Some people feel dizzy when they hear certain sounds. We finally know why that happens
His name was Pietro Tullio, and he used to drill tiny holes into birds' heads.
It sounds gruesome (and probably was), but for Tullio – an Italian biologist of the early 20th century – it was pioneering research, not sadism. And the strange experiment produced an even stranger result, which ultimately became known as the Tullio phenomenon.
What Tullio discovered was that when his poor pigeon subjects received these tiny holes in the semicircular canals of their inner ear, they became acutely sensitive to sound, with exposure to certain tones causing them severe balance problems.
Unfortunately, it's not only Tullio's perforated avians who are susceptible to these unsettling bouts of dizziness and vertigo.
It's estimated that around one in 100 people have abnormally thin or non-existent bones overlying their superior semicircular canal – a congenital condition known as superior or semicircular canal dehiscence, and the most common cause of the Tullio phenomenon in humans.
"It's very much like the feeling when they've had too much to drink," says biomedical engineer Richard Rabbitt from the University of Utah.
"They get dizzy, and they feel nauseous, and they can't see well and lose their balance."
But why does it happen?
In new research, Rabbitt and his team have quantified what's actually going on inside people's heads to explain these intense topsy-turvy sensations, evoked by nothing other than musical notes, the sounds of conversation, or sometimes subtle changes in atmospheric pressure.
"What our paper is about is the biophysics of how that happens," he explains.
"How does sound excite the inner ear balance organs causing them to send the wrong head-motion signals to the brain?"
Since Tullio first struck upon the strange phenomenon, scientists observed the condition causes a rapid involuntary movement of the eyes, known as ocular nystagmus.
This kind of eye jitter is usually an automatic reflex designed to restore balance and visual stabilisation during body movement, but for people with superior canal dehiscence, sound triggers the same eye motions even when they're perfectly still, which is why they feel sensations of dizziness and vertigo.
"Your eyes will counter-rotate the wrong way," Rabbitt explains, "and it will look like the world is spinning."
To investigate why this takes place, the researchers used computer modelling to simulate fluid motion in the "vestibular labyrinth" of the inner ear, and also examined the biophysics of oyster toadfish, which have similar inner ear balance organs as humans.
Normally, when we move our heads, fluid in the inner ear moves smoothly in a predictable way.
But the team's analysis suggests that in cases of canal dehiscence – where there are holes or deformations in inner ear bones – the smooth motion is disrupted by mechanical waves in the fluid, which can be triggered by sound waves entering the ear.
According to the team's analysis, these waves can trick our inner ear nerves, with irregularly pumped inner ear fluid fooling our senses into thinking we're experiencing angular head acceleration – all because somebody hit a particular key on a piano, or laughed at a joke a certain way.
Or, in paper speak: "Sound energy entering the inner ear at the oval window excites fluid motion at the location of the defect, giving rise to travelling waves that subsequently excite mechano-electrical transduction in the vestibular sensory organs by vibration and nonlinear fluid pumping".
The good news is many patients who experience semicircular canal dehiscence can actually have the bone issue successfully treated via surgery.
But for researchers like Rabbitt, it's scientifically satisfying at last to have the bizarre mystery of the Tullio phenomenon soundly solved, almost a century after it was first discovered in those poor, pin-cushion pigeons.
"What wasn't known was the 'Why?' What exactly causes the symptoms patients have?" Rabbitt says.
"This finally connects the symptoms and the dehiscence in a precise biophysical way."
The findings are reported in Scientific Reports.
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