Speech recognition in noise as a function of highpass-filter cutoff frequency for people with and without low-frequency cochlear dead regions.

Regions in the cochlea with very few functioning inner hair cells and/or neurons are called "dead regions" (DRs). Previously, we measured the recognition of highpass-filtered nonsense syllables as a function of filter cutoff frequency for hearing-impaired people with and without low-frequency (apical) DRs [J. Acoust. Soc. Am. 122, 542-553 (2007)]. DRs were diagnosed using the TEN(HL) test, and psychophysical tuning curves were used to define the edge frequency (fe) more precisely. Stimuli were amplified differently for each ear, using the "Cambridge formula." The present study was similar, but the speech was presented in speech-shaped noise at a signal-to-noise ratio of 3 dB. For subjects with low-frequency hearing loss but without DRs, scores were high (65-80%) for low cutoff frequencies and worsened with increasing cutoff frequency above about 430 Hz. For subjects with low-frequency DRs, performance was poor (20-40%) for the lowest cutoff frequency, improved with increasing cutoff frequency up to about 0.56fe, and then worsened. As for speech in quiet, these results indicate that people with low-frequency DRs are able to make effective use of frequency components that fall in the range 0.56fe to fe, but that frequency components below 0.56fe have deleterious effects.