Lateral suppression preserves speech intelligibility at high intensities

Abstract

The intelligibility of narrowband speech decreased 23% when its level was raised from 45 to 75 dB. However, when flanking bands of low-pass and high-pass white noise were added, intelligibility at the higher speech level recovered by as much as 65%. Recovery appears to be due to lateral suppression counteracting overloading effects of auditory-nerve (AN) firing-rate saturation at high speech intensities. Findings supporting this hypothesis include: (1) the absence of intelligibility enhancement at the lower speech level; (2) a greater effect of the higher frequency flanking noise band at a low noise level; and (3) equivalent effects of continuous versus gated flanking noise, inconsistent with firing-rate adaptation. In addition, there was no intelligibility increase when the flanking noise and narrow-band speech were delivered to opposite ears of listeners. These behavioral results are consistent with previous physiological observations and models of lateral suppressive interactions occurring in the lower auditory pathway. It appears that the noise-induced intelligibility recovery is produced via lateral inhibition of saturated AN-fiber input to neurons of the cochlear nucleus, with a possible additional contribution from mechanical (two-tone) suppression evoked within the cochlea and likely not involving olivocochlear feedback. [Work supported by NIH.]