Modulation Detection by Normal and Hearing-impaired Listeners

In five normally hearing subjects and seven subjects with damaged cochleas, detection thresholds for sinusoidal frequency modulation (FM) and amplitude modulation (AM) were measured using 1 s stimuli with a 500 Hz carrier frequency (Fc) at a 'comfortable' loudness (given by subject-dependent SPLs and SLs). The modulation frequency (Fmod) was 2 Hz or 10 Hz. FM (but not AM) detection was poorer in the hearing-impaired group, especially when the hearing loss at Fc exceeded 50 dB. Fmod had a different effect on FM and AM detection. The corresponding interaction was essentially identical for the two groups of subjects. Previous studies strongly suggested that normal listeners use mainly neural phase-locking cues for the detection of FM when Fmod = 2 Hz, but mainly tonotopic cues when Fmod = 10 Hz. The present results suggest that cochlear damages reduce the usefulness of these two types of cues to an approximately equal degree.