Some characteristics of tactile channels.

The four information-processing channels of glabrous skin have distinct tuning characteristics which appear to be determined in the periphery at the level of sensory receptors and their afferent nerve fibers. The four-channel model [J Acoust Soc Am 84 (1988) 1680] has been updated to include measurement over a wider frequency range of tuning of the P and NP I channels, psychophysically determined by forward-masking and adaptation tuning curve methods. In addition to differences in their tuning, the P and NP channels differ in the following ways: (1) the P channel, but not NP channels, has been found to be capable of temporal summation, which operates by neural integration; (2) the capacity for spatial summation is also an exclusive property of the P channel; (3) sensitivity declines with age at a greater rate in the P channel than in the NP channels; (4) the masking or adaptation of a channel has no effect on the sensitivity of the other channels, although the channels interact in the summation of the perceived magnitudes of stimuli presented to separate channels.

Passive, active and intra-active (self) touch.

A series of experiments are described in which magnitude estimates of the perceived size of steel balls were made when the balls were actively rolled between the fingertip and several other body sites (thumb, thenar eminence, forearm). This movement, called scripting, involves actively moving an object by a touching surface over another surface of the body which is passively being touched. We define this active/passive activity as "intra-active touch" and the results show that the perceptual size of the balls is dependent upon the body part passively being activated. An additional series of experiments decoupled the actively generated and passively received tactile information by having subjects either perform the scripting on another individual's body site or by having the other individual roll the balls on the subject's various sites. The latter experiments showed that the passive body can contribute to the overall impression of the size of the balls, but only when the intra-active touching involved the glabrous skin of the hands. Intra-active touch between the active finger and the passively touched hairy skin of the forearm showed no effect of the touched surface on the perceived size of the balls. The results suggest that the mechanisms of intra-active touch are different when glabrous skin activates glabrous skin than when glabrous skin activates hairy skin.

Vibrotactile temporal summation: probability summation or neural integration?

Temporal summation, a decrease in the detection threshold that occurs when either the duration of a stimulus or the number of stimuli in a sequence is increased, has been attributed to the operations of either the mechanism of neural integration or of probability summation. Our experiments indicate that under certain conditions, both mechanisms may operate, but that the process of neural integration is an exclusive characteristic of the Pacinian (P) channel. The P channel was isolated by applying 250 Hz stimuli through a 1.5 cm2 contactor to the thenar eminence of the hand and the NPII channel was isolated by applying the stimuli through a 0.01 cm2 contactor. The finding that the slopes of the psychometric functions were the same within both channels indicated that probability summation could not account for temporal summation for stimulus durations less than 1 s. The finding that the threshold for the detection of multiple-pulse stimuli increased as the interpulse interval increased indicated that, for time intervals less than 800 ms, temporal summation results from neural integration. But for interstimulus intervals greater than 800 ms, probability summation accounts for temporal summation.

Passive, active and intra-active (self) touch.

A series of experiments are described in which magnitude estimates of the perceived size of steel balls were made when the balls were actively rolled between the fingertip and several other body sites (thumb, thenar eminence, forearm). This movement, called scripting, involves actively-moving an object by a touching surface over another surface of the body which is passively being touched. We define this active/passive activity as "intra-active touch" and the results show that the perceptual size of the balls is dependent upon the body part passively being activated. An additional series of experiments decoupled the actively generated and passively received tactile information by having subjects either perform the scripting on another individual's body site or by having the other individual roll the balls on the subject's various sites. The latter experiments showed that the passive body can contribute to the overall impression of the size of the balls, but only when the intra-active touching involved the glabrous skin of the hands. Intra-active touch between the active finger and the passively touched hairy skin of the forearm showed no effect of the touched surface on the perceived size of the balls. The results suggest that the mechanisms of intra-active touch are different when glabrous skin activates glabrous skin than when glabrous skin activates hairy skin.

Subjective magnitude of tactile roughness.

The subjective experience of tactile roughness was judged by subjects using the method of absolute magnitude estimation (AME). The stimuli were 11 grades of sandpaper having particle diameters ranging from 16 to 905 microm. All of the estimates resulted in power functions when assigned numbers were plotted as a function of particle diameter. It was determined that on the finger pad of the index finger and the thumb there was no difference between the active and passive modes of stimulation and that there was no difference in roughness estimates made on the finger and on the thumb. When the finger and thumb were stimulated simultaneously, higher numbers were assigned for a given stimulus indicating the presence of a form of spatial summation at these sites. The pleasantness of the tactile sensation, as assessed using AME, was inversely related to the roughness estimates. Furthermore, hydration of the stratum corneum with water and three concentrations of surfactant solutions reduced the sensation of roughness below that of normally hydrated skin.

Effects of hydration on tactile sensation.

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.

The effects of aging on information-processing channels in the sense of touch: III. Differential sensitivity to changes in stimulus intensity.

Detection thresholds and difference limens were measured for 16 subjects ranging from 19 to 91 years of age. The stimuli were 250-Hz bursts of vibration applied through a 3.0-cm2 contactor to the thenar eminence of the right hand. Detection thresholds were higher in older than in younger subjects, as were the absolute values of difference limens. When the difference limen was expressed in relative terms as the proportion by which two stimuli had to differ in amplitude to be discriminated (delta alpha/alpha), discriminative capacities were unaffected by aging except for stimuli slightly above the detection threshold, in which case the limens of older subjects were significantly higher than those of younger subjects. The results are consistent with the hypothesis that elevations in the detection thresholds of older subjects are the results of reduced afferent input to central brain centers that, with regard to their capacity to detect the presence of threshold-level stimuli and to discriminate differences among suprathreshold stimuli, are relatively unaffected by aging.

The effects of aging on information-processing channels in the sense of touch: I. Absolute sensitivity.

Thresholds for detecting vibrotactile signals of variable frequency applied to the thenar eminence of the hand by small and large contactors were measured in subjects ranging in age from 10 to 89 years. Thresholds were found to increase as a function of age, but the rate of increase was greater after than before the age of 65 years. The rate of loss of vibrotactile sensitivity was substantially greater in the P channel (mediated by Pacinian corpuscles) than in the NP I channel (mediated by rapidly adapting fibers), the NP II channel (mediated by slowly adapting type II fibers), or the NP III channel (mediated by slowly adapting type I fibers). Women were frequently found to have greater sensitivity than men.

The effects of aging on information-processing channels in the sense of touch: II. Temporal summation in the P channel.

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.

Hairy skin: psychophysical channels and their physiological substrates.

Experiments were conducted in which threshold-frequency characteristics were measured on the hairy skin of the forearm of human observers. Thresholds were measured with two stimulus probe areas (2.9 and 0.008 cm2) at three skin-surface temperatures (15 degrees, 30 degrees, and 40 degrees C). The results suggest that whereas glabrous skin uses four distinct channels of information, only three channels may be involved in mediating the sense of touch for hairy skin. The three channels are defined as Ph, (Pacinian, hairy skin), NPh low (non-Pacinian, hairy skin, low frequencies) and NPh mid (non-Pacinian, hairy skin, middle frequencies). In addition, it is proposed that the neural substrates for the three psychophysically characterized channels are, respectively, the Pacinian corpuscle (PC) nerve fibers, the slowly adapting type II (SAII) fibers, and the rapidly adapting (RA) fibers.

Vibrotactile forward masking as a function of age.

Thresholds for the detection of a 50-ms test stimulus delivered to the thenar eminence were measured as a function of the time interval between the offset of a 500-ms masking stimulus and the onset of the test stimulus (delta t). The frequency of the masker and the test stimulus was the same during a particular testing session and was either 25 or 250 Hz. At all values of delta t, older subjects exhibited significantly more masking than did young subjects. The effects of age were greater for stimuli that primarily affect the Pacinian system (250 Hz) than those that primarily affect non-Pacinian systems (25 Hz). Psychophysical measurements of the apparent duration of tactile sensations suggest that both sensory persistence and adaptation are affected by aging. Since adaptation seemed to be the more dominant factor for stimuli with durations as long as 500 ms, it was concluded that the effects of aging on forward masking seen in our study were due mainly to increased amounts of adaptation produced by the masker.

Effects of noise on detection of amplitude increments of sinusoidal vibration of the skin.

Vibrotactile thresholds for detecting a 300-Hz signal in the presence of both a 300-Hz sinusoidal pedestal and a background noise were measured as a function of the amplitudes of the pedestal and noise. Threshold increased monotonically as a function of the amplitude of the noise, but was a nonmonotonic function of the amplitude of the sinusoidal pedestal. Negative masking, in which the pedestal facilitated detection of the test stimulus, was observed in the absence of background noise and in the presence of subthreshold background noise when the pedestal was near or below threshold. Negative masking disappeared when the experiment was conducted in the presence of moderately intense to intense background noise. The results are consistent with a peripheral high-energy threshold for taction.

Vibrotactile temporal gap detection as a function of age.

The ability of subjects to detect temporal gaps between bursts of sinusoids or bursts of bandlimited noise was measured to evaluate the phenomenon of tactile "sensory persistence" in older persons. Vibratory stimuli were delivered to the right thenar eminence of 27 subjects ranging in age from 8-75 years. The subjects' task was to detect the presence of a silent interval or "gap" between flanking 350-ms vibrotactile stimuli. The gap-detection threshold, expressed as the amplitude of vibration relative to the absolute detection threshold, decreased as the gap duration increased and was higher for gaps in noise than for gaps in sinusoids. The threshold for detecting short gaps increased with age for noise stimuli, but not for sinusoidal stimuli. Furthermore, the gap-detection threshold recovered more rapidly in older subjects for noise stimuli, but less rapidly in older subjects for sinusoidal stimuli. Because of these differences, it appears that the effects of age on gap detection cannot be due to a simple increase in sensory persistence, but may be due to multiple processes.

Vibrotactile intensity discrimination measured by three methods.

The difference threshold for the detection of changes in vibration amplitude was measured as a function of the intensity and frequency of stimuli delivered through a 2.9-cm2 contactor to the thenar eminence. Stimuli were either 25- or 250-Hz sinusoids, narrow-band noise centered at 250 Hz, or wideband noise. Thresholds were measured by two-interval, forced-choice tracking under three methods of stimulus presentation. In the gated-pedestal method, subjects had to judge which of two 700-ms bursts of vibration separated by 100 ms was more intense. In the continuous-pedestal method, subjects had to detect a 700-ms increment in the amplitude of an ongoing pedestal of vibration. In the two-burst-continuous-pedestal method with 1500-ms pedestals, the subject had to detect which of two successively presented pedestals contained a 500-ms amplitude increment. Thresholds were consistently lower for detecting increments in the amplitude of a continuous pedestal of vibration than for detecting amplitude differences between briefly presented successive pedestals or amplitude increments in successive pedestals. A "near miss" to Weber's law was found both for sinusoidal and for noise stimuli. The difference threshold was not affected by stimulus frequency condition.

Vibrotactile masking: effects of stimulus onset asynchrony and stimulus frequency.

Vibrotactile thresholds for the detection of a 50-ms vibratory stimulus on the thenar eminence of the hand were measured in the presence of and in the absence of a 700-ms suprathreshold vibratory masking stimulus. When thresholds were measured in the presence of the masking stimulus, stimulus onset asynchrony (SOA) was varied so that backward, simultaneous, and forward masking could be measured. The amount of masking, expressed as threshold shift, was greatest when the test stimulus was presented near the onset or offset of the masking stimulus. For both backward and forward masking, the amount of masking decreased as a function of increasing stimulus onset asynchrony. Comparisons were made of the amounts of masking measured when the test and masking stimuli were both sinusoids, and when the test stimulus was a sinusoid and the masking stimulus was noise. In all conditions, the masked threshold decreased approximately 4.0 dB when SOA was increased from 100 to 650 ms with reference to the onset of the 700-ms masking stimulus. More simultaneous masking was observed when sinusoidal test stimuli were detected in the presence of noise than when they were detected in the presence of sinusoidal maskers of the same frequency. The functions were essentially identical for detection of a low-frequency (20 Hz) test stimulus mediated by a non-Pacinian channel and detection of a high-frequency (250 Hz) test stimulus mediated by the Pacinian channel.

Four channels mediate the mechanical aspects of touch.

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.

A device for measuring tactile spatiotemporal sensitivity.

A tactile stimulator array constructed from 88 piezoelectric ceramic plates is described. The array can produce sinusoidal traveling waves with arbitrary temporal frequency, spatial wavelength, and amplitude. Detection thresholds were measured for five temporal frequencies (1, 4, 16, 64, and 256 Hz) and five spatial wavelengths (1.81, 3.62, 7.23, 14.5, and infinity mm), and were plotted as a three-dimensional, spatiotemporal threshold surface. Detection thresholds were also measured using a conventional vibrator and a large circular contactor simulating the infinite wavelength stimulus produced by the array. The results describe the spatial and temporal frequency sensitivity of the P and NP I cutaneous receptor populations.

The effects of skin temperature on the psychophysical responses to vibration on glabrous and hairy skin.

Psychophysical detection thresholds for vibration were measured at the thenar eminence and volar forearm using a 0.008-cm2 (1.0-mm-diam) contactor. Measurements were made at 14 sinusoidal frequencies between 12 and 500 Hz at six skin temperatures between 15 degrees and 40 degrees C. The results are consistent with the hypothesis that three functionally discrete non-Pacinian afferent systems mediate vibrotactile responses. It was possible to identify the response characteristic of the rapidly adapting (Meissner) system, but it was not possible to isolate the responses of two slowly adapting (SAI, SAII) systems.

Vibrotactile forward masking: psychophysical evidence for a triplex theory of cutaneous mechanoreception.

Threshold shifts for the detection of vibrotactile test stimuli were determined as a function of the intensity of a masker. A 50-ms sinusoidal test stimulus was applied to the thenar eminence of the hand 25 ms after the termination of a 700-ms sinusoidal masker applied to the same site. The frequency of the test stimulus and the frequency of the masker were varied. To eliminate the influence of the Pacinian receptor system, stimuli were delivered through a 0.01-cm2 contactor. The results support the hypothesis that the detection of vibration delivered through a small contactor is determined by two separate populations of non-Pacinian receptors. The study constitutes a psychophysical demonstration of the existence of three receptor systems responsible for the detection of vibration.