Modulation detection thresholds (MDTs) vary across stimulation sites in a cochlear implant (CI) electrode array in a manner that is subject and ear specific. Previous studies have demonstrated that speech recognition with a CI can be improved by site-selection strategies, where selected stimulation sites with poor modulation sensitivity are removed from a subject’s processor MAP*. Limitations of site-selection strategies are that they can compromise spectral resolution and distort frequency-place mapping because the frequencies assigned to the removed sites are usually reallocated to other sites, and site bandwidths are broadened. The objective of the present study was to test an alternative approach for rehabilitation that aimed at improving the across-site mean MDTs by adjusting stimulation parameters at the poorly performing sites. On the basis of previous findings that modulation detection contributes to speech recognition and improves significantly with stimulus level, the authors hypothesized that modulation sensitivity at the poor sites could be improved by artificially increasing stimulation levels at those sites in the speech processor, which then would lead to improved speech recognition.
Nine postlingually deafened ears implanted with Nucleus CIs were evaluated for MDTs, absolute-detection threshold levels (T levels), and the maximum loudness levels (C levels) on each of the available stimulation sites. For each ear, the minimum stimulation level settings in the speech-processor MAP were raised by 5%, and alternatively by 10%, of the dynamic range (DR) from true thresholds on five stimulation sites with the poorest MDTs. For comparison, a 5% level raise was applied globally to all stimulation sites. The C levels were fixed during these level manipulations. MDTs at the five poorest stimulation sites were compared at 20% DR before and after the level adjustments. Speech-reception thresholds (SRTs), that is, signal to noise ratios required for 50% correct speech recognition, were evaluated for these MAPs using CUNY sentences. The site-specific level-adjusted MAPs were compared with the global-level–adjusted MAP and the MAP without level adjustment. The effects on speech recognition of adjusting the minimal stimulation level settings on the five poorest stimulation sites were also compared with effects of removing these sites from the speech-processor MAP.
The 5% level increase on the five electrodes with the worst MDTs resulted in an improvement in the group mean SRT of 2.36 dB SNR relative to the MAP without level adjustment. The magnitude of level increase that resulted in the greatest SRT improvement for individuals varied across ears. MDTs measured at 20% DR significantly improved on the poor sites after the level adjustment that resulted in the best SRT for that ear was applied. Increasing the minimal stimulation levels on all stimulation sites or removing sites selected for rehabilitation, the parsimonious approaches, did not improve SRTs.
The site-specific adjustments of the T level settings improved modulation sensitivity at low levels and significantly improved subjects’ SRTs. Thus, this site-rehabilitation strategy was an effective alternative to site-selection strategies for improving speech recognition in CI users.
Based on previous findings that Modulation detection threshold (MDT) improves with stimulus level and the hypothesis that modulation sensitivity contributes to speech recognition, the present study presents a site-rehabilitation strategy that increases the minimum stimulation level from true threshold on sites with the poorest MDTs. The modification of the stimulation levels improved modulation sensitivity at the lower end of the dynamic range on the poorly performing sites. The site-specific level manipulation with a magnitude of 5% of the dynamic range significantly improved the subjects’ speech reception thresholds relative to the control condition without level manipulation.
Department of Otolaryngology, Kresge Hearing Research Institute, University of Michigan, Ann Arbor, MI.
This work was supported by National Institutes of Health-National Institute on Deafness and Other Communication Disorders grants R01 DC010786, T32 DC00011, and P30 DC05188.
The authors declare no conflicts of interest.
Address for correspondence: Ning Zhou, Department of Otolaryngology, Kresge Hearing Research Institute, University of Michigan, Ann Arbor, MI 48109, USA. E-mail: firstname.lastname@example.org