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.

Attenuating the haptic horizontal-vertical curvature illusion.

In a number of experiments, blindfolded subjects traced convex curves whose verticals were equal to their horizontal extent at the base. Overestimation of verticals, as compared with horizontals, was found, indicating the presence of a horizontal-vertical illusion with haptic curves, as well as with visible curves. Experiment 1 showed that the illusion occurred with stimuli in the frontal plane and with stimuli that were flat on the table surface in vision and touch. In the second experiment, the stimuli were rotated, and differences between vision and touch were revealed, with a stronger illusion in touch. The haptic horizontal-vertical illusion was virtually eliminated when the stimuli were bimanually touched using free exploration at the body midline, but a strong illusion was obtained when curves were felt with two index fingers or with a single hand at the midline. Bimanual exploration eliminated the illusion for smaller 2.5- through 10.2-cm stimuli, but a weakened illusion remained for the largest 12.7-cm patterns. The illusion was present when the stimuli were bimanually explored in the left and right hemispace. Thus, the benefits of bimanual exploration derived from the use of the two hands at the body midline combined with free exploration, rather than from bimanual free exploration per se. The results indicate the importance of haptic exploration at the body midline, where the body can serve as a familiar reference metric for size judgments. Alternative interpretations of the results are discussed, including the impact of movement-based heuristics as a causal factor for the illusion. It was suggested that tracing the curve's peak served to bisect the curve in haptics, because of the change in direction.

Sex differences in the haptic change task.

Three experiments examined gender differences in picture location memory in the sense of touch involving the change task. In Experiments 1 and 2, blindfolded sighted participants felt 15 tangible raised-line pictures and memorized their locations for 8 min. Subsequently, they felt another set of raised-line pictures that were identical to the original set, but locations were exchanged for 6 of them. The change task required participants to indicate which raised-line pictures were moved to new locations and which were not moved. It was expected that females would show superior picture location memory compared with males. The results showed that females had a significantly higher number of correct picture location judgments than males in Experiment 1. No differences appeared between males and females on the change task when the task was much more difficult and the raised-line pictures were irregularly arrayed in the second experiment. Task difficulty was too high in Experiment 2, and a third experiment reduced the number of pictures in the irregular array to 12. Females performed better than males in Experiment 3. Gender differences in the change task are small in magnitude in touch as well as in vision. The results of the present experiments using touch are consistent with the larger literature in vision.