The sense of flutter-vibration: comparison of the human capacity with response patterns of mechanoreceptive afferents from the monkey hand.

Abstract

Human thresholds for the perception of oscillatory movement, when sine wave stimuli are delivered to the glabrous skin of the hand were determined over the frequency range 5 to 300 per sec. The double-limbed function and its dissociation by cutaneous anesthesia indicated that it depends upon input in 2 distinct sets of primary afferent fibers, one innervating the glabrous skin and sensitive over the range 5-40 cps, the 2nd terminating in subcutaneous tissue and sensitive in the range 60 to 300 per/sec. In an exactly similar experimental paradigm, the frequency sensitivity of the 2 relevant sets of myelinated mechanoreceptive afferents innervating the monkey''s hand were measured, by study of single fibers isolated by microdissection of the median nerve. For each, in the appropriate frequency range, the response changes with intensity from a zone of demultiplication to a tuning point, at which one impulse is discharged with each cycle of the stimulus, A tuning curve can be constructed for each fiber, over the range of frequencies for which tuning occurs. Those of the quickly adapting fibers innervating the glabrous skin overlap the low frequency of the human function, those of the pacinian afferents its high frequency end. Thus the sense of flutter-vibration is dual, and is served by 2 distinct types of afferents with overlapping sensitivity ranges. Consideration is given to the problem of the central mechanisms of frequency and amplitude discrimination of oscillatory stimuli on the basis of the available afferent input.