Although previous physiological and anatomical experiments have identified four afferent fiber types (PC, RA, SA II, and SA I) in glabrous (nonhairy) skin of the human somatosensory periphery, only three have been shown to mediate tactile (mechanoreceptive) sensation. Psychophysical evidence that four channels (P, NP I, NP II, and NP III) do, indeed, participate in the perceptual process is presented. In a series of experiments involving selective masking of the various channels, modification of the skin-surface temperature, and testing cutaneous sensitivity down to very low-vibratory frequencies, the fourth psychophysical channel (NP III) is defined. Based on these experiments and previous work from our laboratory, it is concluded that the four channels work in conjunction at threshold to create an operating range for the perception of vibration that extends from at least 0.4 to greater than 500 Hz. Each of the four channels appears to mediate specific portions of the overall threshold-frequency characteristic. Selection of appropriate neural-response criteria from previously published physiological data and correlation of their derived frequency characteristics with the four psychophysical channels indicates that each channel has its own physiological substrate: P channel and PC fibers, NP I channel and RA fibers, NP II channel and SA II fibers, and NP III channel and SA I fibers. These channels partially overlap in their absolute sensitivities, making it likely that suprathreshold stimuli may activate two or more of the channels at the same time. Thus the perceptual qualities of touch may be determined by the combined inputs from four channels.

The routine tasks of washing usually necessitates the immersion of parts of the body in water, which causes hydration and changes in the mechanical properties of the superficial layer of skin. To determine how hydration affects tactile sensations, the hydration and skin-surface temperature of glabrous and hairy skin was first measured under normal conditions (air), after submersion in distilled water alone and after submersion in a surfactant-water solution. In these experiments, measurements were made of the time to achieve complete hydration and the recovery time to normal levels. The uptake of water in hairy skin was found to be considerably greater than in glabrous skin, and retention was significantly prolonged by the surfactant additive. Subsequent experiments on glabrous skin, based on the results of the preceding hydration studies, measured in-air and hydrated tactile thresholds and sensation magnitudes to vibratory stimuli and to the roughness of textured surfaces. Vibrotactile detection thresholds were not affected by skin hydration, nor were sensation magnitudes to suprathreshold vibratory stimuli. However, suprathreshold perceptions of roughness were substantially altered by hydration. It is concluded that hydration and the mechanics of the skin play a major role in the perception of spatiotemporal (i.e., textured) surfaces and, thus, must be taken into account in any physiological/psychophysical model based on using such stimuli. This may not be required for models based on predominantly temporal (i.e., vibratory) stimuli.

Thresholds for detecting 250-Hz vibrotactile signals of variable duration applied to the thenar eminence of the hand were measured in 16 subjects ranging in age from 19 to 81 years. Detection thresholds were higher in older than in younger subjects. Correlation coefficients for the relation between threshold and age ranged from 0.94 to 0.96, depending on signal duration. In addition, the amount of temporal summation was negatively correlated with age. Both the elevated detection thresholds and the reduced amount of temporal summation in elderly subjects may be partially due to the decrease in the number of Pacinian corpuscles in the hand that occurs with aging. Another factor that could be responsible for reduced temporal summation in older as compared to younger subjects is impairment of the temporal integrator.

Pacinian corpuscles (PCs) in cat mesentery have been studied extensively to help determine the structural and functional bases of tactile mechanotransduction. Although we, like many other investigators, have found that the mesenteric receptors are anatomically very similar to those found in mammalian skin, few physiological characteristics of the mesenteric PCs and those of the skin have been compared. Action-potential rate-amplitude and frequency characteristics (10 Hz-1 KHz), as well as interval (IH) and peri-stimulus-time (PSTH) histograms in response to sinusoidal displacements were obtained from nerve fibers innervating mesenteric PCs and from PC fibers innervating cat glabrous skin. The intensity characteristics obtained on both preparations showed similar response profiles, including equal slopes for low stimulus intensities (approximately 10, with impulse ratios/20 dB displacement) and one and two impulse/cycle entrainment. The frequency characteristics of both groups were U-shaped with similar low-frequency slopes (-12.5 dB/octave) and bandwidths (Q(3dB) = 1.4). The best frequency for both the tactile PCs' and mesenteric PCs was 250 Hz, which is in the expected range. The IHs showed entrainment and the PSTHs showed neither transient responses nor adaptation to steady-state sinusoidal stimuli. The functional similarity between mesenteric PCs' nerve responses and those of tactile PC afferents, as well as the receptors' anatomical similarity, lead us to suggest that the mesenteric PC can act as a model for those in the skin. Furthermore, since the frequency characteristics of the two PC types are similar, it is concluded that the skin, while attenuating stimulus intensity, does not impart temporal filtering of vibratory stimuli.

Based on physiological and psychophysical data, it has been suggested that the neural code for threshold detection in the P channel, mediated by Pacinian corpuscles (PCs), may be 2-4 neural impulses/stimulus (Bolanowski et al., 1988). To further test the efficacy of this code, responses from PCs were measured at different vibratory burst durations. Since it is known that the P channel has the capacity to summate vibratory stimuli temporally within the central nervous system, the effect should also be present in the physiological results. Temporal summation predicts a decrease in threshold at a rate of -3 dB/doubling of stimulus duration. Responses from single PCs were integrated by counting neural spikes over the entire burst duration. Furthermore, real-time responses were integrated with a low-pass filter, more accurately modeling the central process. For the spike-counting scheme, a criterion of 4 impulses/stimulus showed a decrease in stimulus amplitude for increases in duration similar to that obtained psychophysically. The amplitude-duration function obtained with the low-pass filter, however, resulted in a function which did not follow that obtained psychophysically, regardless of the number of impulses/stimulus. Since the P channel is known to have a central integrator, it has been concluded that activity of a single PC afferent probably is not sufficient to signal threshold in the P channel.