Tinnitus is Associated with Disturbed Nerve Function
•Nucleus accumbens (NAc) hyperactive to tinnitus-frequency-matched stimuli
•Structural anomaly in ventromedial PFC correlates with NAc hyperactivity
•Moderate core and posterior auditory cortex hyperactivity in patients
•Limbic corticostriatal dysregulation may cause chronic tinnitus
Tinnitus is a common disorder characterized by ringing in the ear in the absence of sound. Converging evidence suggests that tinnitus pathophysiology involves damage to peripheral and/or central auditory pathways. However, whether auditory system dysfunction is sufficient to explain chronic tinnitus is unclear, especially in light of evidence implicating other networks, including the limbic system. Using functional magnetic resonance imaging and voxel-based morphometry, we assessed tinnitus-related functional and anatomical anomalies in auditory and limbic networks. Moderate hyperactivity was present in the primary and posterior auditory cortices of tinnitus patients. However, the nucleus accumbens exhibited the greatest degree of hyperactivity, specifically to sounds frequency-matched to patients' tinnitus. Complementary structural differences were identified in ventromedial prefrontal cortex, another limbic structure heavily connected to the nucleus accumbens. Furthermore, tinnitus-related anomalies were intercorrelated in the two limbic regions and between limbic and primary auditory areas, indicating the importance of auditory-limbic interactions in tinnitus.
From press release:
Tinnitus appears to be produced by an unfortunate confluence of structural and functional changes in the brain, say neuroscientists at Georgetown University Medical Center (GUMC).
The phantom ringing sounds heard by about 40 million people in the U.S. today are caused by brains that try, but fail to protect their human hosts against overwhelming auditory stimuli, the researchers say in the January 13th issue of Neuron. They add that the same process may be responsible for chronic pain and other perceptual disorders.
The researchers say that the absence of sound caused by hearing loss in certain frequencies, due to normal aging, loud-noise exposure, or to an accident, forces the brain to produce sounds to replace what is now missing. But when the brain's limbic system, which is involved in processing emotions and other functions, fails to stop these sounds from reaching conscious auditory processing, tinnitus results.
"We believe that a dysregulation of the limbic and auditory networks may be at the heart of chronic tinnitus," says the study's lead investigator, Josef P. Rauschecker, PhD, a neuroscientist. "A complete understanding and ultimate cure of tinnitus may depend on a detailed understanding of the nature and basis of this dysregulation."
Tinnitus isn't curable, although antidepressants appear to help some patients, as does the use of masking noise to diminish focus on the ringing sensations.
Using functional Magnetic Resonance Imaging (fMRI), the Georgetown researchers tested 22 volunteers, half of whom had been diagnosed with chronic tinnitus. They found that moderate hyperactivity was present in the primary and posterior auditory cortices of tinnitus patients, but that the nucleus accumbens exhibited the greatest degree of hyperactivity, specifically to sounds that were matched to frequencies lost in patients.
The nucleus accumbens is part of the corticostriatal circuit, which is involved in evaluation of reward, emotion, and aversiveness, says Rauschecker. "This suggests that the corticostriatal circuit is part of a general 'appraisal network' determining which sensations are important, and ultimately affecting how or whether those sensations are experienced," he says. "In this study, we provide evidence that these limbic structures, specifically the nucleus accumbens and the ventromedial prefrontal cortex, do indeed differ in the brains of individuals with tinnitus."
Functional lapses in these same areas have also been implicated to altered mood states and to chronic pain. "Both of these conditions may also involve the inability to suppress unwanted sensory signals," Rauschecker says.
Based on their findings, the researchers argue that the key to understanding tinnitus lies in understanding how the auditory and limbic systems interact to influence perception -- be it sound, emotions, pain, etc.
Grant support was provided by the National Institute on Deafness and Other Communication Disorders, the Tinnitus Research Consortium, the Tinnitus Research Initiative, and the Skirball Foundation.
Amber M. Leaver, Laurent Renier, Mark A. Chevillet, Susan Morgan, Hung J. Kim and Josef P. Rauschecker.
Dysregulation of Limbic and Auditory Networks in Tinnitus.