McKeon, J. W., Parrish, A. E., & Beran, M. J. (2024). Object-based warping during distance discriminations by rhesus monkeys (Macaca mulatta) and capuchin monkeys (Sapajus apella). Animal Behavior and Cognition, 11(4), 349-360.  https://doi.org/10.26451/abc.11.04.02.2024

Humans and nonhuman animals misperceive the world. However, they do not always share the same misperceptions. Vickery and Chun (2010) reported a visual illusion, object-based warping, wherein distances between stimuli contained within an object were perceived by adult humans as larger than distances presented against a plain background. Rhesus monkeys and capuchin monkeys were tested on their susceptibility to this illusion. After being trained to choose the pair of dots with the larger distance between them, they were tested on a variety of conditions to assess illusion susceptibility. The results showed that there were no species differences in performance, but there was an effect of how dot pairs were presented. The Congruent condition (only the pair of dots with a longer distance between them was within an object) was performed significantly better than the Baseline condition (both pairs of dots were either within objects or on the plain background) and the Incongruent condition (the pair of dots with the smaller distance between them was within an object, and the other pair was on a plain background). Performance in the Baseline condition was also significantly higher than that in the Incongruent condition. Perhaps most compelling was that on trials where the two dot pairs had equal distances between dots, but one pair was presented within an object and the other was not, all monkeys except one showed a significant bias towards the object-contained array. These results indicate that this illusion is not exclusive to humans, and that the attentional and visual processes that make humans susceptible to the illusion are present in at least some other nonhuman primate species.

Perception, Illusions, Rhesus monkeys, Capuchin monkeys, Distance discrimination