-- KarlinBark - 6 July 2004
Reading some papers on tangential forces vs. normal forces applied to the human skin- Biggs, Srinivasan from MIT,tangential versus normal displacements of skin: relative effectiveness for producing tactile sensations?

At the forearm, subjects were more sensitive to tangential forces than normal force. However, at the fingerpad, sensitivity to tangential forces was lower than sensitivity to normal force, due to the approximately five-fold greater stiffness of the fingerpad to tangential traction...therefore, when an actuator is limited in terms of displacement, then a tangential stimulation is a superior choice. When an actuator is limited in terms of force, tangential stimulation is the superior choice for the hairy skin (forearm) but normal stimulation is the better choice on the fingerpad. -Srinivasan, Biggs, 2002

Subjects can discern the direction of a tangential displacement of the fingerpad- perceived intensity of this increases linearly with increasing tangential force. SA's signal the direction of skin stretch and hence the direction of impending slip; RA's and PC's signal the occurrence of slip with spatiotemporal or intensive codes, - Srinivasan Whitehouse, Lamott, 1990

Neurophysiological recordings in monkeys have shown that rapidly adapting type I and type II (RAI and RAII) afferents, as well as slowly adapting type I (SAI) afferents, signal skin stretch when a smooth glass plate is translated tangential to the skin surface with or without slip. However, only the SAI afferents respond with a sustained discharge outlasting the initial stretching phase and exhibit a distinct directional bias. In contrast, both RAI and RAII afferents are silent even if slips occur after the initial skin stretch - (Srinivasan et al. 1990)

A probe was glued to the forearm skin and 1 mm excursions parallel to the skin surface were given in the lateral-medial or proximal-distal directions, the subjects discriminated opposite directions of skin stretch - Gould, et al, 1979

There was also a study on the ability of human subjects to scale different levels of tangential force applied to the finger. Results showed that most human subjects were able to scale both tangential and normal forces applied to the tip of their finger The magnitude estimates were unaffected by either a threefold increase in the rate of force application or a range of normal forces applied concurrently with the tangential force. This study is a bit dodgy though?some parts of the testing were not explained in great detail, and Li and I have some doubts as to whether or not we can take what this paper concludes to be absolutely true. - Pare, 2001

This suggests that the peripheral cutaneous receptors can encode the mechanical shear force applied to the skin through the resulting stretching of the skin. Furthermore, the results of the present study as well as those of Smith and Scott (1996) suggest that the shear forces applied to the skin can be evaluated independently of the normal forces applied simultaneously with the tangential force. (same suspicions as above) -Pare, 2001

When the forearm was indented 1 mm with the 13-mm diameter disc at a rate of 0.2 mm/ set, the stimuli were sometimes not felt. The primary finding in this study is that the perceived depth of a skin indentation is not greatly altered by changes in the velocity with which the skin is indented. The fact that cutaneous indentation sensations are largely independent of velocity allows a more accurate judgment of indentation depth than would be possible otherwise. However, the adequacy of this rate compensation varies with stimulus conditions, failing most strikingly when a stimulus is sufficiently slow and shallow that it is not felt at all. On the dorsal surface of the forearm a 7-mm diameter stimulator which indents the skin 1 mm at 0.2 mm/set is not reliably sensed (Horch et al., 1975). -P. R. BURGESS, 1983

Estimates of displacement have been found to be less affected by velocity than are estimates of intensity (Burgess et al. 1983)