Cochlear implantation, as a means of restoring hearing to profoundly and totally deaf people, has now become a routine clinical procedure. Implant users can perform remarkably well in many aspects of speech perception. However, current implant devices do not provide all speech information equally well. One major limitation is in providing voice fundamental frequency (fO), known to be problematic even for non-tonal languages in which intonation plays an important role in speech communication. This causes even more difficulty in tonal languages, such as Mandarin Chinese or Thai, in which pitch variations are used to convey lexical meanings. This thesis is mainly concerned with how implant users perceive and use tonal information. Studies were first conducted in normal-hearing listeners to investigate the nature of tone and the importance of voice fO in understanding running speech. Three acoustic cues (fO, amplitude envelope, and duration) were examined for their contributions to tonal perception in syllables. The results clearly demonstrated voice fO to be the dominant cue. To determine the effect of explicit fO in sentence recognition, vocoded stimuli with various degrees of spectral information were presented to four age groups of listeners (aged 6, 9, 12, and 20). Information about natural fO variations enhanced sentence recognition significantly even when spectral information was severely degraded, and the effect was strong across all ages. The investigation in implanted children first examined tone recognition performance and the acoustic cues used for recognising tonal contrasts, especially the use of amplitude envelope. These implanted children appeared to make some use of amplitude changes in recognising tonal contrasts, though the overall effect was rather small. They also showed some evidence for the use of duration and temporal pitch information. For the effect of fO in sentences, no significant difference was found in performance on sentences with their original fO contours and those with uninformative fO contours. This indicates that the voice pitch information provided by current implant devices is too limited to allow listeners to take advantage of the presence of natural fO. In the light of the significance of fO both in signalling tonal identity in syllables and in perceiving sentences, it is likely that implant users will further benefit with a better representation of voice pitch.

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Mismatch negativity (MMN) is a negative component in the human event related potential that occurs in response to a change in the nature of a repeating stimulus. Since it occurs even in the absence of attention, it reflects an automatic cerebral process for detecting change. The MMN is clinically helpful in detecting sensory memory and central processing deficits in groups, but its use as a clinical tool in individual patients is yet to be established. Late Discriminative Negativity (LDN) is another recently reported second negativity that follows MMN and has been suggested to reflect the automatic processing of complex linguistic stimuli in children. The use of cochlear implants in profoundly deaf children is increasing with a trend towards earlier implantation. However the tools used at present to assess and predict outcome of cochlear implantation are primarily based on behavioural tests, which are difficult to use effectively in young children due to limitations in communication and cognitive skills in this age group. MMN has been proposed as a potential test which can be used to assess auditory memory and central auditory processing in cochlear implant patients. The main objective of this study was to assess the correlation of auditory event related potential (ERP) measures with behavioural assessment data to identify if ERPs including mismatch negativity (MMN) can be used to categorize cochlear implant patients into good and poor performers. We carried out an observational, cross sectional, non-randomized, cohort study investigating auditory event related potentials to speech stimuli in 35 cochlear implant patients between the ages of 7 and 17 years and compared the occurrence, latencies and amplitudes of P1, N2, MMN and LDN with overall behavioural outcome in these children. Behavioural measures included category of auditory performance scores (CAP) and speech intelligibility rating scores (SIR). Auditory ERPs in response to standard stimuli were identifiable in 30 out of 35 patients, demonstrating a major positive component (P1) followed by a negativity (N2) with absence of N1 in all patients. The P1 component in pre-lingually deaf patients showed a statistically significant reduction in its latency with increasing duration of implant use. MMN was recorded in 80-85% of star performers but in only 15-20% of poor performers. Patients with higher SIR scores demonstrated statistically significant longer duration of MMN compared to those with a lower SIR score. Patients with higher SIR and CAP scores also demonstrated a statistically significant inverse relationship with the duration of LDN. These results indicate that MMN can be used to assess the functional status of the auditory cortex in young children with cochlear implants and may provide an objective mechanism for differentiating good from poor performers.

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This thesis describes the design and development of an autonomous micro-drilling system capable of accurately controlling the penetration of complaint tissues and its application to the drilling of the cochleostomy; a key stage in the cochlea implant procedure. The drilling of the cochleostomy is a precision micro-surgical task in which the control of the burr penetration through the outer bone tissue of the cochlea is vital to prevent damage to the structures within and requires a high degree of skill to perform successfully. The micro-drilling system demonstrates that the penetration of the cochlea can be achieved consistently and accurately. Breakthrough can be detected and controlled to within 20µm of the distal surface and the hole completed without perforation of the underlying endosteal membrane, leaving the membranous cochlea intact. This device is the first autonomous surgical tool successfully deployed in the operating theatre. The system is unique due to the way in which it uses real-time data from the cutting tool to derive the state of the tool-tissue interaction. Being a smart tool it uses this state information to actively control the way in which the drilling process progresses. This sensor guided strategy enables the tool to self-reference to the deforming tissue and navigate without the need for pre-operative scan data. It is this capability that enables the system to operate in circumstances where the tissue properties and boundary conditions are unknown, without the need to restrain the patient.

Normal-hearing adults have no difficulty in recognising consonants accurately, even in moderately adverse listening conditions. By contrast, users of multichannel cochlear implants have difficulty with the accurate perception of consonants, even in good listening conditions. Cochlear implant users are known to show systematic deficits in recognition of consonant features, with perception of the place feature, which relies on spectral information, being worst. These deficits may be attributed both to signal distortions introduced by the processing of the implants and to other factors, in particular the spectrotemporal distortions which occur at the interface between electrode array and auditory nervous system, including cross-channel interaction. The objective of the work reported here was to attempt to partial out the relative contribution of these different factors to consonant recognition. This was achieved by comparing cochlear implant users’ perceptual errors, analysed in terms of information transmission, with errors made by normal-hearing subjects listening to acoustic models of implant processing, in various conditions. Two initial experiments were undertaken to develop and refine an acoustic model of the Nucleus 24 cochlear implant. Findings from these two experiments informed the design of the main acoustic model experiment, which was undertaken in parallel with a further experiment involving users of the Nucleus 24 device. In both experiments, subjects listened to nonsense syllables with and without the addition of stationary background noise, in three different configurations of implant processing parameters. Additionally, in the acoustic model experiment, a simulation of cross-channel spread of excitation, or “channel interaction”, was varied. Results showed that acoustic model experiments were predictive of the pattern of consonant feature transmission in cochlear implant users with better baseline consonant recognition scores. Deficits in consonant recognition in this subgroup could be explained by the loss of phonemically relevant acoustic information in speech due to the nature of cochlear implant processing, while channel interaction appeared to play a smaller role in accounting for problems in consonant recognition. The work also evaluated the effect of changes in channel number and stimulation rate and failed to find any changes in consonant recognition as these parameters were varied. The lack of a stimulation rate effect was consistent with acoustic measurements of the temporal modulation transfer function of the processor, which showed almost no change across stimulation rates.