About one in 100 people worldwide experience a spike of nausea and vertigo and alarming unconscious rapid eye movement in response to sound waves of certain frequencies hitting their inner ears. The unpleasant reaction, known as the Tullio phenomenon, occurs in individuals with a condition called semicircular canal dehiscence, wherein a birth defect or injury causes an abnormal hole in the tiny bone that covers one of the three fluid-filled tubes within the ear. These tubes serve to detect the up and down motions of the skull, and thus allow our brains to orient themselves in three-dimensional space.
Officially described in 1929, biologists have since observed that the phenomenon can be triggered by sustained musical tones from a variety of instruments, high-pitched conversations, and even changes in atmospheric pressure or coughing. Thankfully, physicians have also developed surgical techniques that alleviate the problem by repairing the gap in the bone.
But why this happens has remained mysterious – until now.
A research team led by Richard Rabbitt at the University of Utah has used a series of experiments in inner ears taken from oyster toadfish, a small oceanic species that have a similar auditory organ structure and function as humans, to discover how sound waves can induce the firing of nerve signals that normally only relate to movement. The work has been published in the journal Scientific Reports.
"It's very much like the feeling when they've had too much to drink. They get dizzy, and they feel nauseous, and they can't see well and lose their balance," Rabbitt explained about what it’s like to have the condition in a statement. "What our paper is about is the biophysics of how that happens. How does sound excite the inner ear balance organs causing them to send the wrong head-motion signals to the brain?"
The answer is surprisingly intuitive, if you’re well-versed in fluid mechanics, that is. After exposing fish whose ear canals had been altered to resemble semicircular canal dehiscence to auditory stimuli, Rabbitt and his colleagues observed that the sound waves entering the inner ear induced an unusual pumping of the fluid (endolymph) within the tubes, giving rise to traveling waves that subsequently excite specialized, mechanically triggered cells that in turn signal nerve cells. This stimulation leads the brain to mistakenly conclude that the head is rotating.
To compensate, "[y]our eyes will counterrotate the wrong way, and it will look like the world is spinning," said Rabbitt. "This finally connects the symptoms and the dehiscence in a precise biophysical way."
You can listen to the types of sounds that can trigger Tullio phenomenon in the clips below, though if you think you have the condition you probably shouldn't.