Exploring the Jastrow Illusion in Humans (Homo sapiens), Rhesus Monkeys (Macaca mulatta), and Capuchin Monkeys (Sapajus apella)

Abstract

In the Jastrow size illusion, two vertically stacked but offset stimuli of identical size are misperceived such that the bottom stimulus is overestimated relative to the top stimulus due to their spatial layout. In this study, we explored whether nonhuman primates perceive this geometric illusion in the same manner as humans. Human adults, rhesus macaques, and capuchin monkeys were presented with a computerized size discrimination task including Jastrow illusion probe trials. Consistent with previous results, humans perceived the illusory stimuli, validating the current experimental approach. Adults selected the bottom figure as larger in illusion trials with identical shapes, and performance was facilitated in trials with a true size difference when the larger figure was positioned at bottom. Monkeys performed very well in trials with a true size difference including difficult discriminations (5% difference in stimuli size), but they did not show evidence of the Jastrow illusion. They were indifferent between top and bottom stimuli in the illusory arrangement, showing no evidence of a human-like (or reversed) bias. These results are considered in light of differences in perceptual processing across primates and in comparison to previous comparative studies of the Jastrow and other size illusions.

Assessing the perception of face pareidolia in children (Homo sapiens), rhesus monkeys (Macaca mulatta), and capuchin monkeys (Sapajus apella).

“…Alternatively, this result could demonstrate a bias against selecting illusory faces, suggesting that monkeys recognize something unique about these images that differs from nonface objects. However, given that monkeys have shown biases toward local features in past research (Agrillo et al, 2019;Hopkins & Washburn, 2002;Spinozzi et al, 2003), we think the former explanation is more likely.…”

Size Illusion

“…Then, the Jastrowlike illusory pair of stimuli were presented on some trials, and the chimpanzees selected the top stimulus more often than expected by chance, suggesting that they perceived the top stimulus as the narrower option. Finally, Agrillo et al (2019) presented the same computerized task to human adults, rhesus macaques, and capuchin monkeys. All species learned to discriminate shapes based on size, and when presented with Jastrow illusion trials, humans perceived the illusory stimuli as predicted.…”

Dogs (Canis lupus familiaris) are susceptible to the Kanizsa’s triangle illusion

“…Discussion of visual illusions dates back to Aristotle and nowadays a variety of illusions tackling different perceptual mechanisms have been scientifically investigated (Eagleman 2001 ). For instance, recent literature provides experimental evidence about visual illusions eliciting misperception of relative size (Agrillo et al 2019 ; Doherty et al 2010 ), quantity (Agrillo et al 2016 ; Kirjakovski and Matsumoto 2016 ), colour (Rizzi and Bonanomi 2012 ; Schlaffke et al 2015 ), brightness (Blakeslee and McCourt 2012 ) and motion (Ashida et al 2012 ; Kanazawa et al 2013 ). Most of the studies in this field have concentrated on humans, but it has been suggested that some of the underlying mechanisms of illusory perception might be shared across species (Feng et al 2017 ).…”