Consider this letter from Joan (last name withheld), a hyperbaric nurse from Rhode Island: "I too can hear my eyeballs,” she writes. “However, it is not a loud noise, just a swishing back and forth when I move them and usually at night before sleep when it is quiet. I’m certain there are many more [such bodily noises]…we’re just not 'tuned in'!"
Joan is right. Charles Limb of the National Institute of Health tells me that some people hear their eyeballs not just at bedtime, but all the time! In this case, the noise is not imaginary; it is caused by the tugging of eye muscles. The skull bone between the inner ear canals and the brain is very thin, and in patients with this condition, disease or injury has perforated the thin shield, allowing the semicircular canals of the inner ear to bulge into the brain cavity. This extra hole in the head acts as a third ear tuned to internal sounds.
But perhaps the most exciting connection to emerge from the case of the noisy eyeballs came from Martin Kronberg, of Ottawa, Canada. A few months ago, I received a note from him: "Please find attached a paper that is about to be published in the journal Biosience Hypothesis,” he wrote. “The paper was significantly inspired by your article in Scientific American. Kronberg’s scientific paper suggests that sulfites could cause tinnitus by acting on the nucleus accumbens, a small cortical area located in the center of the brain.
Sulfites are used as antioxidants to prevent foods from turning brown and as preservatives to kill harmful microorganisms. In fact, I know, as an amateur wine maker, that the first step in making wine is to add sodium sulfite to kill the wild yeast on the grapes so that the juice can be inoculated with the preferred strain. Sulfites are used this way in many food products and sulfa drugs (sulfonamides) are also used as antibiotics.
So what do sulfites have to do with tinnitus and noisy eyeballs? Kronberg noticed that the enzymes blocked by sulfites in bacteria—this property is why sulfites are such powerful antiobiotics—have very similar chemical properties to the enzymes that help regulate the production of dopamine and serotonin, both of which are important neurotransmitters in the nucleus accumbens. In particular, Kronberg suggests that sulfites could impair the function of the enzyme making dopamine (dihydroxyphenylalanine), and also the enzyme tryptophane hydroxylase required for synthesis of melatonin and serotonin. If so, then sulfites might disrupt the normal functioning of the nucleus accumbens, and prevent it from properly regulating the flow of information.
Only time will tell if there is anything to this hypothesis springing from the curious cross pollination among the readers of Scientific American Mind, but it is wonderful to be doing science collaboratively and serendipitously. Who knew that my noisy eyeballs would one day lead to so many intriguing scientific ideas?
Are you a scientist? Have you recently read a peer-reviewed paper that you want to write about? Then contact Mind Matters editor Jonah Lehrer, the science writer behind the blog The Frontal Cortex and the book Proust Was a Neuroscientist. His next book, How We Decide, will be available in February 2009.