Can tiny bumps help fingers detect a surface slipping?

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Researchers investigated how the microgeometry of flat surfaces affects slip detection on human and monkey fingerpads. Using a servo-controlled tactile stimulator, they pressed and stroked glass plates against passive human fingerpads to test whether people could distinguish the direction of friction-induced skin stretch and detect slip, with or without micrometer-sized surface features. To identify the associated peripheral neural codes, they recorded responses to the same stimuli from individual low-threshold mechanoreceptive afferent fibers supplying the fingerpads of anesthetized macaque monkeys.

Humans could not detect a smooth glass plate slipping on the fingerpad. They could perceive the direction of skin stretch from information carried by slowly adapting afferents that respond differently to different stretch directions. Stretch direction signaled the direction of impending slip, but perceiving relative motion between the plate and finger required detectable surface features.

Barely detectable micrometer-sized protrusions enabled slip detection: depending on their geometry, they exclusively activated rapidly adapting fibers of either the Meissner (RA) or Pacinian (PC) type. A smooth plate bearing a single raised dot, 4 micrometers high and 550 micrometers in diameter, sequentially activated neighboring RA fibers along the dot's path, providing a reliable spatiotemporal code. Stroking a plate with a fine, homogeneous matrix of dots, each 1 micrometer high and 50 micrometers in diameter and spaced 100 micrometers center-to-center, caused fingerpad vibrations that activated only PC fibers and produced an intensive code.

The findings show that humans detect surprisingly small features on smooth surfaces, allowing them to detect slip, and that feature geometry governs the associated neural codes. When surface features exceed the response thresholds of all the receptors, redundant spatiotemporal and intensive information is available for slip detection.