Abstract
Nagasaka, Lazareva, and Wasserman (2007) reported that prior visual experience with geometric 2-D shapes affects pigeons' perception of partly occluded shapes (amodal completion). Here, we investigated the effect of experience on amodal completion in bonobos by adapting the methods used in the study of Nagasaka et al. Specifically, we explored amodal completion in bonobos by analyzing the errors they committed in the process of learning a 3-alternative simultaneous discrimination task. Bonobos were trained to discriminate among occluded, complete, and incomplete stimuli. The occluded stimulus comprised two colored shapes, one of which occluded the other (e.g., a blue circle occluded a red triangle); the complete and incomplete stimuli comprised the single shape that had been partially covered in the corresponding occluded stimulus (e.g., a whole triangle or a partial triangle, respectively). The correct response was to select the occluded stimulus; the dependent measure was the percentage of errors to the complete stimulus. Bonobos were trained until they reached criterion with one set of stimuli; novel sets of stimuli were repeatedly introduced. At the beginning of training, the percentage of errors to the complete stimuli was similar to the incomplete stimuli and did not differ from chance (50%). After repeated training with different shapes, the percentage of errors to the complete stimulus increased above chance. Moreover, when the bonobos were re-exposed to the initial set of training stimuli, they committed 90% of their errors to the complete stimulus. Background perspective cues on half of the stimuli did not influence discriminative performance. These results suggest that the bonobos came to view the complete stimulus as being more similar to the occluded stimulus than the incomplete stimulus. Extensive experience with two-dimensional images appears to facilitate amodal completion of partially occluded stimuli in bonobos, just as is the case in pigeons.