Neuroscience: How cochlear implants improve hearing

The neural mechanisms that enable cochlear implants to successfully restore hearing are elucidated in a study in rats, published in Nature this week. The study provides a pathway to help to improve the performance of these widely used medical devices.

Cochlear implants can restore hearing in people with profound deafness, but responses vary widely. Some recipients can understand speech within hours of the implant’s activation, whereas others experience little recovery even after many months of use. To understand why, Robert Froemke and colleagues fitted 16 deaf rats with cochlear implants and then studied the patterns of brain activity associated with hearing restoration. Similar to people, rats varied in their responses to the implant: in the study, activity in the locus coeruleus (a region of the brainstem associated with learning) predicted a positive reaction. When the same brain region was activated artificially, the variation observed between animals disappeared — every rat stimulated in this way displayed responses to sounds after only a few days of being fitted with the implant.

Neurons in the locus coeruleus make and release the neuromodulatory chemical noradrenaline, which can then influence the structure and function of diverse neural networks. This ‘rewiring’ of the brain is a key feature of learning; when cochlear implants fail, it may be because the locus coeruleus is not adequately engaged and the brain is less able to rewire itself. Strategies that help to engage this target area could therefore help to optimize the functioning of neuroprosthetic devices, the authors suggest.