) and the effects of different stripe luminances (10, 30, and 350 cd/m(2) respectively).
Results: The optomotor response could be used to measure vision in one eye only even if both eyes opened. The optomotor gain was affected by refractive error. Younger animals (one month old) had lower optomotor gain than older ones (0.61 ± 0.2 in one-month, 0.77 ± 0.13 in two-month and 0.80 ± 0.11 in three-month old). For a spatial frequency of 0.6 cyc/deg, the effects of stripe contrast were tested in two months old guinea pigs. At an average stripe luminance of 30 cd/m(2), the optomotor gain dropped from 0.95 ± 0.20 at maximal contrast, to 0.94 ± 0.16 at 50% contrast, and 0.70 ± 0.10 at 25% contrast. At three different luminances, gains were 0.81 ± 0.25 (10 cd/m(2)), 0.95 ± 0.20 (30 cd/m(2)), and 0.80 ± 0.09 (350 cd/m(2)), measured with gratings of 100% contrast, respectively.
Conclusions: Myopic guinea pigs have reduced grating acuity compared to hyperopic ones. The optomotor gain increases with age. For a given grating contrast, the highest optomotor gains are obtained at the highest luminance of 30 cd/m(2). In summary, spatial vision in guinea pig declines with the magnitude of myopia, and increases with luminance and age.