Effect of current focusing on the sensitivity of inferior colliculus neurons to amplitude-modulated stimulation.

In multichannel cochlear implants (CIs), current is delivered to specific electrodes along the cochlea in the form of amplitude-modulated pulse trains, to convey temporal and spectral cues. Our previous studies have shown that focused multipolar (FMP) and tripolar (TP) stimulation produce more restricted neural activation and reduced channel interactions in the inferior colliculus (IC) compared with traditional monopolar (MP) stimulation, suggesting that focusing of stimulation could produce better transmission of spectral information. The present study explored the capability of IC neurons to detect modulated CI stimulation with FMP and TP stimulation compared with MP stimulation. The study examined multiunit responses of IC neurons in acutely deafened guinea pigs by systematically varying the stimulation configuration, modulation depth, and stimulation level. Stimuli were sinusoidal amplitude-modulated pulse trains (carrier rate of 120 pulses/s). Modulation sensitivity was quantified by measuring modulation detection thresholds (MDTs), defined as the lowest modulation depth required to differentiate the response of a modulated stimulus from an unmodulated one. Whereas MP stimulation showed significantly lower MDTs than FMP and TP stimulation (P values <0.05) at stimulation ≤2 dB above threshold, all stimulation configurations were found to have similar modulation sensitivities at 4 dB above threshold. There was no difference found in modulation sensitivity between FMP and TP stimulation. The present study demonstrates that current focusing techniques such as FMP and TP can adequately convey amplitude modulation and are comparable to MP stimulation, especially at higher stimulation levels, although there may be some trade-off between spectral and temporal fidelity with current focusing stimulation.

Evaluation of focused multipolar stimulation for cochlear implants in acutely deafened cats.

OBJECTIVE: The conductive nature of the fluids and tissues of the cochlea can lead to broad activation of spiral ganglion neurons using contemporary cochlear implant stimulation configurations such as monopolar (MP) stimulation. The relatively poor spatial selectivity is thought to limit implant performance, particularly in noisy environments. Several current focusing techniques have been proposed to reduce the spread of activation with the aim towards achieving improved clinical performance. APPROACH: The present research evaluated the efficacy of focused multipolar (FMP) stimulation, a relatively new focusing technique in the cochlea, and compared its efficacy to both MP stimulation and tripolar (TP) stimulation. The spread of neural activity across the inferior colliculus (IC), measured by recording the spatial tuning curve, was used as a measure of spatial selectivity. Adult cats (n = 6) were acutely deafened and implanted with an intracochlear electrode array before multi-unit responses were recorded across the cochleotopic gradient of the contralateral IC. Recordings were made in response to acoustic and electrical stimulation using the MP, TP and FMP configurations. MAIN RESULTS: FMP and TP stimulation resulted in greater spatial selectivity than MP stimulation. However, thresholds were significantly higher (p < 0.001) for FMP and TP stimulation compared to MP stimulation. There were no differences found in spatial selectivity and threshold between FMP and TP stimulation. SIGNIFICANCE: The greater spatial selectivity of FMP and TP stimulation would be expected to result in improved clinical performance. However, further research will be required to demonstrate the efficacy of these modes of stimulation after longer durations of deafness.

Acute cochlear nucleus compression alters tuning properties of inferior colliculus neurons.

Auditory brainstem implants (ABI) have been used in neurofibromatosis type 2 (NF2) patients in an attempt to restore hearing sensation, with limited clinical success. Factors associated with poor clinical outcomes for NF2 ABI patients include larger tumour size, longer duration of hearing loss, and brainstem distortion and/or deformation caused by tumours that compress the brainstem. The present study investigated changes in tuning properties of inferior colliculus (IC) neurons following compression of the contralateral cochlear nucleus (CN). The left CN in adult rats (n = 8) was exposed and a 32-channel acute recording probe inserted along the tonotopic gradient of the right IC. In 4 animals, an ethylene vinyl acetate bead was applied to the exposed CN. Three recordings were made corresponding to T(1) = 0 min (before compression), T(2) = 45 min (during compression) and T(3) = 225 min (following bead removal/recovery). Recordings consisted of a response area protocol using pure tones of various frequencies and intensities (1-44 kHz; 10-70 dB SPL) to determine the characteristic frequency for each probe site. Compression of the CN led to sharpened tuning curves, decreased spike rate, and increased threshold and characteristic frequency in the IC. Reversal of compression enabled these variables, excluding threshold, to recover to baseline. NF2 patients may have poorer ABI performance due to damage to the physical structure of the CN, resulting in alterations to the tonotopic organisation of the auditory pathway which may complicate ABI implantation and activation.