Spectrotemporal modulation sensitivity in cochlear implant users: Impact of noise carrier and modulation bandwidth on reaction time

Spectrotemporal modulation tests probe spectral and temporal resolution in cochlear implant (CI) users. This study investigated how carrier type and bandpass modulations influenced modulation detection performance. Nineteen CI users performed a reaction-time task involving the detection of spectral (0.25-2 cycles/octave) and/or temporal (4-16 Hz) modulations embedded in a broadband carrier. Carriers were either (1) a complex tone composed of 87 random-phase sinusoids spaced linearly at 100 Hz and weighted by a pink spectrum or (2) pink noise. Surprisingly, stimuli with dense spectral modulations were more readily detected when paired with the complex tone carrier. In contrast, the pink noise carrier yielded the expected low-pass spectral modulation transfer function profile. Electrodogram simulations based on CI sound processing strategies suggest that using a complex tone carrier with more closely spaced, logarithmically arranged tones may reduce unintended cues, such as spectral aliasing. Additionally, 2-octave limited bandwidth stimuli with a fixed temporal modulation rate (4 Hz) and spectral densities ranging from 0-2 cycles/octave were tested within broadband pink noise, centered at frequencies from 500 to 4000 Hz. Detection sensitivity was lowest at 500 Hz-a result supported by electrodograms-suggesting potential device processing limitations for spectrotemporal modulations at apical electrode sites.