Psychophysical investigation of the perception of hand-transmitted vibration

Abstract

It is desirable to reduce the unwanted vibration from some machinery but retain the wanted (i.e. useful) vibration for the vibrotactile perception of some stimuli. This thesis seeks to understand the sensations produced by hand-transmitted vibration. Four types of receptor that innervate the glabrous skin of the hand are thought to be responsible for mediating tactile stimuli; they are referred to as either Pacinian or non-Pacinian receptors. A literature review revealed the known characteristics of the Pacinian and non-Pacinian receptor systems. The sensitivity of the Pacinian system changes systematically with changes of contact area, stimulus duration, skin temperature and age, whereas the non-Pacinian systems are less dependent on contact area and stimulus duration, but dependent on stimulus gradients delivered to the skin. A series of eight psychophysical experiments has been conducted in the laboratory to investigate the perception of hand-transmitted vibration at both threshold and supra-threshold levels so as to investigate factors influencing perception. Four studies determined absolute perception thresholds (minimum magnitude of stimuli that could be perceived by subjects), examining variables influencing absolute thresholds (i.e. the psychophysical method, the contact position and posture, contact area, contact force, contact location). Significant decreases in thresholds due to increasing contact area (extending from the fingertip to the whole hand) were found. Absolute thresholds were also influenced by stimulus frequency, the psychophysical method and contact conditions, but were similar for two different hand postures (grasping a handle and pressing a plate). The results were partially explained by the reported response characteristics of the Pacinian and the non-Pacinian systems. Three further studies investigated sensations at supra-threshold levels. A study investigated the variation in the localisation of sensations with varying contact areas; this study investigated whether the dynamic responses of the hand influenced psychophysical ratings of tactile sensitivity. Sensation magnitudes were also determined using the method of magnitude estimation. The results indicated that the frequency dependence of the sensation magnitude was affected by contact area, showing a spatial summation effect. Nevertheless, a study of difference thresholds (minimal change of stimulus magnitude required for subjects to notice a change) showed no frequency dependence between 8 and 500 Hz at either of two reference magnitudes (2 and 5 ms'^ r.m.s.). Identification of independent responses via Pacinian and non-Pacinian was undertaken in the final study so as to provide further understanding of the experimental results. The experiment involved the determination of masked thresholds (a threshold shift caused by another vibration stimulus) at the fingertip and at the whole hand. Thresholds for four pure tone test frequencies (16, 31.5, 63 and 125 Hz) in the presence of each of two Va-octave bandwidth maskers (at 16 or 125 Hz) were determined at magnitudes from 0 to 30 dB above the absolute threshold of each masker. Independent responses from the Pacinian receptors and the non-Pacinian receptors were obtained from the masking functions. It was concluded that contact area is the most important factor modifying sensitivity of the Pacinian system and that this is responsible for the whole hand having lower vibrotactile thresholds than the finger. Moreover, there seem to be three independent receptors within the Pacinian and the non-Pacinian systems (i.e. FA II, FA I and possibly SA II receptors) combining to detect hand transmitted vibration, with differing sensitivities over the frequency range of interest. The findings from the series of studies extend understanding of the perceptual characteristics of the Pacinian and non-Pacinian systems that are excited by hand-transmitted vibration.

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