Visual perception of planar-rotated 2D objects in goldfish (Carassius auratus).

This study examined the ability of goldfish to visually identify 2D objects rotated in the picture plane. This ability would be adaptive for fish since they move in three dimensions and frequently view objects from different orientations. Goldfish performed a two-alternative forced choice task in which they were trained to discriminate between two objects at 0°, then tested with novel aspect angles (+/- 45°, +/- 90°, +/- 135°, 180°). Stimuli consisted of an arrow and half circle (Experiment 1) and line drawings of a turtle and frog (Experiments 2 and 3). In the first two experiments, the S+ and S- were presented at the same aspect angle. Performance in these experiments exceeded chance on four of seven novel aspect angles. Overall accuracy was not significantly different with complex stimuli (animal drawings) vs. simple stimuli (geometric shapes). In Experiment 3, when fish were tested with the S+ at varying aspect angles and the S- always presented at 0°, the fish failed to discriminate among the stimuli at all but one aspect angle. These goldfish viewing planar-rotated 2D objects did not display viewpoint-invariant performance, nor did they show a systematic decrement in performance as a function of aspect angle.