Robustness against distortion of fundamental frequency cues in simulated electro-acoustic hearing

Abstract

Speech recognition by cochlear implant users can be improved by adding an audible low frequency acoustic signal to electrical hearing; the resulting improvement is deemed "electroacoustic (EAS) benefit". We assessed the role of fundamental frequency (F0) information as a predictor of EAS benefit. Normal hearing listeners were presented with vocoded speech tokens with differing manipulations of the F0 signal, specifically: a pure tone with the correct mean F0 but with smaller variations around this mean (i.e. a smaller modulation depth), or a narrowband of white noise centered around F0, at varying bandwidths; a pure tone downshifted in frequency but keeping overall frequency modulations. Speech recognition thresholds significantly improved (p<0.05) when tones with reduced frequency modulation, or noise bands maintaining F0 information via their centre frequency were presented alongside the vocoded speech. Addition of a pure tone downshifted by 50 Hz or only a tone to indicate voicing, showed no significant EAS benefit. These results confirm that the presence of the target's F0 is beneficial for electro-acoustic hearing in a noisy environment and they indicate that the benefit is robust to a certain decrease in frequency selectivity, so long as mean F0 and frequency modulations of F0 are preserved.