MODULATION RATE DETECTION AND DISCRIMINATION BY NORMAL-HEARING AND HEARING-IMPAIRED LISTENERS

Modulation detection and modulation rate discrimination thresholds wer e 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 de tection thresholds increased with modulation rate, but significantly m ore so for HI than for NH subjects. Similarly, rate discrimination thr esholds (Delta r) increased with increases in f(m) and decreases in mo dulation depth. When compared to NH subjects, rate discrimination thre sholds for HI subjects were significantly worse for all rates and for all depths. At the fastest modulation rate with less than 100% modulat ion depth, most HI subjects could not discriminate any change in rate. When valid thresholds for rate discrimination were obtained for HI su bjects, they ranged from 2.5 semitones (Delta r=12.7 Hz, f(m)=80 Hz, m =100%) to 8.7 semitones (Delta r=214.5 Hz, f(m)=320 Hz, m=100%). In co ntrast, average rate discrimination thresholds for NH subjects ranged from 0.9 semitones (Delta r=4.2 Hz, f(m)=80 Hz, m=100%) to 4.7 semiton es (Delta r=103.5 Hz, f(m)=320 Hz, m=60%). Some of the differences in temporal processing between NH and HI subjects, especially those relat ed to modulation detection, may be accounted for by differences in sig nal audibility, especially for high-frequency portions of the modulate d noise. However, in many cases, HI subjects encountered great difficu lty discriminating a change in modulation rate even though the modulat ion components of the standard and test stimuli were detectable. [S000 1 -4966(98)02208-5].