A detailed understanding of tactile perception is important to the investigation of multimodal human interaction with real or virtual environments. For vibration frequencies in the range 200-300 Hz, tactile thresholds at most body sites are determined by the response Pacinian corpuscles [1,2]. In general, it appears that vibrotactile thresholds are higher for areas with lower densities of mechano-receptors, such as the torso or forearm, and lower for areas with higher densities of receptors, such as the fingertip and palm. The hypothesis in this study was that this relation should be valid when comparing localised stimulation sites within a single fingertip.

MRI investigation

Identified locations within the right index distal phalange. The U-shaped lines indicate the outlines of the finger. The inset shows a single slices from the 3D MRI data set.

Images of the index fingertip were acquired from two subjects, ages 22 and 24 years.
Imaging was performed with a Philips whole-body imager at 1.5 T. A fat-suppression MRI technique was used to produce 3D data sets with a slice thickness of 140 µm and an in-plane resolution of 140 µm × 140 µm.
The objects identified as Pacinian corpuscles are clustered off the midline of the fingertip, in close proximity to the expected locations of the digital nerves, matching the distributions described by Stark et al. [3].

Psychophysics investigation

Tactile stimulator for two point threshold measurements

The vibrotactile stimulator, showing the curved surface on which the finger rests and the two stimulation sites within this surface;

Psychometric curves - the full black line shows mean data from the side of the finger and the full grey line shows mean data from centre of the finger pad. The dotted lines indicate the ranges of standard error.

A purpose-built vibrotactile stimulator was used to measure the detection threshold on the right index finger.
Detection threshold (defined as 50% detection rate) was measured for 14 young-adult subjects.
The mean threshold at the side of the finger was ~ 1.0 µm and at the midline was ~ 2.0 µm.
The mean difference in threshold (defined as 50% detection rate) between the two sites was determined to be (5.4 ± 1.8) dB.

Conclusions

The MRI results indicate a non-uniform distribution of Pacinian receptors in the fingertip, with a lower density towards the centre of the fingerpad than toward the sides. The psychophysics results indicate a non-uniform vibrotactile sensitivity over the fingertip, with a higher threshold on the midline and a lower threshold toward the side. This lends support to the hypothesis that higher receptor density is associated with lower threshold, and vice versa.

References

Bolanowski, S.J., Gescheider, G.A., Verrillo, R.T., Checkowsky, C.M. Four channels mediate the mechanical aspect of the sense of touch. J. Acoust. Soc. Am., 84, 1680-1694. (1988)
Löfvenberg, J., Johansson, R.S. Regional differences and interindividual variablity in sensitivity to vibration in the glabrous skin of the human hand. Brain Research, 301, 65-72. (1984)
Stark, B., Carlstedt, T., Hallin, R.G., Risling, M. Distribution of human Pacinian corpuscles in the hand. A cadaver study. J. Hand Surg. [Br.], 23, 370-372.(1998)

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