Multi-unit activity in sensory fibers is related to intensity of sensation evoked by air-puff stimulation of the glabrous hand in man.

An analysis of a multi-unit activity (MUA) recorded from the median nerve at the wrist was undertaken to study intensity coding in the peripheral somatosensory system. Brief air-puff stimuli were applied over the glabrous hand to obtain neural and psychophysical responses. The detection threshold (So) was determined and 6 above-threshold stimulus intensities (So + 0.25 kg/cm2, So + 1.25 kg/cm2, So + 2.50 kg/cm2, So + 3.75 kg/cm2, So + 5.00 kg/cm2, and So + 6.25 kg/cm2) were adopted for magnitude estimation. MUA was recorded for the 6 stimulus intensities and for the threshold (So) level. Response duration, number and frequency of the MUA were evaluated as candidate coding parameters for stimulus intensity. A simple power function with an exponent of 1.03 provided an adequate description of the magnitude estimation function. Similarly, stimulus-response functions of the 3 parameters defining the MUA were well represented by straight lines in double logarithmic plots, with the function of the MUA number having the highest power exponent (0.98) and also the highest correlation coefficient. The results suggest that the total number of MUA, representing spatial recruitment, is the most relevant coding parameter for the subjective magnitude estimation of the stimulus intensity.