I N N O V A T E 6 2 0 1 1 80 E S S A Y S The cochlear implant has restored partial hearing to over 200 000 profoundly deaf people, making it the most widely distributed and successful neural prosthetic by several orders of magnitude. It has also proven useful as a research tool in expanding the understanding of neural plasticity in children and the mechanisms involved in auditory pattern recognition. Speech perception in certain users is incredible, considering that the low-resolution signal, presented through a limited number of channels, still provides enough information for speech recognition. Subject-specific modelling of neural responses to electrical stimulation of the auditory system by Ramana Govindasamy, Tiaan K Malherbe, Prof Tania Hanekom and Prof Johan J Hanekom In the vicinity of the inner ear lies the cochlea (Latin for “snail”), which is responsible for the conversion of sound waves into neural impulses. This transduction is inclusive of displacements in the nanometre range and can occur at a rate that humbles other peripheral systems. Engineers are familiar with the Fourier transform, which enables the decomposition of complex signals into fundamental sinusoids. This spectral discretisation is explanatory of the cochlea’s ability to represent audible frequencies as a function of length along the cochlea (tonotopy). This organisational structure is preserved through to the inferior colliculus, a metabolically vibrant region in the midbrain involved in acoustic integration and linked to the cochlea via the auditory nerve. Sensorineural hearing loss occurs when the hair cells required for transduction are damaged. The cochlear implant is an engineering feat enabling thousands of people to regain some perception of hearing by the controlled electrical stimulation of the auditory nerve fbres. Existing implants provide varying degrees of speech perception, enabling some users to engage in telephonic communication without associative lip reading. In addition, they have been clinically proven to enhance the implantees’ quality of life. Despite its widespread success, the level of hearing restoration, speech intelligibility and music perception provided by the cochlear implant varies greatly among subjects. Speech intelligibility is particularly diffcult in noise, while music perception is generally poor, although quiet environments also lead to variance in speech perception, suggesting that there may be inherent subject-specifc variables that infuence this. Members of the University of Pretoria’s Bioengineering Group have been working in the feld of cochlear implants for more than two decades. The research group specialises in computational modelling of the electrically stimulated hearing system. The models are used to probe and understand the underlying  Figure 1. Modelling of a specifc subject's auditory neural response.