Vibration intensity difference thresholds

Abstract

The intensity difference threshold is defined as ‘the difference in the intensity of two stimuli which is just sufficient for their difference to be detected’.
The aim of this thesis is to advance understanding of the perception of vibration intensity differences in humans. In addition to increasing understanding of the tactile senses, knowledge of difference perception could inform various applications such as the optimisation of the vibration characteristics of vehicles and the design of human–machine interfaces involving communication via the sense of touch.
Absolute thresholds for the perception of vibration in the glabrous skin have been modelled by ‘channels’ within the somatosensory system that predict the effects of vibration frequency, vibration magnitude, vibration duration and contact conditions. Difference thresholds are less well understood and there is little knowledge of their dependence on vibration characteristics and contact conditions.
In this thesis, psychophysical methods were employed to determine the difference thresholds with various input conditions (whole-body vibration, foot-transmitted vibration, grasping a vibrating handle, and localised excitation of the hand and the forearm). Five experiments investigated the dependence of difference thresholds on vibration magnitude, vibration frequency, the responses of the somatosensory channels of the skin (especially the Pacinian and the non-Pacinian I channels) as well as the location of the vibration input, information from other sensory systems, and the presence of masking vibration.
The first experiment tested the hypothesis that relative difference thresholds (i.e. the percentage change in vibration magnitude required for the change to be detected) for vertical whole-body vibration depend on the frequency and magnitude of the vibration. Relative difference thresholds were found to be independent of vibration magnitude except at the lowest frequency (2.5 Hz) and the highest frequency (315 Hz), where the change in motion may have been perceived by vision and hearing, respectively. The second and third experiments investigated the dependence of difference thresholds on the frequency and magnitude of hand-transmitted vibration and foot-transmitted vibration. The experiments produced similar results, with difference thresholds independent of the frequency of vibration and only dependent on the magnitude of vibration at 125 Hz, where higher magnitudes (18 dB sensation level and above) produced greater relative difference thresholds. The fourth experiment tested the hypothesis that a low-magnitude low-frequency masking vibration (at 16 Hz) would not affect high-frequency difference thresholds (at 125 Hz). It was found that the lowfrequency masker only increased difference thresholds when its magnitude was greater than 12 dB SL. The final experiment with localised vibration at the hand and arm tested the hypothesis that NPI and P channels have different relative difference thresholds. Overall, there was no significant difference between the relative difference thresholds of vibration mediated by the NPI channel (at 10 Hz) and the P-channel (at 125 Hz), but the relative difference thresholds of the P-channel tended to be lower than those of the NPI-channel, as in experiments II and III.
Depending on the test conditions, the median unmasked relative difference thresholds were in a range from 0.1 to 0.6. There was a tendency for the relative difference thresholds to decrease with increasing contact area, with whole-body vibration producing the smallest relative difference thresholds and localised vibration producing the greatest relative difference thresholds. From the results of all five experiments, it was concluded that excitation area and cues from other senses were more likely to cause relative difference thresholds to depend on the frequency and magnitude of vibration, than any differences in discrimination capability between the P and NPI channels. Other findings include a possible reduction in the discrimination capability of the P-channel with increasing magnitude of vibration (in Experiments II and III) and the suggestion of lower relative differencethresholds for the NPII channel (in Experiment V).

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Identifiers

ORCID for M.J. Griffin: orcid.org/0000-0003-0743-9502

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