The Same Burn, Told Twice

AA burn of a given temperature does not produce a pain of a given size. The same stimulus, delivered twice to the same forearm, may be rated mild on one afternoon and close to intolerable on the next, and the difference often traces to nothing more than what the volunteer was told beforehand. Clinicians have known this for a century and have filed it under suggestion, a word that concedes the observation while denying it a mechanism. The vocabulary inherited from nineteenth-century physiology makes that filing inevitable: it treats the nerve ending as a sensor, the spinal cord as a cable and the brain as a reader. On such a picture context is interference: a rating that moves while the stimulus stands still is noise to be averaged away. The variation, however, is orderly, it is large, and it survives every precaution the old picture recommends.

BRosa Nakagawa, a pain physiologist at the Ellsmere Centre for Somatic Research, holds that the size of a pain is set in advance rather than read off afterwards. On her account the nervous system is continuously issuing predictions about how much harm a stimulus will do, and what reaches awareness is that prediction, corrected only partially by the signal arriving from the skin. Between 2014 and 2021 her group put 318 volunteers through a procedure in which a thermal probe at 47°C was preceded by a spoken cue, and found that a cue promising severe heat lifted ratings by 1.8 points on a ten-point scale relative to a cue promising mild heat, the probe being identical in both. Fed nothing but the cue and the stimulus, her model accounted for 71 per cent of the variation in her volunteers' ratings. Nothing in the account requires the person to attend to anything: on Nakagawa's model a cue does its work whether or not the volunteer notices it.

CA claim of that shape can be aimed at its own weak spot, and Nakagawa has aimed it there. If what is felt is chiefly a prediction, a volunteer should gain little from accurate information wherever the prediction has already been formed and the correction arrives late. In her 2019 delayed-disclosure trials, volunteers told the true temperature only after the probe had been lifted rated the episode 0.3 points lower than volunteers told nothing, against 2.1 points lower when the same information was supplied beforehand. The benefit of late disclosure was confined to volunteers who had been given no cue at the start. The pattern reverses in a group she had not set out to study: among 44 patients with back pain of more than five years, late disclosure did more good. Nakagawa calls the figures a fence around one explanation rather than a verdict on pain itself.

DAdaeze Nkemelu, a clinical psychologist at the Brindle Hospital Pain Unit, replies that a volunteer lying still with nothing to do but attend to a probe is placed in circumstances no one meets outside a laboratory, and that a result obtained there describes the laboratory as much as the person. Her own participants receive the same 47°C probe while working through a task that competes for their attention, an arrangement now ordinary but judged careless when she adopted it in 2016. Under those conditions the effect of a misleading cue fell to 0.5 points, and 68 per cent of her participants said afterwards that they had not registered the cue at all. Nkemelu concedes two weaknesses. Her 61 participants were recruited from the staff of one hospital, and the attentional competition she treats as the active ingredient was inferred from task performance and from the ratings themselves, since nothing yet allows attention to be measured directly while a person is in pain.

EThe dispute has left the clinic. Wound-care firms have taken up cue-setting as a design principle: a 2020 trial reported that dressings described to patients as engineered for comfort drew ratings 22 per cent lower than identical dressings described in plain terms, although the gap closed whenever the nurse applying them had not been told which was which. Three health services now train staff in pre-procedure wording, and one was criticised for advertising the practice as pain relief without a drug. Nakagawa points out, awkwardly for her own position, that her model was fitted entirely on brief heat delivered to healthy skin and predicts nothing about pain that has lasted for years. Nkemelu, for her part, insists that describing a mechanism and taking seriously what a patient's pain means are not aims anyone has to choose between, and fears that hospital managers will read a lowered rating as a patient treated. Both dislike the habit of publishing the average rating without the spread beneath it.

FA settlement of a kind is taking shape, in which the two accounts are given different territory rather than ranked. Where a threat is announced and then delivered within seconds, the rating tracks the announcement almost as closely as it tracks the stimulus, which would account for the speed of the shift and its indifference to considered belief. Where pain must be lived with for months, attention has moved to what the pain is taken to mean — damage accumulating, or a body guarding itself — a reading formed well away from the moment of injury. What remains open is whether the laboratory effect and the clinical one are one phenomenon at two timescales, and no measure now available can settle it, since the ten-point scale that makes the brief experiments workable turns almost meaningless when a person is asked to apply it to a year. What both researchers reject is the assumption behind the old cable diagram: that a pain which moves with its context must be either an error of measurement or an invention of the sufferer.