SO THESE TWO GUYS walk into a bar. No—this is a true story! One is a neuroscientist (yours truly), and the other is a writer for Scientific American Mind. (We'll call him Bill.) We are at the recent meeting of the Society for Neuroscience in Atlanta. A gentle bear of a man comes over, licking suds from his lips and praising the local brew with an authoritative German accent. As he raises his glass to the light to inspect the beer's clarity, I think to myself, “This is my chance.” So I tell him about a bizarre thing my eyes do.
“My eyeballs make this horrid grating noise when they move,” I say. (This sound effect may be amusing in cartoons, but it's a real pain in the neck when I am trying to fall asleep.) Now usually when I make a public confession about my noisy eyeballs, my audience gets a queer look on their faces, as if I am sprouting fur under a full moon. But Josef Rauschecker of Georgetown University, one of the world's authorities on the auditory cortex as well as on local brews, listens intently.
“The weird thing,” I add, “is that it only happens in that twilight between falling asleep or waking up.” (You know, that dreamy, peaceful Neverland that you wish would last just a bit longer before the alarm clock rudely smacks you out of it.) “It's a loud, irritating noise, but as soon as I awaken my eyeballs go silent.” In fact, I have tried to experiment with the phenomenon, darting my eyes left and right and up and down. But as soon as my mind becomes lucid enough to investigate, the noise vanishes.
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“Me, too!” Bill shouts above the barroom din. I look at him as if he has just sprouted fur. But, Bill tells us, his eyeballs do not shriek when he is waking up. Instead it happened years ago, when he stopped taking his medication for depression: “It freaked me out!”
Rauschecker considers us. “I know what your problem is,” he says. I listen in astonishment. What are the chances of meeting someone who shares my oddball noise in front of the only person in the world who could understand it?
“Serotonin,” Rauschecker pronounces. Serotonin is a neurotransmitter that stimulates neurons in the nucleus accumbens to activate it. He explains that his recent brain scans of people suffering from tinnitus, a constant ringing in the ears, show that a particular part of their brain—the nucleus accumbens—is smaller than normal [see box on opposite page]. This nugget of nerve cells is a throttle point regulating the flow of sensory information to the cerebral cortex via another major relay center, the thalamus. Now it all makes sense to me as a neuroscientist, but let me explain it for you.
Like a valve on a water faucet, the nucleus accumbens prevents an overload of sensory input from reaching our conscious mind. Controlling this information flow is important for sleep, attention and anxiety and for suppressing unwanted noise. That guy yapping on his cell phone, oblivious to the conversations around him, is an example of the nucleus accumbens in action. All he hears is the person talking on the phone; he is deaf to sounds in his other ear, because his nucleus accumbens shuts off the input to his auditory cortex. People suffering from ringing in their ears are unable to shut off the irritating noise because of their feeble nucleus accumbens valve.
Sleep and Serotonin
The same sensory shutoff function is essential for sleep. Serotonin levels fluctuate in the sleep-wake cycle, and serotonin is a well-known sleep aid. The sleep centers of the brain (raphe nuclei) connect to the nucleus accumbens. The emotional center of the brain (amygdala) also connects to it. Inputs to the nucleus accumbens from both the emotional and sleep centers of our brain explain why our sensations sharpen with arousal and stress but are blunted by sleepiness.
People with depression or bipolar disorder also are experiencing a kind of sensory blockage. In depression, no amount of sensory stimulation seems to move the patient. The trickle of input from the world is too feeble to excite the cerebral cortex normally. On the other hand, when the nucleus accumbens valve is stuck wide open, a veritable fire hose of input blasts the cortex in response to any sensory input, leading to mania.
In my case, nerve fibers must have sprouted to repair my hearing loss after I had suffered damage to my inner ear, but some of the sprouts “cross-wired” to circuits that normally connect my eyes to my vestibular system (responsible for the sense of balance). Thanks to this vital connection the world does not blur like an amateur video when you turn your head; cells in your inner ear sense that your head is moving, and they send signals to your eyeball muscles to adjust their position, always keeping the same point in space focused on the same spot on your retina at the back of your eye. In my case, “crosstalk” developed between these nerve circuits after my hearing injury, so now when my eyes move, signals reach my auditory cortex and I perceive the input as noise.
Normally, dysfunctional noise gets filtered out. For example, the thunderous sounds in our head caused by talking or chewing would disrupt hearing if they were not shut off before reaching the auditory cortex. In cases of people who have tinnitus, doctors may someday prescribe a small dose of selective serotonin reuptake inhibitor (SSRI) drugs—the same ones used for treating depression—a bit of serotonin, or the diet supplement tryptophan, which the body uses to make serotonin.
“We must write you two up as a case history for the medical literature,” Rauschecker tells us. I am just delighted to know at last that my eyeballs' sound effects are simply my nucleus accumbens still snoozing. As soon as I wake up, it wakes up, too, and gets back to work filtering out important incoming messages from the junk.
I left the party stunned by the serendipity. What if we three had not happened by chance to meet in a bar? Had we shared coffee instead of beer, would Bill and I have felt forthcoming enough to reveal our peculiar secrets? Something as important as science should not be left to chance—but so often that is the way it is.
