Modulation rate detection and discrimination by normal-hearing and hearing-impaired listeners

Modulation detection and modulation rate discrimination thresholds were obtained at three different modulation rates (f(m)= 80, 160, and 320 Hz) and for three different ranges of modulation depths (m): full (100%), mid (70%- 80%), and low (40%-60%) with both normal-hearing (NH) and hearing-impaired (HI) subjects. The results showed that modulation detection thresholds increased with modulation rate, but significantly more so for HI than for NH subjects. Similarly, rate discrimination thresholds (Δr) increased with increases in f(m) and decreases in modulation depth. When compared to NH subjects, rate discrimination thresholds for HI subjects were significantly worse for all rates and for all depths. At the fastest modulation rate with less than 100% modulation depth, most HI subjects could not discriminate any change in rate. When valid thresholds for rate discrimination were obtained for HI subjects, they ranged from 2.5 semitones (Δr= 12.7 Hz, f(m) = 80Hz, m=100%) to 8.7 semitones (Δr=214.5Hz, f(m)=320 Hz, m=100%). In contrast, average rate discrimination thresholds for NH subjects ranged from 0.9 semitones (Δr=4.2 Hz, f(m)=80 Hz, m= 100%) to 4.7 semitones (Δr= 103.5 Hz, f(m)=320 Hz, m=60%). Some of the differences in temporal processing between NH and HI subjects, especially those related to modulation detection, may be accounted for by differences in signal audibility, especially for high- frequency portions of the modulated noise. However, in many cases, HI subjects encountered great difficulty discriminating a change in modulation rate even though the modulation components of the standard and test stimuli were detectable.