What contributes to our perception of the Shepard Tabletop Illusion?

A vertical parallelogram placed next to an identical one but rotated horizontally by 90 degrees gives rise to one of the most powerful geometrical illusions: the Shepard Tabletop Illusion. In this illusion, the two parallelograms, or tabletops, are perceived as having different aspect ratios: the vertical one appears longer and thinner than the horizontal one. Although the exact mechanisms responsible for the illusion remain unclear, it is commonly understood that the brain treats the vertical tabletop as a three-dimensional object receding into the distance, and the horizontal one as an object closer in view. Here, we investigated the role of depth cues, texture, shape constancy, mental rotation, and autistic-like traits in the illusion. Using the method of adjustment, we measured susceptibility to four variants of the illusion: (i) plain parallelograms; (ii) plain tables; (iii) wood-like parallelograms; (iv) wood-like tables. We also measured participants’ shape constancy and mental rotation abilities, along with autistic-like traits. The results revealed that illusion strength increased with the inclusion of additional depth cues but decreased when wood-grain texture was added to the table configurations. Additionally, illusion strength correlated with mental rotation but not with shape constancy abilities. Also, there were instances in which illusion strength decreased with an increase in autistic-like traits. These findings demonstrate that the Shepard Tabletop Illusion cannot be explained solely by theories of depth perception. Processes related to the analysis of low-level visual features and mental rotation, as well as individual differences in autistic-like traits, seem to play a role as well.