Studies on the visual acuity of sheep using shape discrimination learning

Two experiments were conducted to determine the visual acuity of sheep. Three ewes were trained to discriminate between a Landolt ring and an ordinary ring of equal size associated with feed. The left and right positions of the 2 targets were shifted according to the Gellermann series. After the ewes were fully trained, their visual acuity was determined by changing the size of the rings or the distance from the target. The ewes were subjected daily to 1 or 2 sessions which consisted of 30 trials each. The criterion of successful discrimination was 2 (Exp. I) or 3 (Exp. II) consecutive sessions with more than 21 correct choices (P<0.05, Chi-square test). The best scores of visual acuity in the 3 ewes were 0.085, 0.095 and 0.19, respectively.