Detection of tactile stimuli. Thresholds of afferent units related to psychophysical thresholds in the human hand.

Abstract

Psychophysical thresholds were determined at 162 points in the glabrous skin area of the human hand when slowly rising, triangular indentations of controlled amplitudes were delivered with a small probe. Constant stimuli was used with either the 2 alternative forced choice or the yes-no procedure. The distribution of the psychophysical thresholds varied with the skin region. Nerve impulses were recorded from single fibers in the median nerve of subjects with percutaneously inserted tungsten needle electrodes. The thresholds of 128 mechanosensitive afferent units were determined when stimuli were delivered to partly the same points as stimulated for the assessment of the psychophysical thresholds. In the region of low psychophysical thresholds there was good agreement between the thresholds of the rapidly adapting and Pacinian corpuscle units and the psychophysical thresholds, particularly at the lower ends of the samples. In the skin regions of high thresholds practically all psychophysical thresholds were higher than the thresholds of the most sensitive afferent units. Simultaneous recording of nerve impulses during a detection task indicated that subjects did not detect stimuli strong enough to elicit several impulses in afferent units in this region. Detection was dependent on 1 impulse in 1 or a few rapidly adapting units under optimal conditions in the region of low psychophysical thresholds, whereas it seemed unlikely that activity in Pacinian corpuscle units was crucial. Humans are able to detect an input consisting of a single impulse in a single rapidly adapting unit.