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Phantom percepts, such as subjective tinnitus, are driven by fundamental changes in spontaneous brain activity. Sleep is a natural example of major shifts in spontaneous brain activity and perceptual state, suggesting an interaction between sleep and tinnitus that has so far been little considered. In a new collaborative review article from DPAG’s auditory and sleep neuroscientists, tinnitus and sleep research is brought together for the first time, and, in conclusion, they propose a fundamental relationship between natural brain dynamics and the expression and pathogenesis of tinnitus.

A graphical depiction of tinnitus as ‘local wakefulness’ in the sleeping brain. If tinnitus related activity in the brain persists during sleep it may locally interfere resulting in local wakefulness, which in turn, may hinder the brain from entering global, restorative sleep. Not depicted in this figure, but discussed in the review, is the possibility that neural activity under high sleep pressure may temporarily suppress tinnitus and associated 'local wakefulness'. © Linus Milinski
A graphical depiction of tinnitus as ‘local wakefulness’ in the sleeping brain. Tinnitus related activity in the brain during sleep may cause local wakefulness and hinder the brain from entering global, restorative sleep.

Subjective innitus – or tinnitus for short - is a very common phenomenon defined by a constant phantom sound generated by the brain, usually in the form of a persistent ringing or hissing. Many people experience temporary tinnitus after, for example, a music concert or a bad case of the common cold. However, permanent tinnitus affects more than 250 million people worldwide, which severely impacts their quality of life causing many to experience depression or anxiety. There is currently no cure for tinnitus, so treatments presently focus on helping people to cope with the condition. Common triggers of tinnitus include intense exposure to noise and any form of hearing loss or damage to the ear. It is widely understood that multiple parts of the brain, including, but not limited to, the auditory system, become hyperexcited and hyperactive in the event of ear damage, leading to the sensation of tinnitus. However, precisely what happens in the brain as tinnitus develops and progresses is currently unclear.

The brain also undergoes a widespread alteration of spontaneous activity when we sleep, and sleep disruption is a common symptom experienced by people with tinnitus. Yet, we know very little about this apparent link between tinnitus and sleep. We also do not know how brain activity caused by tinnitus is affected by the state of being awake or asleep, nor how the effects of sleep on brain plasticity may contribute to the consolidation of tinnitus in affected people. A new review led by Linus Milinski and Associate Professor Victoria Bajo Lorenzana addresses the relationship between tinnitus and sleep for the first time. In bringing together recent developments in the fields of tinnitus and sleep research, they have identified a clear relationship between phantom sounds, sleep and sensory disconnection. They have thus proposed a fundamental interaction between the phantom percepts caused by tinnitus-aberrant brain activity and natural brain state dynamics. These findings have important implications for tinnitus research, diagnostics and potential therapeutic interventions.

About 75% of our total sleep time is spent in non-rapid eye movement (NREM) sleep, during which the brain produces a stereotypical slow oscillatory activity that spreads across the cortex. Research has identified regions impacted by tinnitus that are known to prominently express  slow-wave activity during NREM sleep. This spatial overlap between brain regions suggests a dynamic interaction between the two seemingly separate activities. First author Linus Milinski said: “It could explain why interrupted sleep is such a common symptom in tinnitus patients. Tinnitus activity might be reduced during intense NREM sleep. But as sleep pressure decreases, and with it the drive of the brain to express slow-wave activity, aberrant brain activity could regain its potential to affect the brain on a wide scale as observed during wakefulness in tinnitus patients.

Watch a complete outline from Linus Milinski in the below video abstract (via the BRAIN Journals YouTube channel):

Follow this link for the full text of Linus Milinski's narration

The researchers not only describe the mechanism underlying the interference of tinnitus with sleep; they also issue a framework for future research that could ultimately lead to the development of new clinical guidelines for treating tinnitus. Linus Milinski said: “While research towards understanding tinnitus has progressed in recent years, a breakthrough for treatment development is still not in sight. The role of natural brain state dynamics has - surprisingly - been ignored in this endeavour. We make the case that widening the scope in tinnitus research towards the brain's natural dynamics will provide fruitful ground for understanding those of pathological nature.”

Associate Professor Bajo Lorenzana concluded: “Our proposed mechanism could explain comorbidities so prominently seen in tinnitus patients and lead to a new angle in clinical and basic research. Furthermore, we describe how the brain's natural dynamics during sleep may be harnessed for tinnitus treatment and how sleep is ultimately linked to how tinnitus develops over time. These findings will help researchers to identify a time window where delivering a treatment for tinnitus will be most effective before it develops into a permanent condition. The findings may also provide information about how tinnitus affects sleep quality. This could lead to a new line of research looking at whether sleep could help to correct the abnormal brain activity that is linked to tinnitus.”

The full review, “Tinnitus: at a crossroad between phantom perception and sleep”, available to read in Brain Communications, is a collaboration between Linus Milinski, Dr Fernando Nodal, Professor Vladyslav Vyazovskiy, and Associate Professor Victoria Bajo Lorenzana.

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