Vibrotactile Spatial Acuity and Intensity Discrimination on the Lower Back Using Coin Motors

Tactile vision substitution devices are assistive devices for the blind that redirect visual information to the skin. The amount of visual information that can be presented on a tactile display is limited mainly by the spatial resolution of the skin and the ability to distinguish between various vibration intensities. In this study, we have determined the two-point discrimination (TPD) threshold and intensity-discrimination threshold (just-noticeable difference, or JND) on the lower back using coin motors. Given the importance of stimulus timing, we have determined TPD threshold and JND at different stimulus onset asynchronies (SOAs). The JND was determined between two coin motors with a distance equal to the TPD threshold. In this way, we could establish the contrast sensitivity at the maximal theoretical resolution. TPD thresholds tended to decrease at longer SOAs, from 52 mm edge-to-edge at an SOA of 0 ms, to 28 mm at 200 ms. The JND did not depend on SOA, and the average Weber fraction was 0.14. A median of 5 JNDs was available across the available dynamic range. Together, these data provide the predicted spatial resolution and contrast resolution achievable with a back-worn tactile display based on coin motors.