HETEROTOPIC NOCICEPTIVE CONDITIONING STIMULI AND MENTAL TASK MODULATE DIFFERENTLY 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 comparing its effects to those induced by a mental task (MT). Five test stimuli made from short CO2 laser pulses (duration: 40 msec; diameter: 10 mn; intensity: 0.25-0.8 Joules) were delivered every 30 to 45 sec at random 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 painful, the two least intense stimuli as warm, and the intermediate stimulus as hot or near painful. Perception (VAS), reaction time (T) and cerebral evoked potentials (CEPs) were monitored before, during and after conditioning stimulation consisting either of HNCS (hand submerged in cold water) or of MT (arithmetic substraction). Pain perception (first pain) threshold was increased in both conditioning situations; however, the stimulus-response curve and the neurophysiological correlates were differently affected. During HNCS, the stimulus-response curve was depressed and T was increased mainly for the intermediate stimulus, whilst CEP power density was reduced for all stimulus intensities; discrimination performance near pain threshold was dramatically depressed. During MT, the stimulus-response curve was shifted down toward higher stimulus intensities, T was equally increased for al stimulus intensities, whereas CEP power density was not changed; discrimination performance remained unchanged. These observations bring additional information to show that, compared to MT, HNCS produces distinct changes in perception and physiological correlates and might interfere with transmission processes of the afferent volley evoked by the CO, laser stimuli, possibly via diffuse noxious inhibitory controls (DNICs).