Microstimulation of single tactile afferents from the human hand. Sensory attributes related to unit type and properties of receptive fields.

Tungsten microelectrodes were inserted percutaneously into the median nerve of alert human subjects for recording and stimulation of single nerve fibres. Impulses from mechanoreceptive units in the glabrous skin of the hand were recorded and single afferents were characterized with respect to unit type (FA I, FA II, SA I, and SA II), as well as size and shape of receptive field, and force threshold. The electrode was then reconnected to an electrical stimulator and short pulse trains (0.25 to 0.5 s, 20 to 100 Hz) were delivered at successively increasing current intensity, while the subject was asked to report any sensation that he noticed in the hand. The first sensation was always that of a localized skin deformation within a small area, typically 2 to 3 mm in diameter, often coinciding with the receptive field of the recorded unit. Spatial matching was also found in many cases for the size, shape and orientation of the perceptive and receptive fields, strongly suggesting that the sensation was accounted for by the recorded unit that had been selectively activated by the current pulses. There were clear differences between group data associated with the four types of units with regard to the quality of the percepts. Vibratory sensation was reported with all FA II units and was common with FA I units, whereas a sustained indentation was often associated with SA I units. Indirect evidence suggested that activation of SA II units usually did not elicit a sensation. It was confirmed that a single impulse in a single FA I unit may elicit a sensory response in the attending subject, whereas a much larger input was required from SA I units, which are also less sensitive to mechanical stimuli. This was one of several findings supporting the impression that differential receptive properties, even within a group of afferents, were associated with different sensory responses. It was concluded that a train of impulses in a single tactile unit may produce within the brain of the subject a construct which specifies with great accuracy the skin area of the unit's terminals as well as a tactile subquality which is related to unit properties.