The horizontal-vertical curvature illusion in touch is present in three-dimensional objects and raised lines

The present study compared the strength of the horizontal-vertical illusion in blindfolded sighted people with raised-line curves and three-dimensional (3-D) objects. Although the horizontal-vertical curvature illusion has been reported with raised-line stimuli in both vision and touch, it was not known whether similar haptic distortion would be found with 3-D objects. Similar overestimation of verticals was found with both types of stimuli in Experiment 1. Experiment 2 used bimanual unrestricted exploration at the body midline and stimuli horizontal on the table surface or in the frontal plane. In Experiment 2, illusion strength was substantially stronger when the stimuli were frontal and diminished overall for the horizontal group. The horizontal-vertical illusion was strong with optimal methods of presentation (free bimanual exploration) in Experiment 2, even where radial-tangential scanning could not be a causal factor in the frontal group. The results suggest that illusory distortion in haptics is not the result of the use of raised lines, and these patterns can be effective surrogates for 3-D objects.