Why is it that after listening to music, the last song you hear sometimes replays in your mind for several minutes after the music stops?
—Dave VanArsdale, via e-mail
Andrea Halpern, professor of psychology at Bucknell University, offers this explanation:
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TUNES THAT GET STUCK in the head, evocatively called “earworms,” are probably related to the more common experience of simply being able to call up from memory any familiar tune. For instance, try thinking of a song you know, such as “Happy Birthday.” Most people I test in my lab claim they can do this easily, and the resulting auditory image is fairly vivid. Other scientists and I have been investigating the characteristics of these auditory images and how the brain processes them, which could help explain why some of these images replay persistently.
Familiar tunes that are stored in memory seem to retain characteristics—such as tempo or pitch—that closely match those of their real auditory counterparts. But what is your brain actually doing when you recall a song? My colleagues and I have asked people to carry out tasks involving musical imagery while we recorded their brain activity using neuroimaging. In one study we played sounds of different musical instruments and asked subjects to rate them for similarity. Then we turned off the speakers and asked people simply to imagine the instrument sounds to compare them. We saw similar brain activity in the two conditions: some parts of the auditory system were active both while hearing music and while imagining it, even though the imagined condition was silent.
As we learn more about how the brain processes tunes, we can speculate about why songs are sometimes persistent. As your question points out, earworms often occur for recently heard tunes that are fresh in your memory. The end of a catchy musical phrase reminds you of the beginning again, which can get a cycle going. And contrary to popular belief, earworms are more commonly liked, not disliked, songs. In one of our studies 40 students kept an earworm diary for two weeks. More than half the tunes were rated as pleasant, 30 percent were neutral and only 15 percent were judged unpleasant.
Most of the time the repeating tune just went away by itself—good news for people who do find their earworms annoying. And if that natural fading did not work, the respondents said that engaging in another activity usually made the earworm crawl back into its burrow.
Why doesn’t the human brain have pain receptors?
—Henry Minassian, Brussels, Belgium
Mark A. W. Andrews, professor of physiology and director of the Independent Study Pathway at the Lake Erie College of Osteopathic Medicine, replies:
FOR ANY STIMULUS to be perceived, including pain, specialized cells called sensory neurons must be activated. Internal organs, such as the brain, have few of these neurons; in fact, internal organs house only about 2 to 5 percent of all sensory neurons in the body. This arrangement allows us to closely assess the world around us (possibly because most threats arise externally) while limiting our conscious awareness of internal changes.
Sensory neurons that are specialized for pain perception are known as nociceptors (from the Latin nocere, “to hurt”). The highest concentrations of nociceptors are found in the areas of our bodies that act as direct links to the outside world, such as the skin, bones, joints and muscles. Here the pain receptors act as border sentries, warning us of possible damage so that we may attempt to escape from further harm.
Nociceptors do exist near the brain in its blood vessels and in the meninges, the three thin membranes that wrap around and protect the brain and spinal cord. Recent research has shown that migraine headache pain may arise from the nociceptors in the meninges. Other internal body parts also tend to have their nociceptors in the surrounding tissue, alerting us if they are stretched or squeezed. Interestingly, if the few nociceptors within an organ are stimulated, the pain is “referred” to regions on the surface of the body. This phenomenon explains why the pain that may accompany a stroke is commonly felt in muscles and joints, particularly in the shoulder region. Although the stroke is damaging the brain, the victim does not sense the pain in the brain itself.
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