| Speech-recognition difficulties of sensorineural hearing-impaired listeners cannot be explained solely by degradations of hearing sensitivity. It has been proposed that a decrease in auditory-processing rate may also account for Speech-recognition difficulties. This study employed a backward-masked level-discrimination (BMLD) task to compare auditory-processing rates of frequency regions with normal or impaired sensitivity. The effects of training and of the frequency difference between signal and masker were also investigated.; An adaptive three-interval, two-alternative, forced-choice task was employed. Stimuli consisted of a 25-msec signal tone followed by a 200-msec noise separated by a silent interval (ISI) ranging from 30 to 240 msec. Signal frequencies of 500 or 4000 Hz were presented at 30–40 dB sensation level. Maskers were 1/3-octave band noise bursts centered at either 500 or 4000 Hz. Signal-to-masker spectrum level (S/No) ratios were 0, 5, or 10 dB and 9, 14, or 19 dB for noise bands centered at 500 Hz and 4000 Hz, respectively. BMLD thresholds were measured as a function of the ISI duration to estimate auditory-processing rates. Seven normal-hearing and seven high-frequency impaired-hearing listeners participated in the study.; BMLD thresholds decreased from 12 to 4 dB as ISI increased from 30 to 240 msec, and approached closely the no-mask condition threshold (1–2 dB) at 240-msec ISI. BMLD thresholds decreased as the S/No ratio increased. With 4000-Hz signals, the S/No ratio had to be 14 dB lower than with 500-Hz signals to obtain comparable BMLD thresholds across frequencies; that is, the masking amount was proportional to the overall masker power. For 30–40 dB sensation-level signals, the relation between BMLD threshold and ISI was nearly equal across frequency regions of normal and impaired sensitivity and across normal- and impaired-hearing listeners; that is, auditory-processing rates were similar across listener groups and frequency regions. Inter-individual variability of BMLD thresholds was also similar across normal- and impaired-hearing subjects. BMLD thresholds decreased significantly with training and with large difference between the masker center frequency and the signal frequency. The present results do not support the hypothesis that sensorineural hearing loss is associated with a decrease in auditory processing rate. |
