IS USEFUL SPEECH INFORMATION CARRIED BY FIBERS WITH HIGH CHARACTERISTIC FREQUENCIES

It has been proposed that auditory-nerve fibers with characteristic frequencies (CFs) above the speech-frequency range are important in speech perception when the signal-to-noise ratio in the speech frequency range is low [S. Greenberg, J. Phon. 16, 139-149 (1988)]. If this is true, then it might be expected that recognition of speech at low signal-to-noise ratios would worsen with the addition of high-pass noise sufficiently intense to mask information in high-CF fibers. This hypothesis was tested for closed-set recognition of vowels and spondees. Recognition was measured as a function of signal-to-noise ratio in speech-shaped noise, with and without an intense high-pass noise. The addition of high-pass noise did not degrade vowel recognition. Spondee recognition decreased with the addition of the high-pass noise at the highest levels of the speech-shaped noise. However, this same decrease was seen when the spondees were low-pass filtered to simulate downward spread of masking by the high-pass noise. This indicates that the decrease in spondee recognition with high-pass noise was due to masking of information m fibers with CFs in the speech range. Overall, these results suggest that fibers whose CFs are above the speech range are not necessary for speech perception in noise or in quiet.