Lakshmi N. Mishra

Effects of electrode design and configuration on channel interactions

A potential shortcoming of existing multichannel cochlear implants is electrical-field summation during simultaneous electrode stimulation. Electrical-field interactions can disrupt the stimulus waveform prior to neural activation. To test whether speech intelligibility can be degraded by electrical-field interaction, speech recognition performance and interaction were examined for three Clarion electrode arrays: the pre-curved, enhanced bipolar electrode array, the enhanced bipolar electrode with an electrode positioner, and the Hi-Focus electrode with a positioner. Channel interaction was measured by comparing stimulus detection thresholds for a probe signal in the presence of a sub-threshold perturbation signal as a function of the separation between the two simultaneously stimulated electrodes. Correct identification of vowels, consonants, and words in sentences was measured with two speech strategies: one which used simultaneous stimulation and another which used sequential stimulation. Speech recognition scores were correlated with measured electrical-field interaction for the strategy which used simultaneous stimulation but not the strategy which used sequential stimulation. Higher speech recognition scores with the simultaneous strategy were generally associated with lower levels of electrical-field interaction. Electrical-field interaction accounted for as much as 70% of the variance in speech recognition scores, suggesting that electrical-field interaction is a significant contributor to the variability found across patients who use simultaneous strategies.

Comparison of speech processing strategies used in the Clarion implant processor.

Objective To evaluate the performance of the various speech processing strategies supported by the Clarion S-Series implant processor. Design Five different speech-processing strategies [the Continuous Interleaved Sampler (CIS), the Simultaneous Analog Stimulation (SAS), the Paired Pulsatile Sampler (PPS), the Quadruple Pulsatile Sampler (QPS) and the hybrid (HYB) strategies] were implemented on the Clarion Research Interface platform. These speech-processing strategies varied in the degree of electrode simultaneity, with the SAS strategy being fully simultaneous (all electrodes are stimulated at the same time), the PPS and QPS strategies being partially simultaneous and the CIS strategy being completely sequential. In the hybrid strategy, some electrodes were stimulated using SAS, and some were stimulated using CIS. Nine Clarion CIS users were fitted with the above speech processing strategies and tested on vowel, consonant and word recognition in quiet. Results There were no statistically significant differences in the mean group performance between the CIS and SAS strategies on vowel and sentence recognition. A statistically significant difference was found only on consonant recognition. Individual results, however, indicated that most subjects performed worse with the SAS strategy compared with the CIS strategy on all tests. About 33% of the cochlear implant users benefited from the PPS and QPS strategies on consonant and word recognition. Conclusions If temporal information were the primary factor in speech recognition with cochlear implants then SAS should consistently produce higher speech recognition scores than CIS. That was not the case, however, because most CIS users performed significantly worse with the SAS strategy on all speech tests. Hence, there seems to be a trade-off between improving the temporal resolution with an increasing number of simultaneous channels and introducing distortions from electrical-field interactions. Performance for some CI users improved when the number of simultaneous channels increased to two (PPS strategy) and four (QPS strategy). The improvement with the PPS and QPS strategies must be due to the higher rates of stimulation. The above results suggest that CIS users are less likely to benefit with the SAS strategy, and they are more likely to benefit from the PPS and QPS strategies, which provide higher rates of stimulation with small probability of channel interaction.

System and method for fitting a cochlear implant sound processor using alternative signals

Alternative stimuli, i.e., stimuli other than the constant amplitude stimuli used in prior fitting schemes, are used to set the parameters of a cochlear implant system. The use of such alternative stimuli allows the entire fitting process to be completed in a very short time period, and generally eliminates the need for secondary adjustments. In one preferred embodiment, the alternative stimuli comprise white noise that is internally generated within the speech processor.

Channel interaction and speech processing strategies for cochlear implants

A limitation of multichannel cochlear implants is that electrical channel interactions can arise when multiple electrodes are stimulated simultaneously. Channel interaction can be reduced by decreasing current levels delivered to each electrode, through improved electrode positioning and design, or by using speech processing strategies that stimulate electrodes sequentially. The goal of the present study was to examine the relationship between channel interaction and speech recognition for several speech strategies varying in the number of electrodes stimulated simultaneously. Based on previous research, susceptibility to channel interaction is expected to vary as a function of electrode configuration. Thus patients with strong susceptibility to channel interaction will produce higher speech recognition scores for sequential as opposed to simultaneous speech strategies. Speech strategy performance was evaluated with acute listening trials where patients identified consonants and vowels with each of the fo...

System and method for fitting a hearing prosthesis sound processor using alternative signals

Alternative stimuli, i.e., stimuli other than the constant amplitude stimuli used in prior fitting schemes, are used to set the parameters of a hearing prosthesis, such as a cochlear implant system. The use of such alternative stimuli allows the entire fitting process to be completed in a very short time period, and generally eliminates the need for secondary adjustments. In one preferred embodiment, the alternative stimuli comprise white noise that is internally generated within the speech processor.