Miyuki Morioka

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.

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-mm2 excitation area), and (ii) a 6-mm diameter circular probe with a 2-mm gap to a fixed circular surround (i.e., 79-mm2 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.

Difference thresholds for intensity perception of whole-body vertical vibration: effect of frequency and magnitude

Difference thresholds for seated subjects exposed to whole-body vertical sinusoidal vibration have been determined at two vibration magnitudes [0.1 and 0.5 ms(-2) root mean square (r.m.s.)] and at two frequencies (5 and 20 Hz). For 12 subjects, difference thresholds were determined using the up-and-down transformed response method based on two-interval forced-choice tracking. At both frequencies, the difference thresholds increased by a factor of five when the magnitude of the vibration increased from 0.1 to 0.5 ms(-2) r.m.s. The median relative difference thresholds, Weber fractions (deltaI/I), expressed as percentages, were about 10% and did not differ significantly between the two vibration magnitudes or the two frequencies. It is concluded that for the conditions investigated the difference thresholds for whole-body vibration are approximately consistent with Webers Law. A vibration magnitude will need to be reduced by more than about 10% for the change to be detectable by human subjects; vibration measurements will be required to detect reductions of less than 10%.

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.

Equivalent comfort contours for vertical vibration of steering wheels: Effect of vibration magnitude, grip force, and hand position

Vehicle drivers receive tactile feedback from steering-wheel vibration that depends on the frequency and magnitude of the vibration. From an experiment with 12 subjects, equivalent comfort contours were determined for vertical vibration of the hands at two positions with three grip forces. The perceived intensity of the vibration was determined using the method of magnitude estimation over a range of frequencies (4-250 Hz) and magnitudes (0.1-1.58 ms(-2) r.m.s.). Absolute thresholds for vibration perception were also determined for the two hand positions over the same frequency range. The shapes of the comfort contours were strongly dependent on vibration magnitude and also influenced by grip force, indicating that the appropriate frequency weighting depends on vibration magnitude and grip force. There was only a small effect of hand position. The findings are explained by characteristics of the Pacinian and non-Pacinian tactile channels in the glabrous skin of the hand.

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-mm2 area) with a 1-mm gap to a fixed circular surround (i.e., 7.1-mm2 excitation area), and (ii) a 6-mm diameter circular probe (28-mm2 area) with a 2-mm gap to a fixed circular surround (i.e., 79-mm2 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.

Difference thresholds for the perception of whole-body vertical vibration: dependence on the frequency and magnitude of vibration

When seeking to reduce vibration in transport it is useful to know how much reduction is needed for the improvement to be noticeable. This experimental study investigated whether relative difference thresholds for the perception of whole-body vertical vibration by seated persons depend on the frequency or magnitude of vibration. Relative difference thresholds for sinusoidal seat vibration were determined for 12 males at three vibration magnitudes and eight frequencies (2.5, 5, 10, 20, 40, 80, 160, 315 Hz) using the three-down-one-up method in conjunction with a two-interval-forced-choice procedure. The median relative difference thresholds were in the range 9.5% to 20.3%. There appeared to be a frequency-dependence at the lowest vibration magnitude, such that higher frequencies had higher difference thresholds. The relative difference thresholds depended on the vibration magnitude only at 2.5 and 315 Hz. The influence of both vibration frequency and vibration magnitude on the measured difference thresholds suggests that vision (at 2.5 Hz) and hearing (at 315 Hz) contributed to the perception of changes in vibration magnitude.

Frequency dependence of perceived intensity of steering wheel vibration: effect of grip force

Vehicle drivers receive haptic feedback in response to their movement of the steering wheel and tactile feedback from various sources of vibration of the steering wheel, with the sensations varying depending on the frequency and the magnitude of the movements. From an experiment with 12 subjects, equivalent comfort contours were determined for vertical vibration of the hands with three grip forces. The perceived intensity of vibration on a rigid steering wheel was determined using the method of magnitude estimation at seven frequencies (4 to 250 Hz) over a range of vibration magnitudes (0.1 to 1.58 ms- 2 r.m.s). The comfort contours strongly depended on vibration magnitude, indicating that a frequency weighting for predicting sensation should be dependent on vibration magnitude. At low magnitudes, increased grip force increased sensitivity at high frequencies and enhanced the frequency-dependence of the equivalent comfort contours. The results may be explained by the characteristics of the Pacinian and non-Pacinian tactile channels in the glabrous skin of the hand

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.

Thresholds for the perception of fore‐and‐aft, lateral and vertical vibration by seated persons

Vibration experienced in transport and in buildings can yield discomfort or annoyance if the vibration exceeds the threshold for vibration perception. Knowledge of thresholds makes it possible to determine which frequencies and directions of low magnitude vibration give rise to perception. The effect vibration frequency (2 to 315 Hz) on absolute thresholds for the perception of whole‐body vibration has been determined experimentally with 12 seated persons for each of the three axes of excitation (fore‐and‐aft, lateral and vertical). The frequency‐dependence of the thresholds differed between the three axes. At frequencies, greater than 10 Hz, sensitivity was greatest for vertical vibration. At frequencies less than 3.15 Hz, sensitivity was greatest to fore‐and‐aft vibration. In all three axes, the acceleration threshold contours at frequencies greater than 80 Hz were U‐shaped, suggesting the same psychophysical channel mediated high frequency thresholds for fore‐and‐aft, lateral and vertical vibration. It...