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.

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.

Immunocytochemical identification of proteins within the Pacinian corpuscle.

Light- and electron-microscopic immunocytochemistry (ICC) was performed on Pacinian corpuscles (PCs) obtained from cat mesentery to determine the presence and location of various proteins within the accessory capsule and the neurite. Antibodies to tubulin, neurofilament 200, actin, collagen II and V, glial fibrillary acidic protein (GFAP) and S-100 were used. Type II collagen was localized only in the outer core of the accessory capsule, which is composed of an inner core, an intermediate layer or growth zone, an outer core and an external capsule. Type V collagen was found only in the intermediate growth zone. Intermediate filaments labeled with anti-GFAP were only found in the inner core. The calcium-binding protein that was labeled by anti-S-100 was found only in the inner core. Diffuse and variable staining for actin is present throughout the accessory capsule. The differences in distribution of these various proteins within the capsule suggest different structural/functional properties of the various capsule regions. The neurite was found to contain microtubules (i.e., tubulin) and neurofilaments throughout, but these cellular inclusions were not found within the cytoplasmic extensions (filopodia) that project from the neurite into the hemilamellar clefts formed by the inner-core hemilamellae. The extensions, however, were found to contain actin in a much greater density than that seen in the neurite proper. The presence of actin, but apparent lack of other cytostructural elements within the extensions, is highly reminiscent of the composition of stereocilia found on vestibular and auditory hair cells. Since stereocilia have been shown to play a role in hair-cell mechanotransduction, it is possible that the cytoplasmic extensions are significantly involved with mechanotransduction within the PC.

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.

Under which conditions do the skin and probe decouple during sinusoidal vibrations?

Previous experiments performed on monkey and human fingertips suggested that the skin surface and stimulus probe decouple for sinusoidal displacements applied perpendicularly to the skin surface. From these observations, it was concluded that sinusoidal vibration may not be a suitable stimulus for understanding and modeling the tactile system. We repeated these experiments on human observers using stimulus frequencies ranging from 0.5 to 240 Hz and with displacement amplitudes up to 1 mm peak-to-peak (p-p). The skin and probe movements were measured in the steady-state using stroboscopic illumination and video microscopy. Contrary to previous conclusions, we found that decoupling did not occur for amplitudes less then 0.25 mm p-p, regardless of stimulus frequency. Decoupling was only observed for stimulus amplitudes greater than 0.25 mm over the stimulus-frequency range investigated. To further investigate this effect, a modified stimulus contactor was used, which permitted the measurement of the skin's movement using reflected light. Measurements were made on both the index fingertip and the thenar eminence. Regardless of body site, no decoupling between the skin and stimulus probe was observed for frequencies ranging from 20 to 100 Hz up to displacements of 0.25 mm p-p. These levels are well within the range used in most human psychophysical experiments performed on these parts of the body. We conclude that sinusoidal vibration can be used reliably to stimulate the tactile system and is an appropriate stimulus for developing models of touch.

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.

Mechanisms of attention in touch.

A series of experiments demonstrated the role of higher level cognitive processes, such as attention, in tactile perception. The first series of experiments demonstrated that automatic orienting to a tactile stimulus resulted in inhibition of subsequent stimuli at that body site--inhibition of return (IOR). A possible explanation suggests that inhibition of saccades to a body site can cause the inhibition of subsequent stimuli presented to that same site. In contrast, when the subjects strategically oriented attention to the stimulus, the processing of subsequent stimuli at that body site was facilitated. In both of these experiments the skin received exactly the same test stimuli, (100 Hz sine wave presented for 50 ms), but very different effects were observed depending upon attentional strategy. Experimental manipulations showed that this cannot be due to a peripheral masking of the receptors after cue presentation to the target. Rather the results may be explained centrally by cognitive, particularly attentional mechanisms. Cross-modal interactions suggest that tactile processes are facilitated when vision is oriented to the body site receiving stimulation. Possible explanations come from recent findings of spatiotopic maps of different sensory modalities in the superior colliculus of the midbrain and in the parietal lobe. These are integrated with motor systems that control saccades and head orientation towards sensory inputs. Excitatory links among these maps could be the source of the observed facilitation effects.

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.

A longitudinal study of somesthetic perceptual disorders in an individual with a unilateral thalamic lesion.

Several aspects of tactile, thermal and pain perception were evaluated in an individual (R.S.) with a hemorrhagic lesion centered in her left lateral thalamus. Over a 4-year period, psychophysical evaluations were undertaken every 6-8 months, and five magnetic resonance (MR) studies were conducted. Early tests (1991-1992) revealed large contralateral deficits in R.S.'s perception of touch, innocuous temperature, and mechanically evoked cutaneous pain--more so for the upper versus the lower extremity. R.S. showed a similar pattern for heat pain sensitivity, but a more modest deficit than for mechanically evoked pain. She showed a deficit for cold pain sensitivity on her foot, but not for her hand. Thresholds for all types of stimuli ipsilateral to the lesion were within a normative range. Late in 1993, R.S. demonstrated improvements in sensory capacity for touch and mechanically evoked pain contralaterally, although deficits were still evident. During the same period, heat pain sensitivity improved contralaterally, and strikingly, a permanent, ipsilateral hypersensitivity to heat pain developed in her hand. Throughout the entire testing period, R.S.'s ratings of perceived unpleasantness matched the patterns of perceived pain intensity. Thus, the discriminative and the affective dimensions of her pain would change in tandem. However, perceptible innocuous thermal stimuli evoked no affective response when applied contralaterally, despite being described as pleasant when presented ipsilaterally. Throughout the testing period, R.S. reported a persistent numbness on her right hemi-body. Only during a 3-month period in 1995 did she experience spontaneous pain, which was referred to her right foot. The only change in psychophysical performance related to her right foot was a transient but intense thermal allodynia several months prior to her spontaneous pain. The MR studies over this 4-year period showed changes in the extent of edema, gliosis and/or ischemia that could be related to perceptual changes. Thus, the conspicuous observations in this thalamic lesion case were: (i) differential effects upon the various pain modalities (mechanical, heat and cold); (ii) development of thermal allodynia without mechanical allodynia, including an ipsilateral effect; (iii) a deficit in positive affective responses to temperature; and (iv) the different time courses for changes in evoked somesthetic capacity versus spontaneous paresthesias and pathological pain.

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.

Assessment of vibrotactile sensitivity in patients with carpal tunnel syndrome.

The effectiveness of using vibrotactile threshold measures to aid in the diagnosis of carpal tunnel syndrome (CTS) was evaluated. Thresholds for detecting 1-, 10-, and 300-Hz vibratory stimuli were measured on the fingertips of 24 CTS patients and 20 healthy control subjects. There were no significant differences in threshold for 1- and 300-Hz between the two groups. Although there were significant differences for 10-Hz stimuli, the mean patient threshold was within 1 standard deviation of the mean threshold for the control group. These results indicate that threshold testing is not a suitable diagnostic tool for CTS. Additionally, we examined whether thresholds were elevated in the presence of pain. Seven patients reported experiences of pain and no pain sessions. No significant differences in threshold were found between the two pain conditions, indicating that the presence of pain related to CTS does not affect threshold.

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.

Mitochondrial distribution within the terminal neurite of the pacinian corpuscle.

Electron-microscopic analyses of the mitochondrial organization within the neurite innervating the Pacinian corpuscle (PC) were performed to test the hypothesis that the sites of mechanotransduction are the filopodia projecting from the neurite's surface. Since high concentrations of mitochondria imply the need for metabolic energy, and since transduction mechanisms are heavily dependent on such energy, it was reasoned that the greatest concentration of mitochondria should occur near the filopodia if they are involved in mechanotransduction. The analysis that the mitochondria lie close to the terminal neurite's membrane, on average within 0.4 microns, and thus are ideally located for supplying energy for membrane mechanisms. Although they can be found in a ring-like array, as seen in cross-sections of the terminal neurite, their greatest concentration occurs where the filopodia project from the terminal neurite. A linear algebraic analysis of the data set confirmed a high probability of the joint occurrence of a filopodial base and an increased frequency (number) of mitochondria. Thus the results provide further circumstantial support for the hypothesis that transduction within the PC mechanoreceptors takes place at or near the filopodia.

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.