Rotated object recognition in four domestic horses (Equus caballus)

Horses were tested for recognition of objects under various rotations to better understand how they perceive stimuli. Four horses learned to discriminate positive and negative stimuli for three sets of “real-life” three-dimensional objects always presented in one particular orientation (front to left, top up). When the horses reliably performed at an above chance criterion of 80% correct responses for two consecutive runs of 20 trials (P = .0046), the objects were rotated in depth and/or turned upside down, for example, front to right, top to horse; back to horse, bottom up. Overall performance on rotated trials, as well as Trial 1 scores on novel presentations, indicated that the horses were capable of recognizing objects under all rotational conditions. However, there was a degree of individual variation, with some horses performing better than others on certain rotations. Moreover, objects rotated in certain positions were more easily recognized than others. The most significant effect was seen when rotations with the top of the objects visible were compared with those with the bottom visible—the former being more recognizable than the latter. This suggests that the horses were using certain features of the objects on which to base their decisions. These findings provide new information on object perception in horses and address one common belief about why horses startle at objects that should be familiar to them.