Effect of contact location on vibrotactile thresholds at the fingertip

Vibrotactile thresholds depend on the characteristics of the vibration, the location of contact with the skin, and the geometry of the contact with the skin. This experimental study investigated vibrotactile thresholds (from 8 to 250 Hz) at five locations on the distal phalanx of the finger with two contactors: (i) a 1-mm diameter circular probe (0.78-mm(2) area) with a 1-mm gap to a fixed circular surround (i.e., 7.1-mm(2) excitation area), and (ii) a 6-mm diameter circular probe (28-mm(2) area) with a 2-mm gap to a fixed circular surround (i.e., 79-mm(2) excitation area). With both contactors, especially the smaller contactor at low frequencies (i.e., 8, 16, and 31.5 Hz), thresholds decreased towards the tip of the finger, although there was little variation around the whorl. With low frequencies of vibration, and at all five locations on the finger, similar thresholds were obtained with both contactors, consistent with the NPI channel not changing in sensitivity with a change in the area of stimulation. At high frequencies (i.e., 63, 125, and 250 Hz), thresholds were lower with the larger area of stimulation at all locations, except at the extreme tip of the finger, consistent with spatial summation in the Pacinian channel. It is concluded that with a 6-mm diameter contactor, moderate variations in location around the whorl have little influence on the measured thresholds. With the 1-mm diameter contactor there were greater variations in thresholds and extreme locations, near the nail and the distal interphalangeal joint, may be unsuitable for investigating sensorineural disorders.     

Vibrotactile thresholds at the fingertip, volar forearm, large toe, and heel

Thresholds for the perception of vibration vary with location on the body due to the organization of tactile channels in hairy and non-hairy skin, and variations in receptor density. This study determined vibration thresholds at four locations on the body with two different contactors so as to assist the identification of the tactile channel determining the threshold at each location. Vibrotactile thresholds at six frequencies from 8 to 250 Hz were measured on the distal phalanx of the index finger, the volar forearm, the large toe, and the heel with two contactors: (i) a 1-mm diameter circular probe with a 1-mm gap to a fixed circular surround (i.e., 7.1-mm(2) excitation area), and (ii) a 6-mm diameter circular probe with a 2-mm gap to a fixed circular surround (i.e., 79-mm(2) excitation area). At all frequencies and with both contactors, thresholds on the fingertip were lower than thresholds on the volar forearm, the large toe, and the heel, consistent with a greater density of mechanoreceptors at the fingertip. Thresholds with the larger contactor were lower than thresholds with the smaller contactor on the fingertip at high frequencies (63, 125, and 250 Hz), on the large toe (except at 250 Hz), on the heel (at all frequencies), and on the volar forearm at 250 Hz. It is concluded that at least two tactile channels (Pacinian from 63 to 250 Hz, and non-Pacinian from 8 to 31.5 Hz) determined vibrotactile thresholds at the fingertip, whereas non-Pacinian channels had a dominant influence on vibrotactile thresholds at the volar forearm. The role of Pacinian and non-Pacinian channels could not be confirmed at the large toe or the heel despite some evidence of spatial summation.     

Vibrotactile difference thresholds: Effects of vibration frequency,vibration magnitude,contact area,and body location

It has not been established whether the smallest perceptible change in the intensity of vibrotactile stimuli depends on the somatosensory channel mediating the sensation. This study investigated intensity difference thresholds for vibration using contact conditions (different frequencies, magnitudes, contact areas, body locations) selected so that perception would be mediated by more than one psychophysical channel. It was hypothesized that difference thresholds mediated by the non-Pacinian I (NPI) channel and the Pacinian (P) channel would differ. Using two different contactors (1-mm diameter contactor with 1-mm gap to a fixed surround; 10-mm diameter contactor with 2-mm gap to the surround) vibration was applied to the thenar eminence and the volar forearm at two frequencies (10 and 125?Hz). The up-down-transformed-response method with a three-down-one-up rule provided absolute thresholds and also difference thresholds at various levels above the absolute thresholds of 12 subjects (i.e., sensation levels, SLs) selected to activate preferentially either single channels or multiple channels. Median difference thresholds varied from 0.20 (thenar eminence with 125-Hz vibration at 10?dB SL) to 0.58 (thenar eminence with 10-Hz vibration at 20?dB SL). Median difference thresholds tended to be lower for the P channel than the NPI channel. The NPII channel may have reduced difference thresholds with the smaller contactor at 125?Hz. It is concluded that there are large and systematic variations in difference thresholds associated with the frequency, the magnitude, the area of contact, and the location of contact with vibrotactile stimuli that cannot be explained without increased understanding of the perception of supra-threshold vibrotactile stimuli.     

Thermotactile thresholds at the fingertip: Effect of contact area and contact location

Thresholds for the detection of changes in temperature are used to indicate neuropathy, but a variety of different contact areas and contact locations are used. This study was designed to determine the effects of variations in contact area and contact location on both warm and cool thresholds at the fingertip. With 20 healthy subjects (10 females and 10 males aged 20–30 years), warm thresholds and cool thresholds were determined in two separate sessions using the method of limits. In the first part of each session, thresholds were determined around the centre of the whorl using circular contactors with five different diameters (3, 6, 9, 12, and 55 mm). In the second part of each session, thresholds were determined using two contactors (6- and 12-mm diameter) at three locations along the fingertip: (i) distal (5 mm from the nail), (ii) middle (centre of whorl), and (iii) proximal (3 mm from the distal interphalangeal joint). With increasing contact area, the warm thresholds decreased, the cool thresholds increased, and the inter-subject variability in both warm and cool thresholds decreased. Using the 6-mm diameter contactor, warm thresholds were independent of location but cool thresholds increased from distal to proximal locations. It is concluded that temperature sensitivity at the fingertip increases with increasing area of contact, with the variability in thresholds consistent with the existence of warm and cool “insensitive fields”. The findings show that the influence of contact area and contact location should be considered when assessing thermotactile thresholds at the fingertip.     

Vibrotactile thresholds at the sole of the foot: Effect of vibration frequency and contact location

Studies of vibration perception in the glabrous skin of the human hand have identified four mechanoreceptor channels, with each channel showing characteristic variations in thresholds with variations in the frequency of vibration and the area of vibration excitation. To advance understanding of the channels mediating vibration perception on the sole of the foot, this study determined how thresholds depend on the frequency of vibration, the location on the foot (the big toe, the ball of the foot, and the heel), and the gap between a vibrating probe and a fixed surround. Thresholds at the three locations were obtained at the 12 preferred one-third octave centre frequencies from 20 to 250?Hz using a 6-mm diameter probe with both a 10-mm and a 20-mm diameter surround. With the 10-mm surround, the displacement thresholds at all three locations showed flat responses from 20 to 40?Hz. With both the 10-mm and the 20-mm surround, the displacement thresholds at the three locations showed “U-shaped” responses from 40 to 250?Hz. Relative to thresholds obtained with the 20-mm surround, thresholds obtained with the 10-mm surround were lower at the toe and the heel with 20- and 25-Hz vibration, but higher at the ball of the foot with 31.5- to 250-Hz vibration. It is concluded that absolute thresholds for the perception of vibration at the sole of the foot show important variations with location and with contact conditions and tend to be mediated by the NP I channel in the range from about 20 to 40?Hz and the P channel from about 40 to 250?Hz.     

Vibrotactile thresholds at the sole of the foot: effect of vibration frequency and contact location

Studies of vibration perception in the glabrous skin of the human hand have identified four mechanoreceptor channels, with each channel showing characteristic variations in thresholds with variations in the frequency of vibration and the area of vibration excitation. To advance understanding of the channels mediating vibration perception on the sole of the foot, this study determined how thresholds depend on the frequency of vibration, the location on the foot (the big toe, the ball of the foot, and the heel), and the gap between a vibrating probe and a fixed surround. Thresholds at the three locations were obtained at the 12 preferred one-third octave centre frequencies from 20 to 250 Hz using a 6-mm diameter probe with both a 10-mm and a 20-mm diameter surround. With the 10-mm surround, the displacement thresholds at all three locations showed flat responses from 20 to 40 Hz. With both the 10-mm and the 20-mm surround, the displacement thresholds at the three locations showed "U-shaped" responses from 40 to 250 Hz. Relative to thresholds obtained with the 20-mm surround, thresholds obtained with the 10-mm surround were lower at the toe and the heel with 20- and 25-Hz vibration, but higher at the ball of the foot with 31.5- to 250-Hz vibration. It is concluded that absolute thresholds for the perception of vibration at the sole of the foot show important variations with location and with contact conditions and tend to be mediated by the NP I channel in the range from about 20 to 40 Hz and the P channel from about 40 to 250 Hz.     

Relationship between vibrotactile detection threshold in the Pacinian channel and complex mechanical modulus of the human glabrous skin

The goal of this study was to investigate the relationship between the psychophysical vibrotactile thresholds of the Pacinian (P) channel and the mechanical properties of the skin at the fingertip. Seven healthy adult subjects (age: 23–30) participated in the study. The mechanical stimuli were 250-Hz sinusoidal bursts and applied with cylindrical contactor probes of radii 1, 2, and 3.5?mm on three locations at the fingertip. The duration of each burst was 0.5?s (rise and fall time: 50?ms). The subjects performed a two-interval forced-choice task while the stimulus levels changed for tracking the threshold at 75% probability of detection. There were significant main effects of contactor radius and location (two-way ANOVA, values of p?<?0.001). The thresholds decreased as the contactor radius increased (i.e., spatial summation effect) at all locations. The thresholds were lowest near the whorl at the fingertip. Additionally, we measured the mechanical impedance (specifically, the storage and loss moduli) at the contact locations. The storage moduli did not change with the contactor location, but the loss moduli were lowest near the whorl. While the loss moduli decreased, the storage moduli increased (e.g., more springiness) as the contactor radius increased. There was moderate and barely significant correlation between the absolute thresholds and the storage moduli (r?=?0.650, p?=?0.058). However, the correlation between the absolute thresholds and the loss moduli was high and very significant (r?=?0.951, p?<?0.001). The results suggest that skin mechanics may be important for locally shaping psychophysical detection thresholds, which would otherwise be expected to be constant due to uniform Pacinian innervention density at the fingertip.     

Vibrotactile Perception in Finger Pulps and in the Sole of the Foot in Healthy Subjects among Children or Adolescents

Aims

To evaluate vibrotactile perception at different frequencies in fingers and in foot in healthy girls and boys.

Methods

Vibration perception thresholds (VPTs) were measured in 283 healthy (8–20 years), consecutively included, girls (n=146) and boys (n=137); i.e., 269 children after excluding those with diseases or disorders possibly affecting the nervous system. Thresholds were measured in finger pulps of index and little fingers (seven frequencies; 8–500 Hz) and at first and fifth metatarsal head and at heel in the sole of the foot (six frequencies; 8–250 Hz;) using Multi Frequency Tactilometry.

Results

VPTs, divided in six groups by age and gender (i.e., 8–10 years, 11–15 years and 16–20 years), at all three sites in the sole increased with higher frequencies, but without gender differences. Thresholds at 64 and 125 Hz were generally higher at heel compared to metatarsal heads. VPTs in finger pulps of index and little fingers, with no finger differences, had a different pattern with increasing thresholds with frequency, but with lower thresholds at 64 and 125 Hz. Thresholds at lower frequencies were higher in finger pulps, while at higher frequencies VPTs were lower in finger pulps than in the sole of the foot; thus, vibration perception in the sole was better than perception in finger pulps at lower frequencies and opposite at higher frequencies. VPTs were higher among adolescents than in younger children in the foot, while thresholds were lower in the finger pulps among adolescents, particularly in index finger. Thresholds in finger pulps of index and little fingers, particularly at higher frequencies, correlated with each other, which the three sites in the sole also did.

Conclusions

VPTs in fingers and in feet are different as related to frequency in healthy girls and boys. Multi Frequency Tactilometry is a future valuable method to detect neuropathy in children and adolescents.     

Attenuation of vibrotactile spatial summation.

Masked and quiet thresholds at several frequencies of vibratory stimuli were measured as a function of contactor area. The test site was the left index finger; the masking site was the left little finger. The quiet threshold data were consistent with previous investigations: Low-frequency stimuli showed no spatial summation, whereas high-frequency stimuli did. In the presence of a masker, spatial summation was reduced or eliminated for high-frequency stimuli, i.e., the masked threshold was, under some conditions, independent of contactor area. Low-frequency stimuli continued to show no spatial summation in the presence of a masker. The attenuation of spatial summation appears to be a direct function of the intensity of the masking stimulus. Additional measurements with the left thenar eminence as the test site showed that spatial summation could be attenuated by a masker placed on a contralateral body site. The implications of the results for quantifying the effectiveness of a masking stimulus, for the duplex mechanoreceptor hypothesis, and for the nature of spatial summation on the skin are discussed.     

Population-response model for vibrotactile spatial summation

A computational model based on previous physiological and psychophysical data is presented for the human Pacinian (P) psychophysical channel. The model can predict the probability of detection in simple psychophysical tasks, and hence psychometric functions and thresholds. The model simulates stimulating variable and fixed glabrous skin sites with different-sized contactors and includes spatial variation of monkey P-fiber sensitivities. Therefore, it is especially suitable for studying spatial summation, i.e. the improvement of threshold with increasing contactor area. Selective contributions of neural integration (n.i.) and probability summation (p.s.) are also incorporated into the model. Model predictions are compared to psychophysical results of Gescheider et al. (). The performance of the model regarding the effects of contactor size is very good. In addition to predicting approximately 3?dB improvement of thresholds when the contactor area is doubled, the model also reveals nonlinear contributions of p.s. and n.i. Furthermore, the model asserts that thresholds are largely governed by neural integration when small contactors are used. These and other findings discussed in the article show that the presented model is a helpful tool for formulating testable hypotheses. Although the model can also simulate some temporal summation effects, simulation results do not conform well to previous data on temporal response properties. Thus, the model needs to be refined in that respect.     

Population-response model for vibrotactile spatial summation

A computational model based on previous physiological and psychophysical data is presented for the human Pacinian (P) psychophysical channel. The model can predict the probability of detection in simple psychophysical tasks, and hence psychometric functions and thresholds. The model simulates stimulating variable and fixed glabrous skin sites with different-sized contactors and includes spatial variation of monkey P-fiber sensitivities. Therefore, it is especially suitable for studying spatial summation, i.e. the improvement of threshold with increasing contactor area. Selective contributions of neural integration (n.i.) and probability summation (p.s.) are also incorporated into the model. Model predictions are compared to psychophysical results of Gescheider et al. (2005). The performance of the model regarding the effects of contactor size is very good. In addition to predicting approximately 3 dB improvement of thresholds when the contactor area is doubled, the model also reveals nonlinear contributions of p.s. and n.i. Furthermore, the model asserts that thresholds are largely governed by neural integration when small contactors are used. These and other findings discussed in the article show that the presented model is a helpful tool for formulating testable hypotheses. Although the model can also simulate some temporal summation effects, simulation results do not conform well to previous data on temporal response properties. Thus, the model needs to be refined in that respect.     

Thresholds for the perception of hand-transmitted vibration: dependence on contact area and contact location

The detection of vibration applied to the glabrous skin of the hand varies with contact conditions. Three experiments have been conducted to relate variations in the perception of hand-transmitted vibration to previously reported properties of tactile channels. The effects of a surround around the area of contact, the size of the area of contact, the location of the area of contact, the contact force, and the hand posture on perception of thresholds were determined for 8-500 Hz vibration. Removal of a surround around a contact area on the fingertip elevated thresholds of the NP II channel (FA I fibres) at frequencies less than 31.5 Hz and reduced thresholds of the Pacinian channel (FA II fibres) at frequencies greater than about 63 Hz. When no surround was present, thresholds reduced systematically as the contact area increased from the fingertip to the whole hand at frequencies from 16 to 125 Hz, although the decrease was not inversely proportional to the increase in contact area. The results are partly explained by spatial summation in the Pacinian channel (FA II fibres) and the involvement of the NP II channel (SA II) with some influence of biodynamic responses and contact pressures. There were regional differences in sensitivity over the hand within the NP I channel but not within the Pacinian channel: the NP I thresholds (less than 31.5 Hz) decreased from proximal to distal regions of the hand, whereas the Pacinian thresholds (125 Hz) were independent of contact location over the hand.     

Spatial summation in the tactile sensory system: Probability summation and neural integration

Psychophysical thresholds for the detection of a 300-Hz burst of vibration applied to the thenar eminence were measured for stimuli applied to the skin through 1.5?cm² and through 0.05?cm² contactors. Thresholds were approximately 13?dB lower when the area of the contactor was 1.5?cm² than when it was 0.05?cm². The difference between the thresholds measured with the large and small contactors was significantly reduced when only the lowest thresholds obtained in the testing sessions were considered. This result supports the hypothesis that one component of spatial summation in the P channel is probability summation. In addition, threshold measurements within a session were less variable when measured with the 1.5?cm² contactor. We conclude that spatial summation in the P channel is a joint function of two processes that occur as the areal extent of the stimulus increases: probability summation in which the probability of exceeding the psychophysical detection threshold increases as the number of receptors of varying sensitivities increases, and neural integration in which neural activity originating from separate receptors is combined within the central nervous system rendering the channel more sensitive to the stimulus.     

Vibrotactile thresholds of the Non-Pacinian I channel: I. Methodological issues

Thresholds of the Non-Pacinian I (NP I) channel were measured using a two-interval forced-choice paradigm, a technique independent of the subject's criterion. The studies were performed using the terminal phalanx of the human middle finger with a 40-Hz vibratory stimulus. Unlike most of the previous experiments performed in our laboratory, a contactor surround was not used. This was done to enable comparison with population models of mechanoreceptive fibers in the literature. Since the Pacinian (P) channel and NP I channel have similar vibrotactile thresholds at 40?Hz, a forward-masking procedure was used to elevate the thresholds of the P channel with respect to the NP I channel. While it has been established that the Pacinian fibers are entrained at high stimulus levels, the P channel can be perceptually masked using a 250-Hz stimulus presented prior to the 40-Hz test stimulus. The masking functions were found to be approximately linear on log-log axes and the threshold shifts were found to increase as the masking-stimulus levels increased. The results are discussed in relation to previous studies that were performed at various stimulation sites by using a contactor surround or not. A companion paper presents the variation of NP I-channel thresholds, measured using the methods described herein, and addresses the effects of stimulation along the proximo-distal axis of the phalanx. The companion paper also discusses the predictions of a computational model, recently proposed, in light of the empirical results presented.     

Vibrotactile thresholds of the Non-Pacinian I channel: I. Methodological issues

Thresholds of the Non-Pacinian I (NP I) channel were measured using a two-interval forced-choice paradigm, a technique independent of the subject's criterion. The studies were performed using the terminal phalanx of the human middle finger with a 40-Hz vibratory stimulus. Unlike most of the previous experiments performed in our laboratory, a contactor surround was not used. This was done to enable comparison with population models of mechanoreceptive fibers in the literature. Since the Pacinian (P) channel and NP I channel have similar vibrotactile thresholds at 40?Hz, a forward-masking procedure was used to elevate the thresholds of the P channel with respect to the NP I channel. While it has been established that the Pacinian fibers are entrained at high stimulus levels, the P channel can be perceptually masked using a 250-Hz stimulus presented prior to the 40-Hz test stimulus. The masking functions were found to be approximately linear on log-log axes and the threshold shifts were found to increase as the masking-stimulus levels increased. The results are discussed in relation to previous studies that were performed at various stimulation sites by using a contactor surround or not. A companion paper presents the variation of NP I-channel thresholds, measured using the methods described herein, and addresses the effects of stimulation along the proximo-distal axis of the phalanx. The companion paper also discusses the predictions of a computational model, recently proposed, in light of the empirical results presented.     

Independent responses of Pacinian and Non-Pacinian systems with hand-transmitted vibration detected from masked thresholds

This study was designed to identify psychophysical channels responsible for the detection of hand-transmitted vibration. Perception thresholds for vibration (16, 31.5, 63 and 125?Hz sinusoidal for 600?ms) at the distal phalanx of the middle finger and the whole hand were determined with and without simultaneous masking stimuli (1/3 octave bandwidth Gaussian random vibration centered on either 16?Hz or 125?Hz for 3000?ms, varying in magnitude 0 to 30?dB above threshold). At all frequencies from 16 to 125?Hz, absolute thresholds for the hand were significantly lower than those for the finger. Changes in threshold as a function of masker level were used to estimate the thresholds of three psychophysical channels (i.e. P, NP I, and NP II channels). Increased vibrotactile sensitivity of the hand compared to the finger seems to be not entirely due to increased spatial summation via the Pacinian system (P channel); non-Pacinian system (NP I and NP II channels) also contributed to perception. Differing transmission of vibration between the hand and the finger may have also influenced the thresholds.     

Independent responses of Pacinian and Non-Pacinian systems with hand-transmitted vibration detected from masked thresholds

This study was designed to identify psychophysical channels responsible for the detection of hand-transmitted vibration. Perception thresholds for vibration (16, 31.5, 63 and 125 Hz sinusoidal for 600 ms) at the distal phalanx of the middle finger and the whole hand were determined with and without simultaneous masking stimuli (1/3 octave bandwidth Gaussian random vibration centered on either 16 Hz or 125 Hz for 3000 ms, varying in magnitude 0 to 30 dB above threshold). At all frequencies from 16 to 125 Hz, absolute thresholds for the hand were significantly lower than those for the finger. Changes in threshold as a function of masker level were used to estimate the thresholds of three psychophysical channels (i.e. P, NP I, and NP II channels). Increased vibrotactile sensitivity of the hand compared to the finger seems to be not entirely due to increased spatial summation via the Pacinian system (P channel); non-Pacinian system (NP I and NP II channels) also contributed to perception. Differing transmission of vibration between the hand and the finger may have also influenced the thresholds.     

Time course and action spectrum of vibrotactile adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation. In Experiment 2, test stimuli of 10 Hz and 50 Hz were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min. In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.     

Spatial summation in the tactile sensory system: probability summation and neural integration

Psychophysical thresholds for the detection of a 300-Hz burst of vibration applied to the thenar eminence were measured for stimuli applied to the skin through 1.5 cm2 and through 0.05 cm2 contactors. Thresholds were approximately 13 dB lower when the area of the contactor was 1.5 cm2 than when it was 0.05 cm2. The difference between the thresholds measured with the large and small contactors was significantly reduced when only the lowest thresholds obtained in the testing sessions were considered. This result supports the hypothesis that one component of spatial summation in the P channel is probability summation. In addition, threshold measurements within a session were less variable when measured with the 1.5 cm2 contactor. We conclude that spatial summation in the P channel is a joint function of two processes that occur as the areal extent of the stimulus increases: probability summation in which the probability of exceeding the psychophysical detection threshold increases as the number of receptors of varying sensitivities increases, and neural integration in which neural activity originating from separate receptors is combined within the central nervous system rendering the channel more sensitive to the stimulus.     

Time Course and Action Spectrum of Vibrotactile Adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation.

In Experiment 2, test stimuli of 10 H_z and 50 H_z were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min.

In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.