Modality-specific facilitation and adaptation to painful tonic stimulation in humans

The study assessed the influence of stimulus modality on adaptation or facilitation of pain during tonic cold and tourniquet pressure stimulation. Experimental set-up for the cold stimulation consisted of a thermo-tank with water, cooled to 3 degrees C, circulation pump, electronic thermometer and an electronic 10 cm visual analogue scale (VAS). Experimental set-up for the tonic pressure stimulation consisted of a pneumatic tourniquet cuff, a computer-controlled air compressor, and an electronic VAS. The first experiment assessed temporal profiles of pain intensity and skin temperature during immersion of the non-dominant hand and lower arm into cold water for 3 min or until the pain tolerance limit was reached. The second experiment assessed temporal profile of cuff pain intensity during constant compressions for 10 min beginning at pain intensities of 2, 4, and 6 cm on the VAS ("VAS 2", "VAS 4" and "VAS 6" sessions). Subjects enduring cold stimulation for less than 3 min were defined as non-adapting to cold and vice versa. The intensity of cold pain in non-adapting subjects increased significantly faster than in adapting subjects and reached significantly higher magnitude. The course of pain intensity during constant compression, estimated by a linear regression line, was increasing or decreasing, representing facilitation or adaptation of pain, respectively. The typical profile of adaptation consisted of an "overshoot" in pain intensity, followed by a decrease in pain intensity. There was significant correlation in VAS slopes between sessions separated by 2-5 days, suggesting consistent pattern in pain responses to tonic pressure stimulation. Adaptation or facilitation rates and the overshoot magnitude were dependent on the initial pain intensity (2, 4, or 6 cm on the VAS). The facilitation rate was highest and the adaptation rate was lowest during the "VAS 2" session, while the facilitation rate was lowest and the adaptation rate was highest during the "VAS 6" session. The overshoot magnitude was lowest during "VAS 6" session. Adapting and non-adapting/facilitating responses to cold and to pressure during "VAS 6" session were not correlated, suggesting that pain course and therefore stimulus tolerance during tonic stimulation are modality-specific. The results of the study suggest that tolerance of tonic painful pressure and cold stimulations is specific to stimulus modality and may represent separate nociceptive mechanisms.