Neuro-Mechanical Frequency Analysis in the Cochlea

Abstract

It has become increasingly evident that the mechanical filter action of the cochlea is sharpened before neural spikes reach a recording microelectrode in the auditory nerve. When the sharpening mechanism is damaged, the neural tuning curves more closely parallel the mechanical ones (Kiang, et al., 1970; Evans and Wilson, 1973). This paper summarizes some of our experimental results suggesting one of the ways a neural sharpening may take place. Our conclusions are based on the premise that most if not all microelectrode recordings from the auditory nerve concern the radial fibers which end on the inner hair cells—a consequence of Spoendlin’s (1966, 1970) finding that they constitute 90 to 95% of all auditory-nerve afferents. If this is so, changes in the recorded frequency- or time characteristics following selective elimination of the outer hair cells suggest that, in a normal ear, these cells interact with the radial fibers, either directly, or indirectly via the inner hair cells. Of course, such evidence is not conclusive since elimination of the outer hair cells may not be completely selective and may be accompanied by subtle changes in the inner hair cells. Although anatomical evidence for the interaction is scant (Perkins, 1973), some structural relationships are suggestive. The spiral fibers course quite close to the inner hair cells and join the radial fibers at the entrance to habenula perforata. There seem to be as many spiral fibers as inner hair cells and habenular openings.