Neuroscientists locate gatekeeping system for 'phantom' sensations
Tinnitus and chronic pain have more in common than their ability to afflict millions with the very real experience of "phantom" sensations. Scientists noted similarities between the two disorders more than thirty years ago. Now advances in brain imaging and associated techniques have enabled researchers to begin homing in on their structural and functional bases, revealing what appears to be a central gatekeeping system implicated in both chronic pain and tinnitus.
Tinnitus sufferers hear sounds that aren't there, usually described as a ringing or buzzing "in the ears" even though such perceptions clearly arise in the brain. Hearing loss is often associated with tinnitus but does not adequately account for it. Furthermore, not everyone with damaged cochlear nerves or cortical auditory circuits experiences tinnitus. One hypothesis suggests that the decisive factor is whether or not a higher-level cognitive system responsible for "tuning out" negative sensory signals is working normally.
This view of tinnitus was hailed as a paradigm shift by people in the field when it was first proposed, in 2010, by Prof. Josef Rauschecker, Director of the Laboratory for Integrative Neuroscience and Cognition at Georgetown University and a Hans Fischer Senior Fellow of the TUM Institute for Advanced Study. Mounting evidence has strengthened the case, as improved techniques allow higher-resolution studies of changes in the brain.
In the October issue of Trends in Cognitive Sciences, researchers say identifying the problem is the first step to developing effective therapies for these disorders, which afflict millions of people. None currently exist.
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. They traced the flow of these signals through the brain and showed where "circuit breakers" should be working -- but aren't.
In both disorders, the brain has been reorganized in response to an injury in its sensory apparatus, says Josef Rauschecker, PhD, DSc, director of the Laboratory for Integrative Neuroscience and Cognition at GUMC. Tinnitus can occur after the ears are damaged by loud noise, but even after the brain reorganizes itself, it continues to "hear" a constant hum or drum. Similarly, chronic pain can occur from an injury that often is healed on the outside 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," he says. "Both conditions are extraordinarily common, yet no treatment gets to the root of these disorders."
The affected areas are also important for evaluating and modulating emotional experiences, Rauschecker says. "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 modulates information flow in the brain. Tinnitus and chronic pain occur when this system is compromised."
Other issues often arise in "lockstep" with tinnitus and/or chronic pain, such as depression and anxiety, which are also modulated by the nucleus accumbens, he says. Uncontrollable or long-term stress is another important factor.
"However," Markus Ploner adds, "better understanding might allow not only for early treatment, but also for more targeted and individual treatment with existing as well as novel strategies, such as dopaminergic therapy." Ploner is a consultant neurologist at TUM as well as Heisenberg Tenure Track Assistant Professor of Human Pain Research.