If you look at an fMRI when the brain is experiencing suffering, either from a physical stimulus or a social one, you’ll see two main areas light up: the anterior insula and the anterior cingulate cortex. They’re located right behind your forehead. As scientists have shown, both romantic breakups and social rejection trigger responses in these two areas, an indication that pain isn’t localized to any so-called “pain center” in the brain, but spreads itself across multiple regions that react to a variety of stimuli.
In a study published in Psychological Science, new research looks at this blanketing effect and takes it a step further to learn how positive emotions may also get blunted. Psychologists call this dual reactivity to one factor “differential susceptibility.” Based on the evidence, acetaminophen appears to wipe out not just negative feelings, but any emotional sensitivity a person may have.
Way and his colleagues conducted two experiments for their study. The first involved 82 people, half of whom took a Tylenol pill while the rest took a placebo. After waiting 60 minutes for the drug to take effect, each person saw 40 images — a mix of disturbing, pleasant, and neutral. These included shots of malnourished children, a cow grazing, and babies playing with cats. Each participant rated the photos on a scale of -5 to 5 for their relative negativity and positivity. Then they looked at them all again, this time rating their own emotional reaction to the photos.
What the team found was that the acetaminophen group had a much weaker reaction to the images, regardless of whether they were pleasant or unpleasant in nature. They rated the photos as less extreme objectively and claimed to have less of an emotional response to them.
Thinking the effect could be due to a change in perceived magnitude, not the overlap between brain regions governing pain, the team conducted a second study. This one involved 85 people and proceeded exactly the same as the first, except this time around people reported how blue the photos seemed. If the Tylenol group saw less blue in addition to the other findings, the team thought, the neural mechanisms might be different.
But, the intensity of blue had no effect. Differential susceptibility seemed to rely on the acetaminophen only going after sensitivity, not some general perception of extremes. In part, this reinforces what other psychologists have already found. In 2010, for instance, scientists put subjects on a Tylenol or placebo regimen for three weeks and took fMRI scans shortly afterward. Again, in the anterior cingulate cortex and anterior insula, they found“acetaminophen reduced neural responses to social rejection in brain regions previously associated with distress caused by social pain and the affective component of physical pain.”
The new study, however, throws something of a wrench in the gears when it comes to understanding. They still don’t know whether ibuprofen or aspirin, two other pain relievers that work through similar mechanisms in the body, will yield the same effects. Also, the specific reason acetaminophen has dulling effects overall, not just on negative feelings and sensations, is still unclear. Way implicates several possible factors.
“Multiple chemical processes are affected by acetaminophen, including altering serotonin levels in the brain, levels of inflammation in the brain, and levels of cannabinoids in the brain,” he said. “We don't know which one is responsible at this point.”
According to the Consumer Healthcare Products Association, 23 percent of Americans take a product containing acetaminophen on any given week. Whether people with greater emotional sensitivity could reduce those feelings with a regular Tylenol regimen is still a pending question, Way says. However, he concedes acetaminophen is still a drug, which can create long-term health problems with overuse.
Source: Durso G, Luttrell A, Way B. Over-the-Counter Relief From Pains and Pleasures Alike: Acetaminophen Blunts Evaluation Sensitivity to Both Negative and Positive Stimuli.Psychological Science. 2015.