A new study found that monkeys and humans have similar perceptions when visual illusions are shown to them.

Investigating visual illusions is an important factor in comprehending common visual perceptions. Researchers from Georgia State University wanted to find out if two species of monkeys namely rhesus (Macaca mulatta) and capuchin (Cebus apella) have the same perceptions of the Delboeuf illusion with Homo sapiens or human adults.

The team conducted the study by subjecting both the human and monkey participants to computer-based experiments. The experts provided the human subjects with a personal computer, a mouse and a monitor to complete the investigation. The monkeys, on the other hand, were given a personal computer, a monitor and a digital joystick. Prior to the experiment, the monkeys were trained to use the joysticks.

A reward system for correct answers was established for both groups. Humans were given written feedback while monkeys were rewarded with food pellets that tasted like bananas.

The experiment, published in the Journal of Experimental Psychology: Animal Learning and Cognition, had two parts. For the first part, both humans and monkeys were presented with images of central dots that were sometimes encircled in rings. The participants were then asked to discriminate between the two dots and identify which is larger. As expected by the researchers, humans exhibited the Delboeuf illusion as they overestimated the size of the dots encircled in small rings and underestimated those that were surrounded by large rings. The monkeys, however, did not demonstrate the same perception.

To investigate further, the researchers performed experiment number 2, which involved asking both groups to classify whether central dots were "small" or "large." The researchers presented several dots encircled in different ring sizes to detect if Delboeuf illusion would still manifest in any of the dot-ring ratios shown. After the experiment, the Delboeuf illusion were found to be present in humans and in the two species of monkeys, as all of them underestimated the size of the central dots as the rings become larger.

"These results, along with others, show that humans and monkeys share similarities in their perceptual systems," commented Audrey Parrish, one of the study authors. Both species had similar perceptions and misperceptions of some physical stimuli types. She added that even though this study does not suggest that both species can see the world in the exact same way, this may still demonstrate that monkeys may be good models in studying how human perception works and that contextual cues have similar impacts across different species.

In the end, the authors concluded that the Delboeuf illusion has been extended to rhesus and capuchin monkeys. They also said that the comparative experiments they have conducted have paved the way for more improved evaluations of hypotheses pertaining to illusion perception of nonhumans.

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