Cochlear implant performance and electrically-evoked auditory brain-stem response characteristics

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Abstract

Objectives: The purpose of this study was to find a correlation between cochlear implant performances in phoneme discrimination and activity of the brain-stem.

Methods: Electrically-evoked auditory brain-stem responses (EABRs) and speech recognition performances were measured in 17 patients implanted with an MXM Digisonic DX10 cochlear implant. Speech recognition performances without lip-reading were tested using lists of isolated French words containing 3 phonemes.

Results: The results indicated statistically significant correlations between phoneme correct-identification scores and the following EABR variables: wave V latency, wave II–V latency interval and wave III–V latency interval. These results, indicate that up to about 48% of the variance in isolated word recognition without lip-reading can be accounted for by EABR variables.

Conclusion: The quality of brain-stem functioning influences central processes in phoneme discrimination.

Introduction

Speech recognition performances are largely variable across cochlear implant (CI) subjects: while some patients can understand running speech in noisy backgrounds without the aid of lip-reading, others cannot recognize simple sentences spoken slowly and quietly. The speech recognition performances of a given CI subject may depend on variable factors, such as the etiology of deafness, the duration of sensory deprivation, and the language-acquisition level before implantation and motivation, and it is difficult to find a reliable predictor. The physiological state of auditory system structures appears to be an important underlying factor of overall performance with the implant. The number of surviving cells in the spiral ganglion, which generally correlates well with the duration of deafness (Otte et al., 1978; Schmidt, 1985), has been pointed out as a possible factor, not only of thresholds for electrical stimulation (Pfingst et al., 1980; Shannon, 1983; Pfingst, 1984), but also of speech recognition performances (Pauler et al., 1986) in CI subjects.

However, although results in the literature support the notion that neural survival might be reflected in the growth of electrically evoked brain-stem responses (EABRs) (Smith and Simmons, 1983; Lusted et al., 1984; Brightwell et al., 1985; Hall, 1990), the results of studies in which relationships between speech recognition performances and measures derived from EABR amplitudes, such as threshold, response growth, refractoriness and strength-duration functions have been investigated, have failed to evidence strong correlations (Abbas and Brown, 1991a, Abbas and Brown, 1991b). The results of recent studies suggest that other estimates of the neurophysiological state of peripheral and intermediate auditory nervous structures, which are related to speech recognition performances, might be derived from late EABR wave latencies (Herman and Thornton, 1992; Gallégo et al., 1997a).

The present study sought to characterize further the relationship between speech recognition performances and EABR wave latencies in subjects implanted with the Digisonic DX10 cochlear implant.

Section snippets

Subjects

Seventeen adult subjects (10 female, 7 male; aged between 24 and 67 years), implanted with an MXM Digisonic DX10 cochlear implant, participated in the study. Except for the fact that a minimum of 12 out of the 15 electrodes of the implant had to be active, i.e. to elicit auditory sensations when stimulated, no particular inclusion criterion was defined regarding the etiology of deafness, the duration of deafness and the time since implantation, so as to allow for a wide range of speech

EABRs

Fig. 2 shows examples of EABR traces. The 3 upper curves (Fig. 3a) correspond to recordings prior to digital processing; the lower curve (Fig. 3b) represents the EABR after filtering and summing. Waves II, III and V can easily be identified.

Relationships between EABR variables and speech recognition performances

Using the Kolmogorov-Smirnov normality test, the wave latencies and latency intervals, averaged across electrodes and levels in each patient, were found to be normally distributed; therefore, Pearson's parametric correlation was used to test for

Discussion

The EABR wave latency values obtained in this study are in agreement with those available in the literature (Abbas and Brown, 1988). The variable-filter processing scheme used in the present and previous studies (Gallégo et al., 1996, Gallégo et al., 1997a) leads to clear EABR traces, allowing for accurate estimation of EABR wave latencies using an automatic detection algorithm.

The main result of the present study consists in a relationship between correct phoneme recognition scores on the one

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