ABSTRACT
As is true in other sensory systems, specific receptors in touch respond optimally to different types of stimulation. For example, Pacinian corpuscles, with their rapidly adapting nerve fibers, respond best to the rapid onset and offset of deformations of the skin and to high-frequency vibration; whereas Merkel cells, with their slowly adapting nerve fibers, respond optimally to steady deformations of the skin and to low-frequency vibration (Vallbo & Johansson, 1984). Inasmuch as the receptor selectively responds to a particular type of stimulus, the afferent nerve fiber from the receptor serves as a communication line to convey specific information to the brain about that particular type of stimulus. Indeed, the earliest psychophysical demonstration of the operation of specific tactile neural systems, each optimally responsive to different types of stimulation, came from two findings: thresholds for the detection of high-frequency vibration are lowest at 250-300 Hz and rapidly rise at lower and higher frequencies; and at low frequencies of vibratory stimulation, thresholds are nearly independent of stimulus frequency (Békésy, 1939; Gescheider, 1976; Talbot, Darien-Smith, Kornhuber, & Mountcastle, 1968; Verrillo, 1963; Verrillo, Fraioli, & Smith, 1969).
