Comparison of length judgments and the Müller-Lyer illusion in monkeys and humans

Visuo-spatial magnitude judgements are abstract in that they are detached from the specific sensory parameters on which they are based. Nevertheless, the visual system is actively reconstructing and interpreting the outside world, which sometimes causes reproducible geometric illusions. Here, we investigated the visual length perception of rhesus macaques, an Old World monkey species, in a delayed match-to-sample task and compared the non-human primates' performance to the length judgment of human participants under identical conditions. The quantitative analysis of the length discrimination shows that humans and macaques both show a distance and size effect in judging length and have almost identical length judgment characteristics as determined by the widths of the discrimination functions and the Weber fractions. Moreover, both monkeys and humans were subject to the geometric Müller-Lyer illusion caused by inward or outward pointing 'arrows' at the ends of a line, resulting in over- or underestimation of length, respectively. The strength of the illusion effects (i.e., the magnitude of length misjudgement for stimuli with inward and outward pointing arrows at the end of the lines) was in the range between 1.17 and 1.57° of visual angle for both monkeys and the human participants, and thus very similar between the two primate species. Our results suggest that the visuo-spatial mechanisms underlying simple horizontal line-length perceptions in the human and macaque monkey are qualitatively and quantitatively similar, offering the possibility to investigate the neural correlates of geometric illusions in the monkey and to translate the findings to the human visual system.