[Changes in cortical evoked potentials in relation to cutaneous sensory thresholds. Application to a study of...

The potentials (EPs) evoked by cutaneous electric shock of increasing intensity and the corresponding sensitivity thresholds were studied in 25 healthy volunteers: analgesic electrical stimulation (AES) was performed on only 10 of these. The stimuli (0.5 msec; 0-140 V) applied in a random way with a needle electrode implanted in the horny layer of the tip of the middle finger can produce 4 sensations: initial awareness, pressure sensation, diffusion and pain. The EP recorded on the somatosensory areas had the typical somatosensory (SEP) morphology only at the pressure stage. At the pain thresholds, the response was a well-demarcated 'nociceptive' EP (NEP), with main components at P100, N140, P240. After 45 min of AES, the increase of the sensitivity thresholds was peculiarly pronounced for that of pain, homolaterally (80%) and contralaterally (45%) to the AES. With respect to the EPs, the NEP and especially its late waves showed a noteworthy reduction in amplitude, more significant on the homolateral side (66%) than on the contralateral one (50%), whilst the SEP was only slight modified. The evolution of the thresholds and of the EPs was followed for 45 min after cessation of AES and revealed a significant residual effect of comparable values for the two parameters.