Neuroscientists at Georgetown University Medical Center (GUMC) and Germany’s Technische Universität München report that they’ve identified the brain mechanism responsible for tinnitus and chronic pain — the symptoms that can persist long after an initial injury.
In an article slated to appear in the October 2015 issue of Trends in Cognitive Sciences, researchers explain that identifying the underlying problem is the first step to developing effective therapies for tinnitus and chronic pain. In their article, the scientists describe how the neural mechanisms that normally “gate” or control noise and pain signals can become dysfunctional, leading to a chronic perception of these sensations. In their study, the researchers traced the flow of these signals through the brain and showed where “circuit breakers” should be working, but aren’t.
In both disorders, according to the research team, the brain has been reorganized in response to an injury in its sensory apparatus. Tinnitus can occur after the ears are damaged by loud noise or other issue, but even after the brain reorganizes itself, it continues to “hear” a constant hum or drum. Chronic pain can occur from an injury that often is healed elsewhere in the body but persists inside the brain.
“Some people call these phantom sensations, but they are real, produced by a brain that continues to ‘feel’ the initial injury because it cannot down-regulate the sensations enough,” said Josef Rauschecker, PhD, DSc, director of the Laboratory for Integrative Neuroscience and Cognition at GUMC. “Both conditions are extraordinarily common, yet no treatment gets to the root of these disorders.”
The researchers report that areas of the brain responsible for these errant sensations are the nucleus accumbens, the reward and learning center, as well as other brain regions that serve “executive” or administrative roles, such as the ventromedial prefrontal cortex (VNPFC), and the anterior cingulate cortex. All of these areas are also important for evaluating and modulating emotional experiences, according to Rauschecker.
“These areas act as a central gatekeeping system for perceptual sensations, which evaluate the affective meaning of sensory stimuli — whether produced externally or internally — and modulate information flow in the brain. Tinnitus and chronic pain occur when this system is compromised,” Rauschecker says. He notes that other issues often arise in concert with tinnitus and/or chronic pain, such as depression and anxiety, which are also modulated by the nucleus accumbens. Uncontrollable or long-term stress is another important factor in these symptoms.
The brain plasticity that produces some of these changes provides hope that this gatekeeping role can be restored. Because these systems rely on transmission of dopamine and serotonin between neurons, drugs that modulate dopamine may help restore sensory gating.
“Better understanding could also lead to standardized assessment of individuals’ risk to develop chronic tinnitus and chronic pain, which in turn might allow for earlier and more targeted treatment,” said Markus Ploner, MD, PhD, a consultant neurologist and Heisenberg Professor of Human Pain Research at the Technische Universität München (TUM) in Germany.
Rauschecker, an expert in tinnitus, collaborated with Ploner, who studies chronic pain, during his senior fellowship at the Institute of Advanced Study at TUM. Co-authors include Audrey Maudoux, MD, PhD, from GUMC and Elisabeth May, PhD, from TUM.
For a related story, read this article describing a research study from McGill University Medical Centre that looks at the neuropathic pathways of pain–or, “pain in the brain.”