Coil Efficiency for Inductive Peripheral Nerve Stimulation.

Magnetic stimulation of peripheral nerves is evoked by electric field gradients caused by high-intensity, pulsed magnetic fields created from a coil. Currents required for stimulation are very high, therefore devices are large, expensive, and often too complex for many applications like rehabilitation therapy. For repetitive stimulation, coil heating due to power loss poses a further limitation. The geometry of the magnetic coil determines field depth and focality, making it the most important factor that determines the current required for neuronal excitation. However, the comparison between different coil geometries is difficult and depends on the specific application. Especially the distance between nerve and coil plays a crucial role. In this investigation, the electric field distribution of 14 different coil geometries was calculated for a typical peripheral nerve stimulation with a 27 mm distance between axon and coil. Coil parameters like field strength and focality were determined with electromagnetic field simulations. In a second analysis, the activating function along the axon was calculated, which quantifies the efficiency of neuronal stimulation. Moreover, coil designs were evaluated concerning power efficacy based on ohmic losses. Our results indicate that power efficacy of magnetic neurostimulation can be improved significantly by up to 40% with optimized coil designs.

The effect of fluctuating maskers on speech understanding of high-performing cochlear implant users.

OBJECTIVE: The present study evaluated whether the poorer baseline performance of cochlear implant (CI) users or the technical and/or physiological properties of CI stimulation are responsible for the absence of masking release. DESIGN: This study measured speech reception thresholds (SRTs) in continuous and modulated noise as a function of signal to noise ratio (SNR). STUDY SAMPLE: A total of 24 subjects participated: 12 normal-hearing (NH) listeners and 12 subjects provided with recent MED-EL CI systems. RESULTS: The mean SRT of CI users in continuous noise was -3.0 ± 1.5 dB SNR (mean ± SEM), while the normal-hearing group reached -5.9 ± 0.8 dB SNR. In modulated noise, the difference across groups increased considerably. For CI users, the mean SRT worsened to -1.4 ± 2.3 dB SNR, while it improved for normal-hearing listeners to -18.9 ± 3.8 dB SNR. CONCLUSIONS: The detrimental effect of fluctuating maskers on SRTs in CI users shown by prior studies was confirmed by the current study. Concluding, the absence of masking release is mainly caused by the technical and/or physiological properties of CI stimulation, not just the poorer baseline performance of many CI users compared to normal-hearing subjects. Speech understanding in modulated noise was more robust in CI users who had a relatively large electrical dynamic range.

Comodulation masking release induced by controlled electrical stimulation of auditory nerve fibers.

Normal-hearing listeners can perceptually segregate concurrent sound sources, but listeners with significant hearing loss or who wear a cochlear implant (CI) lag behind in this ability. Perceptual grouping mechanisms are essential to segregate concurrent sound sources and affect comodulation masking release (CMR). Thus, CMR measurements in CI users could shed light on segregation cues needed for forming and grouping of auditory objects. CMR illustrates the fact that detection of a target sound embedded in a fluctuating masker is improved by the addition of masker energy remote from the target frequency, provided the envelope fluctuations across masker components are coherent. We modified such a CMR experiment to electrically-induced hearing using direct stimulation and measured the effect in 21 CI users. Cluster analysis of our data revealed two groups: one showed no or only small CMR of 0.1 dB ± 2.7 (N = 14) and a second group achieved a CMR of 10.7 dB ± 3.2 (N = 7), a value that is close to the enhancement observed in a comparable acoustic experiment in normal-hearing listeners (12.9 dB ± 2.6, N = 6). Interestingly, we observed that CMR in CI users may relate to hearing etiology and duration of hearing loss pre-implantation. Our study demonstrates for the first time that a substantial minority of cochlear-implant listeners (about a third) can show significant CMR. This outcome motivates the development of physiologically inspired multi-band gain control and/or different coding strategies for these groups in order to better preserve coherent modulation and thus to take advantage of the individual remaining capabilities to analyze spectro-temporal patterns.