DOES root canal surgery feel more painful because you can hear the dentist's drill? That is one implication of new research suggesting that sounds, sights and smells can subtly alter our response to treatment, in a way akin to the placebo effect. The finding reinforces the idea that much of our behaviour may be the result of our minds responding subconsciously to cues that are reminders of past experiences.
The placebo effect occurs when people with a medical condition appear to get better after receiving a treatment containing no active ingredients - for example, sugar pills - or even after a chat with a kindly doctor. It is thought to work because recipients have, over their lifetime, become conditioned to feel better when they take a pill or see their doctor.
It had been thought that the effect only occurs when we are conscious of receiving treatment. Now it seems that subliminal cues can trigger something akin to the placebo effect and its dark twin, the nocebo effect, which makes people feel worse.
Researchers led by Karin Jensen of the Massachusetts General Hospital in Charlestown tested the idea by showing 40 volunteers two different images on a computer screen. One image coincided with the volunteers receiving a short but painful heat pulse to their arm. With the other image, they received a milder pulse. Each time they saw one of the two images, they had to rate the extent of their pain, from none to unbearable.
Next, 20 of the participants repeated the test, but this time the researchers applied the same level of heat for both images. Because the volunteers had learned to expect different levels of pain from each image, they continued to rate the pulses that came with the "high pain" image as more severe than those with the "low pain" image.
Finally, the other 20 volunteers viewed the images again, and again the same heat levels were used for both. But this group only saw each image for 12 milliseconds, so the volunteers could not consciously detect which one they were seeing. Yet they managed to tell them apart subconsciously, again associating the "high pain" image with more severe pain (Proceedings of the National Academy of Sciences, doi.org/jb6).
"The research shows our behaviour and emotional experiences are often guided by stimuli out of our consciousness," says Fabrizio Benedetti of the University of Turin Medical School in Italy. "Once again, the placebo response is emerging as an excellent model to understand how our brain works."
Jensen says that if we can identify cues that subliminally aid recovery, "such as background smells or simply a firm, reassuring handshake from a doctor", then we could incorporate them into clinical practice. Likewise, we could try to reduce or eliminate people's exposure to cues that can hamper recovery. "The awareness of these mechanisms, and that they operate in most people, is an important first step. The second will be to test interventions that could benefit patients," she says.
Joel Voss of Northwestern University in Chicago is sceptical that we can improve treatment using subliminal cues, however. It would be too hard to generate them in a hospital or clinic, he says.
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