When we say that we are “in pain”, we usually mean that an injured body part is hurting us. But the phenomenon we call pain consists of more than just physical sensations, and often has mental and emotional aspects, too. Pain signals entering the black box of the brain can be subjected further processing, and these hidden thought processes can alter the way we perceive them.

We still know very little about these non-physical aspects of pain, or about the brain processes responsible for them. We do know, however, that learning and mental imagery can both diminish and enhance the experience of felt pain. Two new studies now extend these findings – one shows that subliminal learning can also alter pain responses, and the other explains how mental imagery can do so.

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It’s well known that simple associative learning procedures can alter responses to pain. For example, newborn babies who have diabetic mothers and are repeatedly exposed to heel pricks in the first few days of life exhibit larger pain responses during subsequent blood tests than healthy infants. Learning also appears to explain the placebo effect, and why it is often so variable.

Several years ago, Karin Jensen, who is now at Harvard Medical School, and her colleagues showed that subliminal cues can reactivate consciously-learned associations to either enhance or diminish pain responses. In their latest study, the researchers set out to determine the extent to which this type of learning can occur non-consciously.

They recruited 49 healthy participants, and presented them with photographs of faces on a computer screen, pairing each image with either a painful high temperature stimulus, or an innocuous stimulus of lower temperature, delivered with a thermal stimulator to their left forearms. When done repeatedly, the participants associate each face with the stimulus to which it was paired, so that the same images enhance or diminish the pain responses when later shown while the heat stimuli are being applied.

The participants had been randomly assigned to one of four groups. For one group, the images used during both the learning and response phase were shown for one-tenth of second, long enough for them to be seen and recognised. For another, they were displayed for about one-hundredth of a second, so that they did not reach the participants’ conscious awareness. Those in the third group were unaware of the images used during learning, but were conscious of them during the response phase, while those in the fourth were conscious of the first set of images but not of the second.

Jensen and her colleagues asked the study participants to rate the intensity of each pain stimulus during the learning phase, and also asked them to report the amount of pain they felt when the images were displayed the second time round.

All of the participants initially rated the high temperature stimuli as painful, and the low temperature ones as non-painful. In the second part of the study, all of them reported that the same painful stimuli felt more intense when the faces that were previously paired with painfully hot temperatures were displayed, regardless of whether the images were ‘masked’ or visible during both phases. Similarly, they all reported feeling less pain when the heat was applied while they viewed faces that had been paired with the non-painful temperatures, but this analgesic effect was significantly smaller.

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These results show quite clearly that non-conscious learning can alter people’s pain responses, so that felt pain is perceived as being more or less intense. It’s still not clear why this effect is so much more pronounced in one direction than in the other, but one possibility is that threat-associated stimuli are processed more rapidly than others.

Francesca Fardo of Aarhus University in Denmark and her colleagues wanted to learn more about exactly mental imagery can alter the perception of pain. Mental imagery is a conscious thought process that involves visualising something in the “mind’s eye.”

We know that chronic pain patients can experience intrusive and distressing pain-related images that exacerbate their painful sensations, and that they sometimes have “coping” images that alleviates the pain. Earlier studies have also shown that imagined movements can temporarily reduce symptoms of chronic regional pain syndrome, as well as the phantom pain experienced by amputees. This may be because mental imagery interferes with the processing of pain signals in the brain. Alternatively, it may divert attention from the source of pain.

Fardo and her colleagues recruited 21 healthy participants and applied electrical stimuli of varying intensities to their forearms, asking them to rate to intensity of each on a scale of one to ten. They then applied painful and non-painful stimuli again, but this time the participants were instructed to imagine either a glove covering their forearm, or a wound on the arm, while they were being delivered, and to rate how much pain they felt each time.

When they imagined a glove on their forearm, the participants reported feeling less pain and unpleasantness. They also rated fewer of the high intensity stimuli as painful, and took longer to detect the ones they rated as painful. But imagining a wound instead increased their pain sensitivity, so that they reported more of the low intensity stimuli as painful, and took longer to judge the non-painful stimuli.

The researchers also used electroencephalography (EEG) to record the participants’ brain activity during the experiment, focusing on a pain-related brain wave pattern called the N2 potential. This revealed that the mental imagery that inhibited pain was associated with larger N2 signals in regions of the cerebral cortex involved in cognitive control, whereas imagery that exacerbated the pain was associated with smaller N2 signals in other regions thought to be involved in emotional processing.



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Both types of neural activity occurred within the same time window after application of the stimuli – between 122 and 180 thousandths of a second – regardless of how long it took the participants to judge the intensity of each one. The N2 signal was thus not specific to the painful stimuli, leading the researchers suggest that it may reflect the integration of sensory information with the participants’ expectations of how they will feel.

Pain is a leading cause of disability worldwide, and conditions such as chronic pain can be extremely difficult to manage, because they often have no identifiable physical cause. These new findings advance our understanding of how the brain processes pain, and offer potential new ways of relieving the suffering of millions without the use of expensive drugs.

References

Jensen, K., et al. (2015). Classical conditioning of analgesic and hyperalgesic pain responses without conscious awareness. Proc. Nat. Acad. Sci. 112: 7863-7867. DOI: 10.1073/pnas.1504567112 [PDF]

Fardo, F., et al. (2015). Neurocognitive evidence for mental imagery-driven hypoalgesic and hyperalgesic pain regulation. Neuroimage, DOI: 10.1016/j.neuroimage.2015.07.008 [Full text]