Absolute thresholds for the perception of fore-and-aft, lateral, and vertical vibration at the hand, the seat, and the foot

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Abstract

Oscillatory motions of handles, seats, and floors produce complex patterns of sensations in the body with the detection of these motions dependent on the sensitivity of the body to the applied vibration. This study examined the effect of input location (the hand, the seat, and the foot) and vibration frequency (8–315 Hz at the hand and foot; 2–315 Hz at the seat) on absolute thresholds for the perception of vibration in each of three axes (fore-and-aft, lateral, and vertical). Perception thresholds were determined with 96 males aged 20–29 years divided into eight groups of 12 subjects; each group received vibration at either the hand, the seat, or the foot in one of the three axes (one group experienced both lateral and vertical vibration at the hand). A frequency dependence in the thresholds was apparent for each of the three directions at each of the three locations; U-shaped acceleration threshold contours at frequencies greater than 80 Hz suggest the same psychophysical channel-mediated high-frequency thresholds at the hand, the seat, and the foot. Among the nine axes, sensitivity was greatest for vertical vibration at the seat at frequencies between 8 and 80 Hz, whereas sensitivity was greatest for vertical vibration at the hand at frequencies greater than 100 Hz. Absolute thresholds for the perception of vibration at the hand, the seat, and the foot are not consistent with the relevant frequency weightings in current standards.

Introduction

In transport, at workplaces, and during leisure and domestic activities, vibration is felt via the hands, the seat, the back, and the feet. Discomfort, annoyance, or interference with activities may occur if the vibration exceeds the threshold for the perception of the vibration. When there is more than one vibration input to the body (e.g. at the hands, the seat, and the feet), the sensation is most easily detected at the location with greatest sensitivity. Knowledge of differences in the thresholds of perception for vibration between the hand, the seat, and the feet should assist the identification of sources of disturbance caused by vibration.

Thresholds for the perception of vibration have been determined in studies of hand-transmitted vibration [1], [2], [3], [4], [5] and in studies of whole-body vibration with seated [6], [7], [8], [9] and standing subjects [6], [7]. However, there has been little investigation of perception thresholds for the foot resting on a vibrating surface. The thresholds of hand-transmitted vibration and whole-body vibration reported in previous studies are not easily compared, partly due to the use of different experimental techniques (e.g. different ranges of frequency, different psychophysical methods, different sitting postures, etc.).

The detection of hand-transmitted vibration mainly involves the somatosensory mechanoreceptive (tactile) channels, often classified as Pacinian (P) and non-Pacinian (NP) channels. The P channel is associated with Pacinian corpuscles (FA II) that provide sensations at high frequencies of vibration (e.g. >40–50 Hz) and summate over the stimulus duration and over the excitation area, known as ‘temporal summation’ and ‘spatial summation’, respectively [10], [11]. The NP channels include the Meissner corpuscles, Merkel disks, and Ruffini endings (i.e. FA I, SA I, and SA II, respectively), and show enhanced sensitivity with increasing stimulus gradients at frequencies less than about 40 Hz [12], [13]. With vibrotactile stimuli (vibration perceived at the fingertip or thenar eminence of the hand), a four-channel model of vibrotactile perception has been proposed [14], [15]. For vibration applied over the entire hand, the identification of the channels responsible for the detection of hand-transmitted vibration has been attempted by Morioka and Griffin [16] who concluded that at least three channels (Pacinian, NP I, and NP II channels) may be involved in detecting hand-transmitted vibration. For the detection of whole-body vibration, several sensory systems are expected to be involved, including the visual, vestibular, auditory, and somatosensory senses [17].

This study was designed to determine differences in the perception of vibration at the hand, the seat, and the feet while these body parts are in contact with vibrating surfaces in a manner similar to that in transport, work, leisure, and domestic activities. Absolute thresholds for perception of vibration were determined to examine the effects of vibration frequency (8–315 Hz for the hand and foot; 2–315 Hz for the seat), vibration direction (fore-and-aft, lateral, and vertical), and input location (the hand, the seat, and the foot) on absolute thresholds for the perception of vibration. There was no backrest and so thresholds were not influenced by the vibration of a surface in contact with the back. The perception thresholds have been presented previously for hand-transmitted vibration [18] and whole-body vibration [19], in experimental studies determining equivalent comfort contours. In this paper, the frequency dependence of vibration perception thresholds at the seat and foot is compared with those at the hand, so as to assist understanding of the mechanisms involved in the detection of vibration at different body locations.

Section snippets

Subjects

The experiment was carried out with a total of 9 conditions (3 axes×3 body locations). Each experimental condition was completed within a session lasting about 1 h. Eight groups of 12 males (total of 96) aged between 20 and 29 years participated in the experiment. Subjects in each group attended a single experimental session determining perception thresholds for fore-and-aft, lateral, or vertical vibration either at the hand, seat, or foot (except subjects in Group B attended sessions with both

Effect of frequency

The median absolute thresholds and the inter-quartile range (25–75th percentiles) of the 12 subjects determined for the hand, the seat, and the foot in each of the three axes of vibration (fore-and-aft, lateral, and vertical) are presented as a function of vibration frequency in Fig. 2. Threshold contours determined from other studies are overlaid for comparison.

With vibration at the hand, the acceleration perception thresholds in all three axes were highly dependent on vibration frequency

Factors influencing the measured perception thresholds

In the present studies, forces between the body and the sources of vibration were not controlled at specific values but the subjects were instructed to maintain specific body postures (sitting upright with their feet supported while grasping handles with forces that they felt most comfortable). Variations in force or pressure at the point of contact with vibration may alter perception thresholds. For the Pacinian channel, the threshold of perception for vibration of a small circular probe

Discussion

The ways in which the perception thresholds in the present results depend on the frequency of vibration are broadly similar to those determined in other studies with vibration of the hand [1], [2], [3], [4], [5] and the seat [6], [7], [8], [9], as seen in Fig. 2. The higher thresholds for hand-transmitted vibration from Reynolds et al. [2] may be partly due to the use of different psychophysical methods. The present study employed a staircase method in conjunction with a ‘yes–no’ procedure in

Conclusions

Perception thresholds for vibration of the hand, the seat, and the foot are highly frequency dependent. Sensitivity to vibration also differs between the three locations. Thresholds for the hand suggest that at frequencies greater than about 20 Hz, perception is mediated by the Pacinian channel. A similar frequency dependence for thresholds at the hand, the seat, and the foot suggests the Pacinian channel may mediate thresholds in all three axes at frequencies greater than about 80 Hz. The

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