gms | German Medical Science

Introduction: For cochlear implant (CI) user’s speech information transmission is reliant upon the ability to detect, track, discriminate, and process the amplitude-modulated (AM) envelope of speech sounds independently in different channels and make comparisons across channels. Information transmission can be hindered at many stages in the auditory pathway due to, for example, spread of electrical current, survival of inner-ear neurons and the neural representation of dynamic spectro-temporal cues. In this work, objective measures of AM envelope discrimination abilities for several electrodes is compared to behavioural measures of AM envelope discrimination.

Methods: A group of adult listeners with CIs (Nucleus) participated in this experiment.

The objective measure that we use captures the neural response to a perceived change (here AM rate) in an auditory stimulus (the so-called auditory change complex (ACC)). The amplitude of the ACC is related to the size of the perceived change in the stimulus. We stimulated single electrodes (4 s, 400 pps pulse trains, 100% AM modulated at 13 and 40 Hz) at the base, middle or apical part of the cochlea using direct CI stimulation via the NIC4 research interface. Participants passively listened while the electroencephalology (EEG) responses were measured with a 64 channel Biosemi (16 kHz) Active Two system. Artefacts caused by CI processing were attenuated using interpolation, and N1-P2 amplitude and latency were extracted from individually averaged epochs.

The behavioural task involves acoustic presentation of sinusoids at two different AM rates (13 versus 40 Hz) discriminated in a three-interval two-alternative forced choice task, where the modulation depth of both modulated sinusoids was adjusted adaptively to derive an AM discrimination threshold. Carrier frequencies were set to the center frequency of the same electrodes as used in the ACC task. Stimuli were delivered through headphones (HD600s) placed over the sound processor of the CIs. All front-end noise reduction features were de-activated during the experiment. It is hypothesized, that CI listeners with higher N1-P2 amplitudes across the electrode channels are better able to behaviorally discriminate between different AM rates.

Results and conclusions: Data collection is ongoing (n=6), correlations were found between the behavioural AM modulation depth thresholds and the P2-N1 Amplitude. Thresholds and N1-P2 amplitude varied across electrodes, and participants. The results so far support the notion that the ACC paradigm for changes in AM rate provides an objective measure of AM discrimination that is related to the behavioral discrimination of the same stimuli. This research will help us to develop objective measures of AM discrimination at the (sub)cortical level, depending on each CI electrode’s viability of the individual electrode neuron interface.

Funding: Supported by UK Medical Research Council Senior Fellowship (MR/S002537/1).