John J. Guinan, Jr.
Dept. of Elec. Eng. and Comput. Sci., Harvard---MIT, Division of Health Sci. and Technol., and Res. Lab. of Electronics, Massachusetts Institute of Technology, Dept. of Otol. and Laryngol., Harvard Med. School; Eaton--Peabody Lab., Mass. Eye & Ear Infirmary, 243 Charles St., Boston, MA 02114
Stimulation of medial olivocochlear (MOC) efferents, either by shocks or sound, inhibits responses of auditory-nerve fibers (ANFs), as if the gain of the ``cochlear amplifier'' were decreased. Adding OC activity, in a quiet background, shifts the dynamic range of ANF responses to higher sound levels. The shift is typically largest for sounds near the characteristic frequency (CF), where the ``cochlear amplifier'' has its biggest effect, and for ANFs with mid- to high-CFs, where the MOC innervation is densest. In a continuous noise background, the response to a transient sound that is masked by the noise can be partially unmasked by MOC activity. By reducing the ANF response to the noise, OC activity reduces adaptation and can enhance the response to the transient, especially for transients well above the masked threshold. Antimasking is present for transient stimuli both near and off CF, however the noise must have energy near CF. MOC mediated anti-masking, which is a reduction of adaptive masking and should be most effective for mid- and high-frequency noise, will be contrasted with stapedius mediated anti-masking, which is a reduction of suppressive masking and is most effective for low-frequency noise.