Cochlear mechanics: implications of electrophysiological and acoustical observations

Implications of the spatial distribution of distortion products (2f1--f2) and (f2--f1) observed from populations of cochlear nerve fibers for cochlear mechanics are reviewed (the terms f1 and f2 represent the primary stimulus frequencies; f1 < f2). Characteristics of the distortion products (2f1--f2) and (f2--f1) in the ear-canal sound pressure of the cat and the chinchilla are investigated. Physiological origin of the acoustic distortion product (2f1--f2) is supported by demonstrations of the vulnerability of the distortion product to anoxia, to overstimulation and to cyanide perfusion of the cochlea. Observations are presented describing the dependence of levels of acoustic distortion products (2f1--f2) and (f2--f1): (1) on primary levels; (2) on f2 with iso-f1; and (3) on f1 and f2 with iso-(2f1--f2). Observations and interpretations are discussed in support of the conclusions: (1) that the distortion product (2f1--f2) in the ear-canal sound pressure observed in our studies is not generated in the experimental apparatus, in the eardrum, or in the middle ear but in the primary-frequency region of the cochlea; (2) that the distortion-product generation requires normal physiological processes in the cochlear sensory apparatus but not the neural activity; and (3) that the distortion-product is mechanically propagated from the generation region in the cochlea toward the distortion-frequency place and toward the stapes, through the middle ear, and into the ear canal involving gross motions of the cochlear partition and the middle-ear ossicles. It is now inevitable that we accept the notion that, in a normal ear, manifestations of significant nonlinear behavior are present in the mechanical response of the middle ear and the cochlea at most of the physiologically normal sound pressure levels.