Attenuating the haptic horizontal—vertical curvature illusion

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.

Touching curvature and feeling size: a contrast illusion

We know that our eyes can be deceiving. Here we demonstrate that we should not always trust our sense of touch either. Previous studies have shown that when pinching an object between thumb and index finger, we can under many circumstances accurately perceive its size. In contrast, the current results show that the local curvature at the areas of contact between the object and the fingers causes systematic under- or overestimation of the object's size. This is rather surprising given that local curvature is not directly related to the object's size. We suggest an explanation in terms of a contrast between the finger separation and an inferred relationship between local curvature and size. This study provides the first demonstration of an illusory haptic size percept caused by local curvature in a pinch grip.

Haptic distal spatial perception mediated by strings: size at a distance and egocentric localization based on ellipse geometry

Participants explored ellipse perimeters defined by fixed-length strings, held taut by their moving finger, with ends attached to two fixed hooks (foci). Participants haptically judged ellipse interfocal distance (IFD; in effect, exocentric separation, or size at a distance) or location of ellipse major axes (i.e., egocentric localization relative to the moving finger). In Experiment 1A, perceivers made reliable and accurate multialternative forced choice IFD judgments. Experiment 1B showed similar reliability for direct estimations but showed greater scaling error. In Experiment 2, perceivers reliably localized ellipse major axes. Both experiments derived from a priori geometrical analysis, consistent with the Gibsonian perceptual research program and previous string-mediated haptic distal spatial studies. Results are discussed with respect to haptic perception as a telemodality and to dynamic touch.