Sensitivity to geometric shape regularity emerges independently of vision.

In a visual intruder task, regular quadrilaterals such as squares and rectangles are easier to process than matched shapes devoid of parallelism, symmetry or right-angles. This geometric regularity effect was found in various human groups, including preschoolers and uneducated adults, but not in non-human primates. It was proposed to reflect a fundamental ability to combine discrete geometric features into structured representations of geometric shapes using an abstract amodal language-of-thought (LoT) that also supports the acquisition of symbolic drawing and formal mathematics. Here, we tested a prediction of this hypothesis: blind participants should have the same intuitions of geometric regularity as sighted ones. To evaluate this prediction, congenitally blind and sighted but blindfolded adults underwent a tactile version of the visual quadrilateral intruder task. Among six tactile shapes, five of which were identical up to small size and rotation changes, participants were asked to identify a deviant shape defined by a fixed displacement of a single vertex, and to rate their confidence in their response. Both variables revealed a geometric regularity effect in both groups, correlated with previous results in the visual domain. Furthermore, a symbolic LoT model was a better predictor of tactile performance than a visual CNN model in blind participants. Thus, the geometric regularity effect develops in the absence of vision.