Perceptual changes accompanying controlled blocking of cutaneous afferents are related to somatosensory evoked potentials.

We examined the effect of local anaesthesia of the skin on somatosensory evoked potentials (SEPs) and psychophysical magnitude estimates for a constant intensity air-puff stimulation (7 kg.cm-2) in 23 normal subjects. Before and after intradermal injection (0.1 ml of 1% xylocain) to the tip of the right index finger, magnitude estimations for the four modalities of skin sensation (touch, pressure, pain and vibration), and SEPs were successively examined at 4-10 min interval until complete sensory and SEP recovery. The time course of sensory recovery was similar for the first three modalities and vibration sense was little affected by local anaesthesia. Immediately after anaesthesia, SEPs either abolished or decreased in amplitude. Thereafter amplitudes increased and peak latencies decreased with the elapsed time. Comparison of the regression lines for the amplitude of P45-N60 component or the time-integrals of N20 and N35 components with that of psychophysical estimates revealed steeper slopes for psychophysical data: 0.4-0.7 for neural versus 1.4-1.7 for psychophysical data. These relations between SEPs and psychophysics following local anaesthesia bear a close parallel to those observed in our previous studies in a normal condition with increasing stimulus intensity. The SEP latencies for N20, P27 and N35 components were better correlated with recovery from anaesthesia. Direct comparisons of SEP measures with subjective magnitudes produced significant correlations for the three modalities of sensation in which again latencies are better correlated than amplitides or time-integrals.