Under which conditions do the skin and probe decouple during sinusoidal vibrations?

Previous experiments performed on monkey and human fingertips suggested that the skin surface and stimulus probe decouple for sinusoidal displacements applied perpendicularly to the skin surface. From these observations, it was concluded that sinusoidal vibration may not be a suitable stimulus for understanding and modeling the tactile system. We repeated these experiments on human observers using stimulus frequencies ranging from 0.5 to 240 Hz and with displacement amplitudes up to 1 mm peak-to-peak (p-p). The skin and probe movements were measured in the steady-state using stroboscopic illumination and video microscopy. Contrary to previous conclusions, we found that decoupling did not occur for amplitudes less then 0.25 mm p-p, regardless of stimulus frequency. Decoupling was only observed for stimulus amplitudes greater than 0.25 mm over the stimulus-frequency range investigated. To further investigate this effect, a modified stimulus contactor was used, which permitted the measurement of the skin's movement using reflected light. Measurements were made on both the index fingertip and the thenar eminence. Regardless of body site, no decoupling between the skin and stimulus probe was observed for frequencies ranging from 20 to 100 Hz up to displacements of 0.25 mm p-p. These levels are well within the range used in most human psychophysical experiments performed on these parts of the body. We conclude that sinusoidal vibration can be used reliably to stimulate the tactile system and is an appropriate stimulus for developing models of touch.