The material-size illusion - The influence of material properties on haptic perception of volume
Haptic matching of the orientation of bars separated by a horizontal distance leads to large systematic deviations (eg Kappers and Koenderink, 1999 Perception 28 781–795). A bar on the right side has to be rotated clockwise in order to be perceived as parallel to a bar at the left side. This finding leads to the following intriguing question which we investigated in this study: Will a bar moving from left to right in a fixed orientation be perceived as rotating counterclockwise? Blindfolded subjects had to touch a bar that moved from left to right or from right to left while it was rotating clockwise or counterclockwise with di erent speeds or did not rotate. For each trial they had to decide whether the rotation was clockwise or counterclockwise. From psychometric curves fitted to the data, we could determine that the results were consistent with the findings in the static case: A bar moving from left to right has to rotate clockwise in order to be perceived as non-rotating (and vice versa). In other words, a translating bar causes the illusory perception of a rotation.