Investigating the effect of focused multipolar stimulation for cochlear implants: preclinical studies

Abstract

Multichannel cochlear implants have been well accepted as an effective and safe treatment for severe to profound sensorineural hearing loss through electrical stimulation of residual spiral ganglion neurons. However, speech intelligibility with existing cochlear implants is thought to be limited by poor spatial selectivity and interactions between channels caused by overlapping activation with contemporary stimulation strategies such as monopolar (MP) stimulation. Focused intracochlear stimulation, resulting in an increase in the number of truly independent stimulating channels available for simultaneous activation, may enable better speech and pitch recognition and also improve temporal resolution.

Various current focusing stimulation strategies such as tripolar (TP) stimulation have been reported to produce sharper excitation patterns and reduced channel interactions compared to MP stimulation at the cost of higher stimulation current levels. Focused multipolar (FMP) stimulation is another such focusing technique; utilizing simultaneous stimulation of multiple channels to create focused electrical fields. FMP stimulation has been validated in a small group of cochlear implant recipients showing that focusing can be achieved, however this was at the expense of higher stimulation currents compared to MP stimulation.

There have been no previous attempts to systematically compare the efficacy of FMP stimulation against TP stimulation or to determine whether factors such as neural survival and the electrode position within the cochlea would affect the performance of FMP stimulation. Controlled preclinical studies in experimental animals can reduce the possible confounding effects of neural survival in human studies. It is also important to test if FMP produces non-auditory sensations since the simultaneous nature of the stimuli would be expected to require greater charge to evoke neural responses.

The primary objectives of this thesis were to determine the efficacy of FMP stimulation, compared to both MP and TP stimulation, by evaluating a) the spatial extent of neural activation b) interactions between cochlear implant channels and c) modulation sensitivity to sinusoidal amplitude-modulated pulse trains. The effects of factors such as degeneration of spiral ganglion neurons, induced by long-term deafness, and the electrode position within the cochlea on the effectiveness of FMP, TP and MP stimulation were also examined. These objectives were achieved by implanting a multichannel cochlear implant into cats and guinea pigs, and recording the neural responses in the inferior colliculus in acute electrophysiological experiments. Neural thresholds and the spread of activation along the tonotopic gradient were measured.

In summary, the main results of this thesis showed that FMP and TP stimulation resulted in more restricted neural activation and reduced channel interaction compared to MP stimulation and these advantages were maintained in cochleae with significant neural degeneration. Moreover, these effects were not adversely affected by the position of the electrode array within the scala tympani. Although greater charge was required to achieve threshold levels, no evidence of ectopic stimulation of non-auditory neurons was observed with FMP or TP stimulation. Systematically varying the degree of current focusing lowered threshold levels for FMP stimulation while still maintaining a selectivity advantage. Modulation detection of MP was found to be significantly better than FMP and TP stimulation at low stimulation levels, but similar at high stimulation levels. Importantly, there was no benefit in terms of restricted neural activation, reduced channel interaction or better modulation sensitivity for FMP compared to TP stimulation. The greater spatial selectivity, reduced channel interactions and the ability to convey modulation using FMP and TP stimulation would be expected to result in improved clinical performance. The insights into current focusing described in this thesis may also be helpful in other neural prostheses such as deep brain stimulation devices and visual prostheses, when more selective stimulation is desired.