Enhancing the intelligibility of high intensity speech: Evidence of inhibition in the lower auditory pathway.

Intelligibility of narrowband speech declines considerably at high intensities, but substantial recovery from this "rollover" occurs when flanking noise bands are added. The present study employed two types of added noise: narrowband noise matching the spectral limits of the rectangular speech band (producing within band masking) versus broadband noise (producing within band masking plus simultaneous enhancement by out of band noise components). When noise added to diotic speech in experiment 1 was interaurally uncorrelated rather than diotic, intelligibility increased 5%, regardless of noise bandwidth. Interestingly, regardless of interaural correlation, intelligibility was 13% higher with broadband rather than narrowband noise, indicating that noise induced recovery from rollover precedes binaural processing. In experiment 2, diotic noise was presented either continuously or gated on and off with individual sentences. Intelligibility was 5% higher with continuous noise, showing adaptation of masking, which occurred regardless of noise bandwidth. Moreover, intelligibility was about 11% higher with broadband rather than narrowband noise, regardless of gating, ruling out peripheral adaptation as a source of recovery from rollover. These and other findings discussed are consistent with previous suggestions that intelligibility at high intensities is preserved by inhibition of rate-saturated auditory nerve input to secondary neurons of the cochlear nucleus.