Effects of Site-Specific Level Adjustments on Speech Recognition With Cochlear Implants

OBJECTIVES: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. DESIGN: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. RESULTS:The 5% level increase on the five electrodes with the worst MDTs resu