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 somatosenso ry 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% xylocai n) to the tip of the right index finger, magnitude estimations for the four modalities of skin sensation (touch, pressure, pain and vibratio n), and SEPs were successively examined at 4-10 min interval until com plete sensory and SEP recovery. The time course of sensory recovery wa s similar for the first three modalities and vibration sense was littl e affected by local anaesthesia. Immediately after anaesthesia, SEPs e ither abolished or decreased in amplitude. Thereafter amplitudes incre ased and peak latencies decreased with the elapsed time. Comparison of the regression lines for the amplitude of P45-N60 component or the ti me-integrals of N20 and N35 components with that of psychophysical est imtes revealed steeper slopes for psychophysical data: 0.4-0.7 for neu ral versus 1.4-1.7 for psychophysical data. These relations between SE Ps 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. Dire ct comparisons of SEP measures with subjective magnitudes produced sig nificant correlations for the three modalities of sensation in which a gain latencies are better correlated than amplitides or time-integrals .