Intraneural microstimulation in man. Its relation to specificity of tactile sensations.

Recent advances in knowledge, and controversy, concerning cutaneous mechanoreception are reviewed. In particular, the question of perceptive specificity is discussed in the light of new experimental approaches, namely microneurography and microstimulation of identified tactile units from the glabrous skin of the human hand. Additional data are also presented. It is concluded that the human brain has an exquisite capacity to detect, localize, delineate, and classify sensations from the input of individual tactile units in the glabrous skin of the hand. The physiological specificity of low-threshold mechanoreceptors in the hand, which has been well documented in previous studies, can now be linked to distinct attributes of very simple tactile sensations which subjects report when a single afferent is stimulated electrically through an intraneural microelectrode. This conclusion does not conflict with the concept that the brain normally makes use of temporal and spatial patterns of impulses from a large number of receptors of various types to form more complex tactile percepts, such as in the recognition of texture, but the microstimulation data demonstrate that particular patterns of nerve impulses are not a necessary determinant of the subjective quality attribute of a simple tactile sensation. On the contrary, the sensations elicited by a train of impulses in a single afferent unit are remarkably distinct and well characterized in a number of respects. A further conclusion is that the afferent inputs are processed differently in tactile subsystems. This observation warrants caution in generalizing results from one system into a common theory of the origin of sensory modality.