HETEROTOPIC NOCICEPTIVE CONDITIONING STIMULI AND MENTAL TASK MODULATEDIFFERENTLY THE PERCEPTION AND PHYSIOLOGICAL CORRELATES OF SHORT CO2-LASER STIMULI

The present study was aimed at examining the specificity of the action of heterotopic nociceptive conditioning stimulation (HNCS) by compari ng its effects to those induced by a mental task (MT). Five test stimu li made from short CO2 laser pulses (duration: 40 msec; diameter: 10 m n; intensity: 0.25-0.8 Joules) were delivered every 30 to 45 sec at ra ndom to 4 different spots on the skin of the upper lip in 3 groups of 10 healthy subjects. The two most intense stimuli were perceived as pa inful, the two least intense stimuli as warm, and the intermediate sti mulus as hot or near painful. Perception (VAS), reaction time (T) and cerebral evoked potentials (CEPs) were monitored before, during and af ter conditioning stimulation consisting either of HNCS (hand submerged in cold water) or of MT (arithmetic substraction). Pain perception (f irst pain) threshold was increased in both conditioning situations; ho wever, the stimulus-response curve and the neurophysiological correlat es were differently affected. During HNCS, the stimulus-response curve was depressed and T was increased mainly for the intermediate stimulu s, whilst CEP power density was reduced for all stimulus intensities; discrimination performance near pain threshold was dramatically depres sed. During MT, the stimulus-response curve was shifted down toward hi gher stimulus intensities, T was equally increased for al stimulus int ensities, whereas CEP power density was not changed; discrimination pe rformance remained unchanged. These observations bring additional info rmation to show that, compared to MT, HNCS produces distinct changes i n perception and physiological correlates and might interfere with tra nsmission processes of the afferent volley evoked by the CO, laser sti muli, possibly via diffuse noxious inhibitory controls (DNICs).