April 2015 issue
April 30, 2015 | Neural engineers and clinical researchers at commercial firms and research laboratories continue to look for new ways to treat tinnitus, which remains one of the greatest unmet medical needs of the 21st century. Nearly 10 percent of the population has experienced ringing in the ear and 10 million Americans seek medical treatment each year. There are few viable therapies available.
One of the earliest neurotech approaches to treating severe tinnitus was epidural stimulation of auditory cortex. A promising clinical trial sponsored by Northstar Neuroscience in 2006 was later scrapped, however, when that company closed its doors following a failed clinical trial for stroke rehabilitation. More recently, a European team conducted a trial of acoustic neuromodulation [NBR Mar15 p3]. Last year, MicroTransponder Inc. began an NIH-sponsored trial of vagus nerve stimulation-induced cortical plasticity. During the therapy, patients wear headphones and hear a series of single frequency tones, paired with stimulation to the vagus nerve.
Early research suggested that tinnitus might be the result of the brain trying to regain the ability to hear lost frequencies by turning up the signals of neurons in neighboring tonotopic frequencies. Because there are too many neurons processing the same frequencies, they fire more strongly, more frequently and in concert with each other, even when the environment is quiet. It is these changing brain patterns that researchers believe could produce the perception that characterizes tinnitus.
Earlier this month, researchers reporting in Current Biology took advantage of a rare opportunity to record directly from the brain of a person with tinnitus in order to find the brain networks responsible. The observations reveal just how different tinnitus is from normal representations of sounds in the brain.
“Perhaps the most remarkable finding was that activity directly linked to tinnitus was very extensive, and spanned a large proportion of the part of the brain we measured from,” said Will Sedley of Newcastle University. “In contrast, the brain responses to a sound we played that mimicked [the subject’s] tinnitus were localized to just a tiny area.”
In the new study, Sedley and The University of Iowa’s Phillip Gander contrasted brain activity during periods when tinnitus was relatively stronger and weaker. The study was only possible because the 50-year-old man they studied required invasive electrode monitoring for epilepsy. He also happened to have a typical pattern of tinnitus, including ringing in both ears, in association with hearing loss.
The researchers found the expected tinnitus-linked brain activity, but they report that the unusual activity extended far beyond circumscribed auditory cortical regions to encompass almost all of the auditory cortex, along with other parts of the brain. The discovery adds to the understanding of tinnitus and helps to explain why treatment has proven to be such a challenge.
“The sheer amount of the brain across which the tinnitus network is present suggests that tinnitus may not simply ‘fill in’ the ‘gap’ left by hearing damage, but also actively infiltrates beyond this into wider brain systems,” Gander said.
These new insights may help to inform treatments such as neurofeedback, where patients learn to control their “brainwaves,” or electromagnetic brain stimulation, according to the researchers.
