Effects of the number of channels and speech-to-noise ratio on rate of connected discourse tracking through a simulated cochlear implant speech processor

Objective: To investigate the effects of number of channels and speech-to-noise ratio on connected discourse tracking (CDT) through simulations of cochlear implant speech processing. Previous studies have used citation-form vowel and consonant materials or simple sentences. CDT rates were expected to be less likely to be limited by ceiling effects and more representative of everyday speech communication. Design: Four normal-hearing subjects were presented with speech processed through a real-time sine-excited vocoder having three, four, eight, or 12 channels. Amplitude envelopes extracted from each band modulated sinusoidal carrier signals placed at each band center frequency. Speech-spectrum shaped noise was added to speech before vocoder processing at three signal to noise ratios based on real-time measurements of speech level (+7, +12, +17 dB). Results: CDT rates increased significantly with number of channels up to eight in both quiet and noise, and decreased significantly with each increase in noise level from quiet. Conclusions: The effects on CDT rates of the number of channels and speech-to-noise ratio are highly correlated with intelligibility measures for Hearing in Noise Test (HINT) sentences, consonants and vowels. However, HINT sentence scores even in noise show ceiling effects that obscure the advantages of processors with eight or more channels. Moderate levels of noise that have only slight effects on other measures significantly affected CDT rate. CDT rates with three or four bands of spectral information were much lower than asymptotic rates, especially in the presence of noise.