Remodeling Sensory and Motor Circuits in the Brain: New Insights from Hearing Loss and Tinnitus

The human brain contains 50 billion neurons with trillions of synaptic connections that allow sensory and motor systems to acquire and integrate many different forms of information. The pioneering electrical stimulation studies of Penfield mapped out the geographical regions of the cerebral cortex occupied by the somatosensory (touch), visual, auditory and olfactory system. Each sensory system occupies a unique location on the cerebral cortex. The sensory and motor circuits in the brain are extraordinarily flexible allowing neuroplastic changes to occur at the molecular, cellular, and structural level. Over the past decade human brain imaging studies and animal models have provided new insights into some of the mysterious sensory-motor interactions that occur in patients with hearing loss. If we attend a loud rock concert and damage our inner ear, a phantom ringing or buzzing sensation known as tinnitus can emerge. Studies of hearing loss and tinnitus have revealed some of the complex multimodal connections that exist among different regions of the brain. We have studied how the cortical territory formerly occupied by the auditory system can shift to respond to somatosensory stimulation of the head, face, and upper torso. Surprisingly, nearly 70% of patients can voluntarily modulate the loudness or pitch of their tinnitus by moving the head, face, and upper torso. How can movements of the head, face, neck, jaw and eyes modulate tinnitus? To identify the neural generators involved in tinnitus, functional brain imaging studies were performed on patients who could voluntarily modulate the loudness of their tinnitus with motor actions. Results from human PET imaging studies led to the conclusion that tinnitus was generated in the brain, not in the inner ear as previously believed. I shall present background and describe recent studies of the remarkable neuroplastic reorganization in the auditory cortex in response to hearing loss, touching on how studies of tinnitus and hearing loss provide a window into the organization and operation of the nervous system.