Measuring relative vibrotactile spatial acuity: effects of tactor type, anchor points and tactile anisotropy

Abstract

Vibrotactile displays can compensate for the loss of sensory function of people with permanent or temporary deficiencies in vision, hearing, or balance, and can augment the immersive experience in virtual environments for entertainment, or professional training. This wide range of potential applications highlights the need for research on the basic psychophysics of mechanisms underlying human vibrotactile perception. One key consideration when designing tactile displays is determining the minimal possible spacing between tactile motors (tactors), by empirically assessing the maximal throughput of the skin, or, in other words, vibrotactile spatial acuity. Notably, such estimates may vary by tactor type. We assessed vibrotactile spatial acuity in the lower thoracic region for three different tactor types, each mounted in a 4 × 4 array with center-to-center inter-tactor distances of 25 mm, 20 mm, and 10 mm. Seventeen participants performed a relative three-alternative forced-choice point localization task with successive tactor activation for both vertical and horizontal stimulus presentation. The results demonstrate that specific tactor characteristics (frequency, acceleration, contact area) significantly affect spatial acuity measurements, highlighting that the results of spatial acuity measurements may only apply to the specific tactors tested. Furthermore, our results reveal an anisotropy in vibrotactile perception, with higher spatial acuity for horizontal than for vertical stimulus presentation. The findings allow better understanding of vibrotactile spatial acuity and can be used for formulating guidelines for the design of tactile displays, such as regarding inter-tactor spacing, choice of tactor type, and direction of stimulus presentation.

Perception of Vibratory Direction on the Back

“…Using somewhat smaller distances, they showed that especially the upward oblique directions were perceived as horizontal. Also relevant here are the results of the study by Hoffmann et al [3] who found an anisotropy in horizontal and vertical acuity: their vertical distances were perceived as smaller than the horizontal distances. In our experiment, such an anisotropy would lead to oblique directions being perceived towards the horizontal and that is what we found for 5 out of the 8 oblique directions.…”

“…Other studies on vibrotactile perception also mention improved performance on or near the spine (e.g. [3,8]).…”

“…Weber [9] already found that vertical two-point pressure thresholds on the back are larger than the horizontal thresholds. Although pressure and vibration do not stimulate the same receptors, and thus Weber's observation on pressure thresholds does not necessarily apply to vibratory stimulation, the study by Hoffmann and colleagues [3] points in the same direction: they found that the accuracy of determining the direction of two subsequent vibration stimuli was higher for horizontal than for vertical directions.…”

Perception of vibrotactile distance on the back

“…Vibrotactile stimulation has been shown to be more accurately localised near the spine and navel than at other positions around the torso 9,10 , although reduced sensitivity has been found directly on the spine 11 . Furthermore, Hoffmann and colleagues have shown that vibrations on the back were more accurately localised with respect to one another in the horizontal direction than in the vertical direction 11 . In the same study it was also found that tactile sensitivity near the spine was higher than further towards the side, but only in the horizontal direction.…”

Development and Testing of a Wearable Vibrotactile Haptic Feedback System for Proprioceptive Rehabilitation

“…For single stimuli at a time, Rantala [48] states the minimum point localization distance to be 15 mm while in case of multiple stimuli, Moore and Breed [49] identifies the minimum distance for two-point discrimination to be more than 20 mm. Hoffmann et al [50] states the closest distance physically possible is 10 mm for vibrotactile elements. They accessed vibrotactile spatial acuity at both 20 mm and 10 mm distance; the 20 mm distance lead to about 64% discrimination accuracy.…”