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.

The frequency selectivity of information-processing channels in the tactile sensory system.

The frequency selectivity of the P, NP I, and NP II channels of the four-channel model of mechanoreception for glabrous skin was measured psychophysically by an adaptation tuning curve procedure. The results substantially extend the frequency range over which the frequency selectivity of these channels is known and further confirm the hypothesis that the input stage of each of these channels consists of specific sensory nerve fibers and associated receptors. Specifically, the frequency characteristics of Pacinian nerve fibers, rapidly adapting (RA) nerve fibers, and slowly adapting Type II (SA II) nerve fibers were found to be the peripheral neurophysiological correlates of the P, NP I, and NP II channels, respectively. The finding that the tuning characteristic for a test stimulus of 250 Hz delivered through a small (0.008 cm2) contactor depended dramatically on the duration of the test stimulus whereas the detection threshold did not, provides new evidence in support of the hypothesis that separate NP II and P channels exist.

The effects of heat-induced pain on the detectability, discriminability, and sensation magnitude of vibrotactile stimuli.

The effects of heat-induced pain on absolute thresholds, sensation magnitudes and amplitude-difference thresholds were measured at 10 and 100 Hz. Consistent with previous results, heat-induced pain elevated the absolute thresholds by approximately 8.0 dB and lessened the magnitudes of tactile sensations during pain as compared to the non-painful condition. In contrast to these effects, the discriminability of change in the intensity of the vibrotactile stimuli was unaffected by the presence of pain indicating that the effect of pain on tactile sensations is more likely due to sensory rather than cognitive processes (i.e., attention) and that the mechanisms underlying tactile sensitivity as compared to discriminability are different.

The effects of stimulus location on the gating of touch by heat- and cold-induced pain.

The influence of heat- and cold-induced pain on tactile sensitivity, a "touch gate", was measured under conditions in which the location of the noxious stimuli was varied with respect to the tactile stimulus applied to the thenar eminence of humans. Vibrotactile thresholds were measured in the absence of pain and during administration of a painful stimulus, with the stimulus frequencies selected to activate independently the four psychophysical channels hypothesized to exist in human glabrous skin. Heat-induced pain produced by spatially co-localizing the noxious stimuli with the tactile stimuli was found, on average, to elevate threshold amplitude by 2.2 times (6.7 dB). Co-localized, cold-induced pain raised the average thresholds by about 1.5 times (3.6 dB). Heat-induced pain presented contralaterally produced no change in vibrotactile sensitivity indicating that the effect is probably not due to attentional mechanisms. Ipsilateral heat-induced pain caused an elevation in tactile thresholds even when the noxious and non-noxious stimuli were not co-localized, and the effect may seem to require that the painful stimulus be within the somatosensory region defined possibly in terms of dermatomal organization. Thus the effect is probably related to somatotopic organization and is not peripherally mediated. A brief discussion as to the possible locus of the touch gate within the nervous system is also given.

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.

The effects of skin temperature on the detection and discrimination of tactile stimulation.

Detection thresholds and intensity-difference thresholds were measured on four subjects ranging in age from 19 to 22 years. 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 substantially higher at 20 degrees C than at 30 degrees or 40 degrees C and were only slightly higher at 40 degrees C than 30 degrees C. When the intensity-difference threshold was expressed in relative terms, as the proportion by which two stimuli must differ in amplitude to be discriminated (delta phi/phi), discrimination capacities were unaffected by surface-skin temperature. The results are consistent with the hypothesis that surface-skin temperature alters the sensitivity of tactile receptors, and that, because of the 'near miss' to Weber's law, the relative difference threshold is unaffected substantially by skin temperature. It was concluded that, at least a partial explanation of the 'near miss' lies in the fact that, at low to moderate sensation levels, the P channel is exclusively activated whereas, at moderate to high sensation levels, because of recruitment of activity in Non-Pacinian channels, neural information for intensity discrimination is additionally provided by channels with superior discriminative capacities.

Effects of stimulus duration on the amplitude difference limen for vibrotaction.

Vibrotactile amplitude difference limens (DLs) were measured by the continuous pedestal and gated pedestal methods. In both cases, the relative DL decreased as a function of the intensity of the stimulus and the results, in most cases, could be described as a near miss to Weber's law. DLs measured by the continuous pedestal method were found to decrease substantially as a function of increases in stimulus duration over a range of 12 to 1000 ms. In contrast, DLs measured by the gated pedestal method were only slightly affected by changes in stimulus duration. It was concluded that the process of temporal summation can manifest itself in reducing the size of the DL in the continuous pedestal, but not the gated pedestal, paradigm.

The effects of static indentation on vibrotactile threshold.

Vibrotactile thresholds were measured on the thenar eminence and the volar forearm at different static depths of skin indentation. Three stimulus frequencies (1, 20, and 200 Hz) were delivered through either a 0.008- or a 2.9-cm2 contractor. The indentation depths ranged from 0 to 1 mm (0.25-mm steps) relative to the point of skin contact with the stimulator. There was a significant effect of indentation in all stimulus combinations of contactor size, location, and frequency. These results resolve an apparent discrepancy in the literature regarding threshold reduction with increasing contactor size observed on the forearm at low frequencies.

Decay in the effect of vibrotactile masking.

Vibrotactile forward masking experiments were conducted on the thenar eminence of the hand to determine the time course of masking in the Pacinian (P) or a non-Pacinian (NP I) channel. Brief masking and test stimuli that contained energy centered either below 27 or at 500 Hz were used to preferentially activate the NP I or P channels, respectively. Test thresholds were measured at different fixed delays ( Delta t) between the masking and test stimuli that ranged from 5 to 995 microseconds. Masking stimulus level was also varied from 5 to 25 dB above threshold. The masking effect followed an exponential decay with different time constants for each of the channels, but a similar asymptote (residual masking) for both channels. At the higher masking stimulus levels, the time constant for the P channel was about 40 microseconds, while the time constant for the NP I channel was about 100 microseconds. Residual masking in both channels increased about 1 dB for every 5-dB increase of masking stimulus level.

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.

Vibrotactile forward masking: effects of the amplitude and duration of the masking stimulus.

Tactile thresholds for detecting a 50-ms signal presented 25 ms after the termination of a masking stimulus increased as a function of the amplitude level and duration of the masking stimulus. The effects were similar in both the P and NP I channels measured at 250 and 20 Hz, respectively. It was concluded that the increased masking caused by increasing the duration of the masking stimulus resulted from processes other than or in addition to temporal integration--the latter being a characteristic of the P, but not the NP I, channel. The slopes of the masking functions, in which threshold shifts were plotted as a function of masking-stimulus sensation level, were consistently greater for 20-Hz than for the 250-Hz stimuli, suggesting that masking efficiency is greater in the NP I than in the P channel.

Some temporal parameters in vibrotactile forward masking.

Tactile thresholds for detecting a 250-Hz signal of variable duration presented at variable times after the termination of a 250-Hz 700-ms masking stimulus were measured. It was found that the threshold shift resulting from the presentation of the masking stimulus declined as functions of the duration of the signal and the duration of the time interval between the masking stimulus and signal. Both of these effects were found to be attributable to an increase in the time interval between the offset of the masking stimulus and the offset of the signal.

Relationship between oral pain and ethanol concentration in mouthrinses.

Previous reports have indicated that certain mouthrinses, even when used as directed can induce oral pain. In order to help determine the causal agent(s), various commercially available mouthrinses, as well as control solutions, were tested in a psychophysical study in which subjects rated categories of pain during and after mouthrinsing. More specifically, the studies tested the effects of ethanol concentration on induced pain. The results show that there is a direct relationship between ethanol content and the amount of induced pain. Furthermore, the amount of pain was found to increase with time of rinsing, and to slowly decrease after cessation of rinsing. Lastly, comparison of ethanol/water controls with a marketed product (Clear Choice) matched for ethanol content showed that, while ethanol was the key factor in mouthwash-induced oral pain, other presently unidentified agents can also add to the effect.

The effects of masking on the growth of vibrotactile sensation magnitude and on the amplitude difference limen: a test of the equal sensation magnitude-equal difference limen hypothesis.

In this study, the hypothesis that the difference limen (DL) for the detection of differences in amplitude of vibrotactile stimuli is independent of the slope of the sensation magnitude function was tested. The slope of the sensation magnitude function was varied by presenting test stimuli in the presence of or in the absence of vibrotactile noise. The slopes of the sensation magnitude functions were determined through a matching technique in which the subject adjusted stimulus amplitudes of a 250-Hz stimulus presented alone and a 250-Hz stimulus presented simultaneously with a masking noise, so that their sensation magnitudes were equated. The slope of the matching function was found to increase as a function of the intensity of the masking noise. In the second phase of the experiment, the amplitude DL was measured by the gated-pedestal method for test stimuli presented under the same stimulus conditions as used in the matching procedure. At all levels of stimulus intensity, the DL was found to be independent of the masking condition provided the sensation magnitudes of the stimuli were the same. This finding supports the hypothesis that the size of the DL is independent of the slope of the sensation magnitude function, provided the sensation magnitudes of stimuli are the same. The generality of this principle, first discovered in hearing, is thus extended to another sense modality.

The effects of masking on vibrotactile temporal summation in the detection of sinusoidal and noise signals.

Thresholds for the detection of vibrotactile signals of varied duration applied to the thenar eminence were measured in the absence of and in the presence of a masking stimulus. Signals were 250- and 500-Hz sinusoids and noise bursts with bandwidth limited to 250-1000 Hz. The masking stimulus was either a 250-Hz sinusoid, which was presented in phase with the signal when it was sinusoidal, or noise. Changes in threshold as a function of changes in signal duration were found which were predicted accurately from Zwislocki's theory [J. Acoust. Soc. Am. 32, 1046-1060 (1960)] of temporal summation when the signal was detected by the Pacinian channel, but not when it was detected by a non-Pacinian channel (NP). However, when either the signal or the masking stimulus or both were noise, NP thresholds were affected by changes in signal duration. Only when the signal and masking stimuli were both sinusoids were NP thresholds independent of signal duration. It is concluded that signal duration effects in the NP channel are not due to temporal integration, but rather to increases in information about the signal content provided to the subject as exposure duration is increased.

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.