Improving the hearing of people with cochlear implants

A cochlear implant, or 'bionic ear', is an electronic device that directly stimulates the auditory nerve, which can help even profoundly deaf people gain a sense of sound. Deafness Research UK recently funded a project aimed at improving the speech understanding and musical perception for people with cochlear implants.

[Project grant: 2004-2006]

Although the technology has improved greatly over the past 20 years, cochlear implants cannot yet restore perfect hearing. In particular, there are difficulties with the way in which they process pitch, making it difficult for users to appreciate music or pick out individual voices from a background of complex noise.

The aim of this project, led by Dr Robert Carlyon at the MRC Cognition & Brain Sciences Unit in Cambridge, was to identify ways in which the sound processing in cochlear implants can be modified so that pitch perception can be improved.

Cochlear implants represent sound by converting it into a series of electrical impulses, replacing the job normally done by the ear. Other researchers have suggested that pitch processing in cochlear implant users could be improved by adding a low-level background of fast pulses to the normal cochlear implant signal.

Dr Carlyon’s team began the study by testing whether this would work and found that it does not – in fact it made hearing a little more difficult. This was a useful result because it suggests that cochlear implant companies and researchers should not direct their efforts at improving pitch perception towards this method!

A second set of experiments showed that even quite small changes in the level of a sound can affect its pitch. However, whether the changes are upwards or down, although consistent within a listener, varies idiosyncratically between different listeners.

The research team have gone on to try and understand why this should be so. By looking to see whether the level-dependent changes in pitch perception are reflected in neural activity, the team hopes to find ways to minimise or at least control them.

Finally, a series of sound pulses was presented to normal-hearing listeners, and the team observed similar results to those found for cochlear implant users. Measuring the neural response to the sound pulses revealed that pitch is affected by the rate at which neurons are able to respond and then respond again.

The team aim to use their findings to gain better control of the pitch resulting from more complex patterns of pulses that are produced by cochlear implant processors.