Functionality of Cochlear Micromechanics – as Elucidated by Upward Spread of Masking and Two Tone Suppression

It is generally accepted that the sharp tuning observed at the characteristic frequency (CF) auditory nerve fibre can be attributed to the sharp mechanical response at the corresponding position of the basilar membrane (at the Best Place BP). This observation has resulted in attention being focused on the basilar membrane and away from other micromechanical structures in the cochlea. However, at regions away from the BP, there is considerable evidence to suggest that the basilar membrane and auditory nerve response diverge. This is especially the case at regions basal to the BP (termed the “tail region” of the spatial response – see Fig 5b for an example of the “tail region”) and introduces possibility of other physiological structures, in addition to the basilar membrane, contributing to the auditory nerve response. While most recent basilar membrane mechanical data show an excellent match to Neural responses at the CF [14], there is a clear and often marked difference in the two responses at the tail. This discrepancy seems especially pronounced in cats, where the slope of neural response in the tail region is about 1 to 4 dB/octave whereas the slope of the basilar membrane response is about 9 dB/octave [15]. This paper reviews various experimental data that suggest that the Upward Spread of Masking (USM) effect is simply a psychophysical observation of what is also observed at the auditory nerve as Neural-2TS. This is a satisfying result as it indicates that there is little difference between what is perceived and the auditory nerve response. Similarly, if the Basilar Membrane Two Tone Suppression (BM-2TS) data were to match Neural-2TS just as exactly, this would indicate that the basilar membrane response is identical to the auditory nerve response and, by the same token, to what is perceived (USM). However, our review shows that there is discrepancy FUNCTIONALITY OF COCHLEAR MICROMECHANICS – AS ELUCIDATED BY UPWARD SPREAD OF MASKING AND TWO TONE SUPPRESSION