Differential effect of supraspinal modulation on the nociceptive withdrawal reflex and pain sensation

Objective To study whether the nociceptive withdrawal reflex (WR) and pain sensation are differentially affected by supraspinal modulation and to determine the nature of this modulation. Methods The WR and pain sensation elicited by electrical stimulation were measured in complete spinal cord injury (SCI) subjects and in intact controls under two different experimental conditions; “facilitation” and “neutral” control. Results Pain sensation was the same under both conditions, whereas the characteristics of the WR were highly dependent on them. In intact body regions the WR threshold was similar to pain threshold under facilitation but was near pain tolerance in neutral conditions. Furthermore, WR was elicited in 100% of trials under facilitation but only in 57% of trials in neutral conditions. Thresholds of WR in paralyzed regions were significantly higher than in intact regions (p < 0.001). The former showed a clear stimulus–response relationship as did pain sensation whereas the WR in intact regions did not. Conclusions The WR and pain sensation are differentially affected by supraspinal modulation. The WR is subject to both excitatory and inhibitory influences, depending on the instructions subjects receive. Significance The experimental setup and subjects’ mental state should be considered when interpreting changes in the WR. Extreme caution should be employed when utilizing reflexive indices as a measure of pain. Verbal report seems a more suitable tool to evaluate pain since it is relatively stable with repeated measurements and in accordance with stimulation intensity.