Efferent neural control of cochlear mechanics? Olivocochlear bundle stimulation affects cochlear biomechanical nonlinearity.

We confirm the report of Mountain (Mountain, D.C. (1980): Science 210, 77-72) that stimulating the crossed olivocochlear bundle (COCB) can change the magnitude of the distortion product (f2-f1) in the ear-canal sound pressure. Our results are extended to include (2f1-f2) as well as (f2-f1) from anesthetized chinchillas with both middle-ear muscles sectioned. In contrast to Mountain's report, the polarity of the change can be either positive, negative or absent, depending on the choice of two-tone stimulus frequencies. The influence of two-tone stimulus level is also complex, but we have not yet seen the polarity of the COCB effect change with stimulus level. The magnitude and polarity of the change in (2f1-f2) are not simply related to those for (f2-f1). The effect of COCB stimulation is abolished when scala tympani is perfused with artificial perilymph containing 10(-5) M d-tubocurarine. These results demonstrate that the COCB effect is postsynaptic, probably mediated by outer hair cells. We suggest that the normal cochlea contains an active biomechanical mechanism which reduces the damping of the cochlear-partition motion and is modulated by activating the efferents. It is thus possible that the central nervous system may be able to control the dynamics of the motion of the cochlear partition.