[Evoked potentials and pain. II. Comparative study of subjective sensations, late components of the cortical potentials and afferent volleys].

The subjective sensations and late components of the cortical somatosensory evoked potential (SEP) have been correlatively studied when elicited by stimulation parameters which were selected in order to activate selectively, either the largest afferent fibers (train of shocks of 10 mA, with a 0.05-0.2 msec duration for each shock) or the several groups of fibers (A alpha beta; A delta and C) of the cutaneous branches of the median nerve (single shock, 1 msec duration; 20 mA). In both cases, a good correlation was found between the quality and intensity of sensations and the amplitude of the late component N150-P220-N400 of the SEP. However, it was also found that this relationship was not linear and suggested that it is possible to saturate the cortical integrative system responsible for the SEP. A selective blockage of the largest fibers (compression) or of the finest ones (lidocaine) have shown that there is no specific involvement of a particular type of peripheral fiber in the genesis of the N150-P220-N400 components. Moreover, these late components could also be recorded with similar latencies when cutaneous stimulations were applied in any part of the body.