The response of tactile nerve endings to varying forces on the skin and to oscillatory motion of the skin. Grasping, holding, and tactile exploration of an object are part of everyday experience. The performance of all these activities is dependent on the dynamic response of specialized nerve endings located in the skin.
Knowledge of the neural and psychophysical processes involved in vibrotaction is necessary to develop effective tactile communication systems, such as vibrotactile pagers, and multipin vibrotactile stimulators used to produce images on an extended skin surface. Potential users range from business people to the visually and hearing impaired: potential applications range from activities involving visual and aural sensory saturation (such as pilots in high stress situations) to those requiring sensory stimulation (for example, virtual reality).
Small electrical impulses, or action potentials, generated by a single nerve ending can be recorded by inserting a microelectrode into the arm of an alert human subject and positioning the electrode tip in a nerve fiber. Studies of the action potentials produced when the skin is locally depressed at a fingertip have identified activity in four networks of distinctive nerve endings lying within 1–2 mm of the skin surface. These nerve endings, which transform skin motion into neural signals, consist of physically and functionally different mechanoreceptors.
Three types of mechanoreceptor in the fingertip have been implicated in vibrotaction. One of these is a population of slowly adapting mechanoreceptors (SA type I, or SAI); their action potentials persist for some time after a transient skin indentation. The two others are populations of rapidly adapting receptors (FAI and FAII types). Type I receptors respond only to skin indentation within a few millimeters of the nerve ending, whereas type II receptors also respond to stimuli at a greater distance from the nerve ending. A fourth mechanoreceptor type appears to respond primarily to skin stretch (denoted SAII). See Cutaneous sensation, Mechanoreceptors