COMPOUND ACTIVITY IN SENSORY NERVE-FIBERS IS RELATED TO INTENSITY OF SENSATION EVOKED BY AIR-PUFF STIMULATION OF THE INDEX FINGER IN MAN

An analysis of a population response recorded from the primary afferents 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 of 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/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 using the stimulus level of So + 2.50 kg/cm2 as the standard stimulus. The subject was asked to give a numerical estimate of the intensity of a series 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 component had the highest power exponent (0.80) and also the highest correlation coefficient (r = 0.59, P < 0.001). The function of other SNAP components had considerably lower power exponents with lower correlation coefficients. Thus, a mismatch between neural and psychophysical functions was obvious when individual neural functions were compared with the psychophysical function. Conversely, it was found likely 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.