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.
Highlights
Vibrotactile devices deploying mechanical stimulation through tactile motors in combination with a sophisticated haptic language are powerful tools with a wide range of applications
For the Parallel rotation eccentric rotating mass motors (P ERMs), there was no significant change in accuracy between 20 mm and 10 mm, for horizontal (t(16) = 1.14, p = 0.270) or vertical presentation (t(16) = 0.48, p = 0.635), and the same was true for the linear resonant actuators (LRAs) (horizontal, t(16) = 1.02, p = 0.321; vertical, t(16) = 0.30, p = 0.771)
Our results indicate that vibrotactile discrimination accuracy differs substantially by tactor type with higher accuracy for the Normal rotation eccentric rotating mass motors (N ERMs) than for the other two tactor types, and higher accuracy for the P ERMs than the LRAs
Summary
Vibrotactile devices deploying mechanical stimulation through tactile motors (tactors) in combination with a sophisticated haptic language are powerful tools with a wide range of applications. We focus on haptic resolution for vibrotactile stimulation for the lower thoracic region, since such passive areas are preferable stimulation sites for tactile devices, because active parts like the tongue, feet and hands should be available for other functions (Dakopoulos and Bourbakis 2010; Kristjánsson et al 2016). Tactile spatial acuity measured with pressure stimuli reflects the response of Merkel disks, which are located in superficial layers of the skin, with very small receptive fields (2–10 mm) and a high number of receptors per nerve ending. Spatial acuity measured with vibratory stimuli at higher frequencies of at least 100 Hz, primarily reflects the responses of Pacinian corpuscles, which are located in the subcutaneous skin tissue and have much larger and less numerous receptive fields, resulting in lower resolution (Gardner and Martin 2013). Since most haptic communication devices are nowadays equipped with vibratory tactors, studies investigating vibrotactile spatial acuity are required for further development of such devices
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