Combining current focusing and steering in a cochlear implant processing strategy.

OBJECTIVE: To evaluate the benefit of combined current focusing and steering to speech recognition in noise with cochlear implants (CIs). DESIGN: Combined current focusing and steering was implemented using focused partial tripolar (pTP) mode with two current steering ranges. The two pTPsteering strategies were compared to a monopolar (MP) strategy without current focusing or steering and a pTP strategy with only current focusing using the Hearing in Noise Test. The strategies differed only in stimulation mode. STUDY SAMPLE: Ten post-lingually deafened adult CI users participated in this study. RESULTS: Compared to the MP strategy, both pTPsteering strategies produced significantly better speech reception thresholds, while the pTP strategy did not. Subjects with better baseline MP performance had less improvements with the pTPsteering strategies. All four strategies were experimental low-rate strategies and none of them outperformed subjects' clinical strategies. CONCLUSIONS: Speech recognition in noise was better with the pTPsteering strategies than with the MP strategy, but the effect of pTP-mode current steering on spectral resolution is yet to be tested.

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.

Comparing Fixed and Individualized Channel Interaction Coefficients for Speech Perception With Dynamic Focusing Cochlear Implant Strategies.

Dynamic focusing cochlear implant strategies aim to emulate normal cochlear excitation patterns by varying the degree of current focusing as a function of input level. Results on the speech perception benefits of these strategies have been mixed. In previous studies, channel interaction coefficients (K), which mediate the relationship between current level and degree of focusing, were fixed across channels and participants. Fixing K without accounting for channel interaction and the current required to accurately stimulate target neurons may elicit suboptimal loudness growth and speech perception. This study tested whether individualizing K improved speech perception relative to fixed-K and monopolar strategies. Fourteen ears of implanted adults were programmed with 14-channel strategies matched on pulse duration, pulse rate, filtering, and loudness. Sentence recognition and vowel identification was measured at 60 dB SPL equivalent in quiet and four-talker babble. On the group level, speech recognition in quiet and noise was similar between strategies. On the individual level, there were participants who benefitted with dynamic focusing strategies for speech perception in noise. Patterns of benefit were generally unclear, beyond associations between focused thresholds, duration of hearing loss, and individual-K benefit. Participants rated dynamic focusing like monopolar in clarity and ease of listening. Almost all participants expressed their willingness to use the strategies in a take-home trial. These results suggest that while individualizing K does not benefit all, there are individuals who benefit, for which the electrode-neuron interface may play a role. Future studies will evaluate acclimatization of dynamic focusing strategies using take-home trials.

Electrical stimulation of the midbrain excites the auditory cortex asymmetrically.

BACKGROUND: Auditory midbrain implant users cannot achieve open speech perception and have limited frequency resolution. It remains unclear whether the spread of excitation contributes to this issue and how much it can be compensated by current-focusing, which is an effective approach in cochlear implants. OBJECTIVE: The present study examined the spread of excitation in the cortex elicited by electric midbrain stimulation. We further tested whether current-focusing via bipolar and tripolar stimulation is effective with electric midbrain stimulation and whether these modes hold any advantage over monopolar stimulation also in conditions when the stimulation electrodes are in direct contact with the target tissue. METHODS: Using penetrating multielectrode arrays, we recorded cortical population responses to single pulse electric midbrain stimulation in 10 ketamine/xylazine anesthetized mice. We compared monopolar, bipolar, and tripolar stimulation configurations with regard to the spread of excitation and the characteristic frequency difference between the stimulation/recording electrodes. RESULTS: The cortical responses were distributed asymmetrically around the characteristic frequency of the stimulated midbrain region with a strong activation in regions tuned up to one octave higher. We found no significant differences between monopolar, bipolar, and tripolar stimulation in threshold, evoked firing rate, or dynamic range. CONCLUSION: The cortical responses to electric midbrain stimulation are biased towards higher tonotopic frequencies. Current-focusing is not effective in direct contact electrical stimulation. Electrode maps should account for the asymmetrical spread of excitation when fitting auditory midbrain implants by shifting the frequency-bands downward and stimulating as dorsally as possible.

Pitch ranking with different virtual channel configurations in electrical hearing.

Monopolar Virtual Channels (MPVCs) use current steering to increase the number of spectral channels provided to cochlear implant users beyond the physical number of electrodes. The current spread created with a current steered channel is similar to the spread found for monopolar stimulation, and this spread may be one of the bottlenecks for improved performance with an increased number of channels. Quadrupolar Virtual Channels (QPVCs) use current focusing in combination with steering in an attempt to increase the number of channels while reducing channel interaction. However, due to the potentially asymmetric current field generated by QPVCs, there may be distortions in the place pitch representation using this mode. A Virtual Tripole (VTP) is introduced as a current focused virtual channel with a relatively symmetrical electric field distribution. In this study, we looked at pitch ranking in cochlear implant users with QPVC, VTP, and MPVC configurations to determine if place pitch shifts similarly across the cochlea or if any of the stimulation modes shift non-monotonically. Results suggest that MPVC and VTP stimulation provide a consistent monotonic shift across cochlear positions while the place shift provided by QPVCs was more variable. The use of VTP stimulation would be recommended instead of QPVC for a speech processing strategy.

Excitation Patterns of Standard and Steered Partial Tripolar Stimuli in Cochlear Implants.

Current steering in partial tripolar (pTP) mode has been shown to improve pitch perception and spectral resolution with cochlear implants (CIs). In this mode, a fraction (σ) of the main electrode current is returned within the cochlea and steered between the basal and apical flanking electrodes (with a proportion of α and 1 - α, respectively). Pitch generally decreases when α increases from 0 to 1, although the salience of pitch change varies across CI users. This study aimed to identify the mechanism of pitch changes with pTP-mode current steering and the factors contributing to the intersubject variability in pitch-ranking sensitivity. The electrical fields were measured for steered pTP stimuli on the same main electrode with α = 0, 0.5, and 1 in five implanted ears using electrical field imaging (EFI). The related excitation patterns were also measured physiologically using evoked compound action potential (ECAP) and psychophysically using psychophysical forward masking (PFM). Consistent with the pitch-ranking results in this study, the EFI, ECAP, and PFM centroids shifted apically with increasing α. An apical shift was also observed for the PFM peak but not for the EFI or ECAP peak. The pattern width was similar with different α values within a given measure (e.g., EFI, ECAP, or PFM), but the ECAP patterns were broader than the EFI and PFM patterns, possibly because ECAP was measured with smaller σ values than EFI and PFM. The amount of pattern shift with α depended on σ (i.e., the total amount of current used for steering) but was not correlated with the pitch-ranking sensitivity across subjects. The results revealed that the pitch changes elicited by pTP-mode current steering were not only driven by the shifts of excitation centroid.

Symmetric Electrode Spanning Narrows the Excitation Patterns of Partial Tripolar Stimuli in Cochlear Implants.

In cochlear implants (CIs), standard partial tripolar (pTP) mode reduces current spread by returning a fraction of the current to two adjacent flanking electrodes within the cochlea. Symmetric electrode spanning (i.e., separating both the apical and basal return electrodes from the main electrode by one electrode) has been shown to increase the pitch of pTP stimuli, when the ratio of intracochlear return current was fixed. To explain the pitch increase caused by symmetric spanning in pTP mode, this study measured the electrical potentials of both standard and symmetrically spanned pTP stimuli on a main electrode EL8 in five CI ears using electrical field imaging (EFI). In addition, the spatial profiles of evoked compound action potentials (ECAP) and the psychophysical forward masking (PFM) patterns were also measured for both stimuli. The EFI, ECAP, and PFM patterns of a given stimulus differed in shape details, reflecting the different levels of auditory processing and different ratios of intracochlear return current across the measurement methods. Compared to the standard pTP stimuli, the symmetrically spanned pTP stimuli significantly reduced the areas under the curves of the normalized EFI and PFM patterns, without shifting the pattern peaks and centroids (both around EL8). The more focused excitation patterns with symmetric spanning may have caused the previously reported pitch increase, due to an interaction between pitch and timbre perception. Being able to reduce the spread of excitation, pTP mode symmetric spanning is a promising stimulation strategy that may further increase spectral resolution and frequency selectivity with CIs.