Researchers have found that the brain is capable of doing something that sufferers from tinnitus cannot do: ignore the ringing sound. The finding suggests key differences in the neural bases of tinnitus and the larger disorder of hearing loss.
Beckman Institute researcher Fatima Husain led the study, which revealed the differences thanks to a novel research approach that included test subjects who had hearing loss but who did not have tinnitus.
It is estimated that tinnitus affects about 50 million Americans, but less than half of hearing impaired people suffer from the disorder, commonly known as ringing in the ears, or “head noise”. Husain decided to explore the relationship between tinnitus and the larger issue of hearing impairment by including measures of brain responses in subjects with only bilateral hearing loss (HL), in addition to subject groups with HL and tinnitus (TIN), and a control group with normal hearing (NH).
The study, published in the journal PLoS ONE, used pure tones and frequency-modulated sweeps (variations in pitch) as auditory stimuli and functional Magnetic Resonance Imaging (fMRI) to measure the brain’s responses in areas that comprise what is known as the short-term memory and attention network: the frontal, parietal and temporal cortices and the anterior cingulate.
The study found that the HL group performed as well on discrimination tasks as the NH group, but the fMRI revealed that their brains had to work harder and were stimulated in more areas than the NH group. There were clear differences in the responses in those groups and the tinnitus group, as the researchers found “decreased activation in the parietal and frontal lobes in the participants with tinnitus compared to the HL group and decreased response in the frontal lobes relative to the NH group. Additionally, the HL subjects exhibited increased response in the anterior cingulate relative to the NH group.”
Husain said the responses of the tinnitus group show that the brain in those with the disorder is able to ignore the “head noise” signal, compared to those with hearing loss alone, in discrimination tasks.
“So when the brain has an internal noise, the tinnitus, that is distracting it from doing other tasks, it is dividing attention. It ignores the distracting internal sound, while still maintaining attention to the external sound,” Husain said. “If I had not included the study of hearing loss without tinnitus, I wouldn't have seen this. Those with hearing loss use all their resources to listen to the external sound.”
These findings are especially meaningful because of what they reveal about how the brains of those with tinnitus and those with hearing loss alone are functioning differently. “Our results,” the researchers wrote, “suggest that a differential engagement of a putative auditory attention and short-term memory network, comprising regions in the frontal, parietal and temporal cortices and the anterior cingulate, may represent a key difference in the neural bases of chronic tinnitus accompanied by hearing loss relative to hearing loss alone.”
Husain, who directs the Auditory Cognitive Neuroscience Lab in the Department of Speech and Hearing Science, is a member of Beckman’s Human Perception and Performance group. She is using this work to build predictive models of the probability of developing tinnitus and for evaluating, modifying, and developing therapies.