SOME SHEAR FACTS AND PURE FRICTION RELATED TO ROUGHNESS DISCRIMINATION AND THE CUTANEOUS CONTROL OF GRASPING

The question of whether friction contributes to the perception of roughness has been overdebated and underinvestigated. A review of the psychophysical literature suggests that roughness and friction can be subjectively distinguished very effectively, although the same rapidly adapting Meissner corpuscles (RA1s) and slowly adapting Merkel receptors (SA1s) are stimulated by both stimuli. It appears that to achieve the subjective appreciation of roughness, the brain must learn to ignore variations in the speed of movement over the skin, the perpendicular force applied to the receptor surface, and the shear forces tangential to the skin generated by friction. Similarly, the subjective appreciation of slipperiness requires selective attention to tangential forces to the exclusion of speed, perpendicular force, and surface texture. A clearer picture is gradually emerging concerning the detection and appreciation of shear forces from investigations of the grasping and lifting of objects of different surfaces against the force of gravity. Although high shear forces provoke larger responses in almost all skin mechanoreceptors, some neurons in both the sensory and motor cortex discharge more vigorously with smooth textures and lower coefficients of friction. Although populations of such neurons sensitive to smooth surfaces and low friction would be very useful in detecting both potential and real slips, just how the afferent signals are derived remains puzzling.