This study examined the latency relationship between mechanically and electrically elicited sensory nerve action potentials (SNAPs) and the somatosensory-evoked potentials (SEPs) they produce. Brief air-puff and electrical stimuli were applied to the tip of the index finger in separate trials and SNAPs from the median nerve at the wrist and SEPs from the scalp were recorded for each stimulus presentation. Air-puff evoked SNAPs were polyphasic, usually consisting of 2 to 4 separate waves, unlike triphasic activity elicited by electrical stimulation. The SEPs produced by these 2 distinct forms of inputs, however, were similar in morphology. The latencies of the initial components of SNAPs and SEPs were longer for air-puff stimulation. The conduction time, however, of the fastest afferent volleys from the wrist to cortex was not significantly different for air-puff (20.52 +/- 1.06 ms, mean +/- SD) and electrical stimulation (20.17 +/- 0.66 ms). It is therefore concluded that the latency delays for air-puff evoked SNAPs and SEPs are due solely to a transduction time at the skin receptors and not due to differences in conduction velocities as suggested in the previous literature.
Brief air-puff stimuli were applied to the volar surface of the right hand to obtain both psychophysical and neurophysiological responses. The detection threshold (So) was first determined (0.56 kg . cm-2 +/- 0.20 kg . cm-2, mean +/- SD) and six levels of the stimulus intensities (So + 0.25 kg . cm-2, So + 1.25 kg . cm-2, So + 2.50 kg . cm-2, So + 3.75 kg . cm-2, So + 5.00 kg . cm-2, and So + 6.25 kg . cm-2) were employed for magnitude estimation using the stimulus level of So + 2.50 kg . cm-2 as the standard stimulus. The subject was asked to estimate numerically the series of stimulus intensities randomly presented. Cortical SEPs were recorded over the hand sensory area in response to a set of 120 air-puffs at the identical intensity level. Thus SEPs for six sets of stimulus intensities given in a random order were obtained from each subject. Six components (N20, P27, N35, P45, N60, and P75) were recorded within 100 ms following stimulation. It was seen that a simple power function with an exponent of 0.81 could be an adequate description of the stimulus-response function for magnitude estimation, as was also revealed by the high correlation coefficient (r = 0.98). Similarly, stimulus-amplitude functions of different SEP components were well represented by straight lines in double logarithmic plots. The function of the early P27-N35 had the highest exponent (0.56) and also the highest correlation coefficient (r = 0.91). Plotting subjective magnitude of the abscissa produced power functions similar to the stimulus-amplitude functions. However, higher correlations were observed for later components. The amplitudes of the four earlier components correlated with stimulus intensity when the effect of subjective magnitudes was removed. In contrast, the correlation between amplitudes and subjective magnitudes with stimulus intensity held constant was positive and significant for the later three components. These results may indicate that early SEP components represent neural coding of physical intensity while later components are more closely related to the subjective judgment of the stimulus.
Sensory funneling of liminal multiple-point air-puff stimulation on the skin was evaluated by measuring somatosensory evoked potentials (SEPs), reaction time (RT) and subjective detection threshold. Single-point air-puffs at the threshold intensity, and 2-point and 3-point air-puffs (10 mm apart) of the same intensity were delivered to the volar hand for recording SEPs, RT and detection threshold. Two- and 3-point stimulation produced a funneled sensation which feels stronger than the sensation produced by the single-point stimulation. Thus, the detection threshold was lower and the detection probability high with multiple-point stimulation. The latency of P300 SEP component showed a trend for shortening, while the P300 amplitude was reduced with multiple-point stimulation. However, these differences did not reach statistical significance. In sharp contrast with the relatively invariant SEP measures, the RT showed a dramatic reduction with multiple-point stimulation. The results suggest that P300 SEP component is less affected by the funneling of sensory input but is related to the detection of the threshold stimulus in an all-or-none fashion. In contrast, the RT is linearly related to the funneling of input and the resultant higher detection probability. We speculate that an intermediate neural process between the P300 elicitation and the motor command was facilitated by the funneling of multiple air-puffs.
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