Cochlear fibre rate--intensity functions: no evidence for basilar membrane nonlinearities.

The presence of inflexions in the slopes of the rate--intensity functions of single cochlear fibres has been cited as evidence for nonlinearities, in the motion of the basilar membrane, or the type described by Rhode. We have carried out a detailed study of these inflexions in the anaesthetized cat. The inflexions were found to be related more to the threshold of the individual fibre than to occur at a common sound pressure level, for fibres of similar and differing characteristic frequency. Likewise, contrary to the earlier evidence, no strong inverse relationship was found to exist between the magnitude of the slope of the rate-intensity function and the threshold of a fibre. What relationship did exist depended upon the presence of fibres having low spontaneous discharge rate activity. The data therefore do not suport the concept of a common input (i.e. basilar membrane) nonlinearity; rather, they suggest that the shape and slope of the rate--intensity functions are more related to the properties of individual neural channels.