One of the human brain's most interesting abilities is the placebo effect when just an expectation of relief can lead to the relief of pain, anxiety, depression, nausea, and many other aversive states.
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However, scientists at University of Gothenburg and University of Oslo showed that the placebo effect may not be limited to pain reduction, but may also enhance pleasure, or hyperhedonia.
The researchers used the placebo effect to improve both painful and pleasant touch sensations in healthy humans - and by comparing brain processing using functional magnetic resonance imaging (fMRI), found that, depending on whether the starting point was painful or pleasant, neurocircuitry associated with emotion and reward underpinned improvement of both pain and pleasant touch by dampening pain but increasing touch pleasantness.
In an interview with Medical Xpress, PhD candidate Dan-Mikael Ellingsen discussed the paper he and his colleagues published in Proceedings of the National Academy of Sciences.
"In recent years, functional brain imaging studies have shown that expecting a treatment to relieve negative symptoms – like pain, anxiety or unpleasant taste – leads to not only subjective reports of relief, but also suppressed brain activity in sensory circuitry during aversive stimuli, such as noxious heat or touch, threatening images, and unpleasant taste.
"However, both aversive and appetitive experiences – for example, tasty food or a pleasant touch – are affected by context and expectation," Ellingsen tells Medical Xpress.
The researchers asked whether improvement of good experiences is encoded entirely in higher-level valuation processing, or whether it would mirror the modulation of early stages of sensory processing that is seen for aversive stimuli.
"If so, we'd expect such positive sensory signals to be up-regulated, in contrast to the down-regulation of sensory signals we see during placebo-induced reduction of aversive experiences."
In the placebo manipulation procedure, participants were shown a short video documentary convincing them that a nasal spray containing the neuropeptide oxytocin would reduce pain and enhance the pleasantness of pleasant touch. Following this video, they self-administered 10 puffs of a placebo nasal that they were told could contain oxytocin.
The pleasant touch stimuli consisted of caress-like light strokes with a soft brush, or a hot/cold pack (resembling a warm hand, applied to the subject's forearm. The pain stimulus was a thermode (~47 degrees Celsius) on the hand.
Ellingsen notes that by comparing brain activation during painful or pleasant touch stimuli after placebo treatment versus no-placebo, the scientists were able to assess differences in activation that was specifically related to having received placebo treatment.
"Importantly, the subjective reports showed that, after receiving placebo relative to no-placebo, touch pleasantness was increased while pain unpleasantness was decreased," he adds.
"When contrasting placebo and no-placebo on brain activation, we found that sensory activation was increased during pleasant touch stimuli and decreased during painful touch stimuli. In other words, the placebo-induced change in sensory processing reflected the placebo-induced change in subjective reports." ■