A compilation of scientific articles on cochlear mechanical measurements, Ménière’s disease and tinnitus
Author: Eric LePage, Ph.D.
Publisher: OAEricle Australia (June 2006), A4 paperback, 385 pages
For over 100 years the field of auditory science has lacked any straightforward explanation for the cumulative nature of acute noise trauma, or for the origins of tinnitus and
Ménière’s disease with their distressing symptoms. key factor limiting progress is the inherent variability of auditory data, suggestive of multiple adaptive mechanisms and involving the descending pathways to the cochlea. With only partial insights yet available as to the nature of outer hair cell activity, progress to describe mechanisms responsible for the ear’s high level performance has been slow. In particular, investigators studying the origins of the sharp tuning and wide dynamic range of the mammalian ear have rejected data, except those from most sensitive ears, in the belief that the apparently artifactual responses represent cochlear pathology. It turns out that targeting the ‘artifactual behaviour’ may be the key to faster progress overall.
The best new clue has come from otologists interested in the origins of Ménière’s disease – working to understand the origin of the swelling of scala media, the centre chamber. It is the rupture of this vessel which is believed to cause the debilitating attacks of vertigo and loss of hearing. In recent times, focus on the cause of this swelling has turned to understanding cochlear homeostasis and potassium recycling. Failure of homeostasis is linked to rise in hydrostatic pressure. It is to be expected that pressure variations in scala media will distort cochlear structures. While the bulk of prevailing discussions about homeostasis is in molecular-biological terms, since 2000 mechanical measurements have suggested that endolymphatic hydrops may be sound-induced. This compilation has been produced because mechanical correlates of hydrops have previously been observed and reported in detail. Between 1973 and 1989, three series of direct basilar membrane measurements were performed by the author. The measurements were directed at the origin of basilar membrane tuning, but they also revealed baseline displacement shifts which were sound-induced and mimicked the behaviour of the summating potential used routinely to diagnose hydrops. In those days no precedent existed for such effects, and they were hotly disputed.
Yet the author’s 1981 doctorial thesis, here reproduced in full, shows that the data were rife with these summating shifts, and analysis of the opposing slow forces needed to explain them suggested a regulatory role for the slow form of outer hair cell motility. Even then, the control of ‘static’ pressure was identified as vital to setting transduction operating points. The articles herein develop a theory which integrates more features of cochlear structure and function than previously, to explain many biophysical and psychophysical phenomena – including the cochlear response to noise trauma, and Ménière’s disease. The auto-regulation model of Klis and Smoorenburg is extended, specifically invoking the outer hair cells as detectors and effectors. Normal hearing is redefined, not in terms of low thresholds and sharp tuning, but in terms of the viability of cochlear homeostasis. The source of variability of many hearing data is thus revealed.
The book is currently available by contacting Eric directly at firstname.lastname@example.org