Effect of noxious stimulation on hand localisation under sensory incongruence
Background. The ability to localise one’s own body parts (self-localisation) in space is fundamental to our sense of physical self, our ability to interact with our environment and to protect ourselves from harm. Self-localisation can rely on multiple sensory channels with the most reliable channel, usually vision, being weighted most heavily. When vision is made unreliable in conditions of sensory incongruence, self-localisation requires reweighting of proprioceptive input. Here we investigated the relationship between self-localisation and processing of painful stimuli. Because of its imperative for protection, we hypothesised that, self-localisation would be quicker in the presence of noxious stimuli than non-noxious or no stimuli. Because pain requires determination of a location for that pain, we also predicted that sensory incongruence would have an analgesic effect. Methods. We induced a mismatch between the seen (visually-encoded) and perceived (proprioceptively-encoded) position of the hands in 18 (8F) healthy volunteers. In four conditions (non-noxious electrical stimuli, noxious electrical stimuli, mixed noxious and non-noxious, no stimuli), participants rated the expectation of receiving a painful stimulus, the intensity of each stimulus, and then immediately estimated the true position of their experimental hand. The primary outcome was the difference between the perceived position of the hand and the true position of the hand – ‘Error’. The secondary outcome was the experience evoked by stimulation, recorded using the Fifty Either Side of Threshold (FEST) numerical rating scale. We recorded electromyographic data from Biceps Brachii and Triceps Brachii (long head) throughout the experiment, to verify that participants did not move their arm. Results. ANOVA showed there was no effect of Condition (noxious, non-noxious, no stimulation, mixed) on Error (p=0.980) –self-localisation was not quicker in the presence of noxious stimulation. For fourteen participants, the noxious stimuli were less painful and for 9, the non-noxious stimuli were less intense during sensory incongruence than during control. EMG data detected no muscle activity during testing. Discussion. Contrary to our prediction, the noxious stimulation did not augment the reweighting of sensory channels. This finding suggests that there is a ceiling effect on top-down reweighting of sensory channels when the reliable channel – vision – is rendered inaccurate, at least under conditions that restrict movement. Our secondary findings corroborate other studies by showing an analgesic effect of sensory incongruence. Further research might pursue a potential therapeutic application of this effect.