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The multichannel auditory brainstem implant: how many electrodes make sense?

OBJECT
Development of multichannel auditory brainstem implant (ABI) systems has been based in part on the assumption that audiological outcome can be optimized by increasing the number of available electrodes. In this paper the authors critically analyze this assumption on the basis of a retrospective clinical study performed using the Nucleus 22 ABI surface electrode array.


METHODS
The perceptual performances of 61 patients with neurofibromatosis Type 2 were tested approximately 6 weeks after an eight-electrode ABI had been implanted. Of eight implanted electrodes 5.57 +/- 2.57 (mean +/- standard deviation [SD] provided auditory sensations when stimulated. Electrodes were deactivated when stimulation resulted in significant nonauditory side effects or no auditory sensation at all, and also when they failed to provide distinctive pitch sensations. The mean (+/- SD) scores for patients with ABIs were the following: sound-only consonant recognition, 20.4 +/- 14.3 (range 0-65%); vowel recognition, 28.8 +/- 18% (range 0-67%); Monosyllable Trochee Spondee (MTS) word recognition 41.1 +/- 25.3% (range 0-100%); and sentence recognition, 5.3 +/- 11.4% (range 0-64%). Performance in patients in whom between one and three electrodes provided auditory sensation was significantly poorer than that in patients with between four and eight functional electrodes in the vowel, MTS word, and City University of New York (CUNY) sentence recognition tests. The correlation between performance and electrode number did not reach the 0.05 level of significance with respect to the sound effect, consonant, and MTS stress-pattern recognition tests, probably because a satisfactory performance in these tests can be obtained only with temporal cues, that is, without any information about the frequency of the sounds. In the MTS word and the CUNY sentence recognition tests, performance was optimal in the patients with eight functional electrodes. Although all top performers had more than three functional auditory electrodes, no further improvement (asymptotic performance) was seen in those with five or more active electrodes in the consonant, vowel, and sound effect recognition tests.


CONCLUSIONS
A minimum of three spectral channels, programmed in the appropriate individual tonotopic order seem to be required for satisfactory speech recognition in most patients with ABI. Due to the limited access to the tonotopic frequency gradient of the cochlear nucleus with surface stimulation, patients with ABI do not receive a wide range of spectral cues (frequency information) with multielectrode (> 5) surface arrays.