Haptic Shape Constancy Across Distance

To explore haptic shape constancy across distance, we measured perceived curvature thresholds of cylindrical shapes, cut out of acetal resin blocks. On each trial, blindfolded observers used their bare finger to scan the surface of two of the shapes consecutively. One shape was close to the observer and the other positioned further away. This spatial displacement changes the available proprioceptive information about the object shape, and therefore the combined proprio-tactile information may signal different objects at the two distances. The results reveal a perceptual compensation for the change in proprioceptive information. However, two distinct patterns of distance compensation emerged: one groups’ data are consistent with predictions from visual object constancy. The other group of observers demonstrate the reverse pattern of response such that objects further away need to have lower curvature to be perceived having equal curvature. We propose that perceived haptic curvature across distance depends on observers’ differential weighting of the multiple available cues.