Compound activity in sensory nerve fibers is related to intensity of sensation evoked by air-puff stimulation of the index finger in man

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An analysis of a population response recorded from the primary afferets was undertaken to study the neural signs of intensity coding in the peripheral somatosensory system. Short air-puff stimuli were applied to the volar aspect ot the tip of the index finger to obtain both neural and psychophysical responses. The detection threshold (So) was first determined and 6 levels of stimulus intensity above threshold(So + 0.25 kg/cm 2, So + 1.25 kg/cm 2, So + 32.5 kg/cm 2, So + 3.75 kg/cm 2 So + 55.00 kg/cm 2, and So + 6.25 kg/cm 2) were adoptes for magnitude estimation using the stimulus level of So + 2.50 kg/cm 2 as the standard stimulus. The subject wasuused to give a numerical estimate of the intensity off the of stimuli randomly presented. Compound sensory nerve action potentials (SNAPs) were also recorded from surface electrodes over the median nerve at the wrist for the 6 stimulus intensities. Eight SNAP components (4 positive and 4 negative) were recorded within 10 msec following stimulation. A simple power function with an exponent of 0.85 provided an adequate description of the magnitude estimation function, as was verified by the high correlation coefficient ( r=0.89, P < 0.001) Similarly, stimulus-amplitude functions of individual SNAP components were well represented by straight lines in double logarithmic plots. The function of the late N2-P3 components had the highest power exponent (0.80) and also the highest correlation coefficient ( r =00.59, P < 0.001). The functions of other SNAP components had considerably lower power exponents with lower correlation coefficients. Thus, a mismatch ebtween neural and psychophysical functions was obvious when individual neural functions were compared with the psychophysical function. Converesly, it was found like y that the total number or time-integral of peaks in the population response was a more pertinent parameter of neural activity and, thus, had a closer correlation with the psychophysical response.