Amplitude-modulation detection and speech recognition in normal and hearing-impaired listeners

Even though temporal speech envelopes may form a salient cue when listening to speech in noisy backgrounds, the relationship between speech intelligibility and the sensitivity to detecting temporal envelopes (i.e., amplitude modulation detection) is not well understood. This study measured speech reception thresholds in quiet, stationary and speech-modulated noise in three listener groups: young (yNH) and older normal-hearing (oNH) listeners and hearing-impaired (HI) listeners. In addition to broadband speech and noise signals, we adopted low and high-pass filtered versions of the stimuli to study the contribution of different coding mechanisms in basal and apical cochlear regions. For the same listeners, amplitude-modulation (AM) detection thresholds were measured in quiet and in the presence of broadband masking noise for 70 dB AM tones of 0.5, 2 and 4 kHz with either 100 or 5 Hz modulation frequency. Even though group trends were clearer than individual differences, AM detection thresholds showed a relationship to speech recognition in the low-pass filtered speech-modulated noise condition, and for yNH and oNH listeners in the high-pass filtered condition. Overall, this study sheds light on the importance of temporal envelope coding sensitivity for speech recognition and its relationship to near-and supra-threshold hearing deficits.