Tactile Displays
The average adult has approximately 2m2 of skin (Gibson, 1968), about 90% hairy, and remainder smooth or glabrous. Although the glabrous areas are more sensitive than the hairy, both types are highly innervated with sensory receptors and nerves (Sinclair, 1981). Tactile displays have utilized both glabrous and hairy skin, the type selected being relative to the sensory display needs of the various investigators. There are several advantages for selecting the skin as the sensory surface to receive information. (1) It is accessible, extensive in area, richly innervated, and capable of precise discrimination. Further, when the skin of the forehead or trunk is used, the tactile display system does not interfere materially with motor or other sensory functions. (2) The skin shows a number of functional similarities to the retina of the eye in its capacity to mediate information. Large parts of the body surface are relatively flat, and the receptor surfaces of the skin, like the retina, are capable of mediating displays in two spatial dimensions as well as having the potential for temporal integration (summation over time). Thus, there is generally no need for complex topological transformation or for temporal coding of pictorial information for direct presentation onto the accessible areas of the skin, although temporal display factors have been explored with the goal of transmitting spatial information across the skin more quickly than is possible with present systems (Kaczmarek et al., 1984; Bach-y-Rita and Hughes, 1985; Kaczmarek et al., 1985; Loomis and Lederman, 1986). Spatial patterns learned visually can be identified factually, and vice versa (Epstein et al., 1989; Hughes et al., 1990). (3) Certain types of sensory inhibition, including the Mach band phenomenon and other examples of lateral inhibition originally demonstrated for vision, are equally demonstrable in the skin (Bekesy, 1967). (4) Finally, there is evidence that the skin normally functions as an exteroceptor at least in a limited sense: Katz noted that to some extent both vibration and temperature changes can be felt at a distance (Krueger, 1970). For example, a blind person can “feel” the approach of a warm cylinder at three times the distance required by the sighted individual (Krueger, 1970).