| Abstrakt: |
Over the last half century, the intense activity upon the mechanics of mammalian hearing falls into two broad categories: 1) Studies by sensory physiologists (e.g., Brownell, Flock, Russell) interested in the holistic active mechanical properties of the outer hair cells (OHC) and 2) studies by theoreticians and practitioners who have been attracted into the field seeking to explain the central question of audition, i.e. What accounts for the extreme sensitivity and vulnerability of mammalian hearing? How can the critical mechanism which goes missing not be best described as amplification, if, when sensitivity is lost, it can be compensated by use of an external amplifier? The historic starting point has been the Bekesy two-chamber model of the cochlea. The general, the Fourier transform of basilar membrane (BM) vibrations would normally have a zero-frequency (or dc-) component. However, because of the historic basis of the standard model, any such component has been systematically disqualified from consideration. This presenter was the first to publish that a dc-component of BM motion not only does exist, but it covaries with the vibrational component in ways which mimicked the polarity variation of the summating potential (LePage, JASA, 1987). The later published data (full method details, LePage, Hear. Res., 1989) did not have the full support of all collaborators beyond the development of the fiber-optic sensor. However, using the finished device, it was shown, to the real-time disbelief of B.M. Johnstone, that the BM displacement displayed properties very similar to the contractile components of the OHC both in temporal dependence and amplitudes equalling fractions of the length of the OHC. These fiber-optic data have been extracted from many paper-chart records, digitised, then reprocessed with MATLAB(TM) to generate a video to bring to new life what was originally only rapid large excursions of the pen on the single channel chart recorder. These BM dc-shifts are indeed highly tuned. Their tuning varies as expected with the manipulations originally described. Moreover, the velocity of these excursions is of the same order with the estimates of velocity of BM vibrations from dozens later investigations which have uniformly assumed the unseen motion was exclusively a vibration at the stimulus frequency. This presentation therefore raises deep questions about whether the two-chamber model has remained a dominant influence on the progressively-refined design of BM measurements – as well as the widespread rejection of data from many studies whose results made no sense in conventional terms of a fragile and "mysterious" cochlear amplifier (CA). Could it be that the expected super fragile tuning mechanism was the oft-employed explanation, when an equally plausible explanation could be a robust mechanism based on the OHC, modulated by an unknown but critical tiny BM baseline bias? [ABSTRACT FROM AUTHOR] |
