Biomedical subjects
Publications and source records attributed to J Tonndorf.
This paper describes, on the basis of animal experiments, that Carhart's notch is caused either by ossicular-chain defects or by stapedial mass-loading. Thus, not all bone conduction impairments are an indication of sensorineural hearing losses, i.e., inner-ear defects.
A new hypothesis is developed concerning the origin of chronic tinnitus. It is based on an analogy between tinnitus and intractable pain, both of their causes being seen in the de-afferentation of nerve fibers. It is suggested that in the control of tinnitus the same interplay exists between large inner-hair-cell fibers, and small outer-hair-cell fibers, provided they are deafferented, that was demonstrated to exist between large and small deafferented fibers of the somato-sensory system in the control of pain.
Békésy's scientific work is assessed, after presentation of a brief biographical sketch. Emphasis is less on his actual achievements (with which the reader is presumed to be generally familiar), and more on his approaches to given problems and the unique solutions he found for many of them. Békésy was the first to study the function of the auditory organ experimentally, replacing theoretical considerations by empirical evidence. He devised novel experimental methods that were suitable for specific problems, often following up on them by an entirely different approach if the latter was more promising for the particular state of his investigation. He was an extremely keen observer, paying attention to seemingly trivial details that often formed starting points of new studies. When he occasionally became bogged down by what appeared to be insurmountable experimental difficulties, he shelved his subject temporarily, often for many years, simply to take it up again when he had found a new approach. It was only to be expected that some of his findings are now being corrected or modified. However, these new studies are conducted many decades after his original observations and measurements, and do not detract from his pioneering and towering achievements.
Evidence for nonlinearities of hydromechanical origin was first found in cochlear models (Békésy, 1928; Tonndorf, 1958). They were of two kinds: (1) amplitude-dependent events, i.e., eddies, leading to harmonic distortion, and (2) amplitude-dependent events, i.e., asymmetrical basilar-membrane displacements, leading to envelope detection, for instance of beats. The evidence on the type 1 event is still circumstantial (e.g., Dallos, 1973). However, the type 2 events appear to be compatible with the results of a number of physiological experiments on actual ears (e.g., Tonndorf, 1958; Goldstein and Kiang, 1968; Sellick et al., 1982; LePage, 1985).
To aid comprehension of cochlear traveling waves, the present article will explain this complex phenomenon in terms of its underlying principles: (1) fluid mechanics, with particular reference to a tangible, analogous event, wave transformation on sloping beaches; (2) the central role of the cochlear stiffness gradient with respect to an energy exchange between travel speed and amplitude of cochlear waves, low pass filter action with distance, and impedance matching for the benefit of wave progression; (3) spatial domain and frequency domain considerations; (4) the solution to the classic dilemma between cochlear tuning and damping; and (5) the task of the helicotrema. I will not discuss the ultimate mechanical stimulation of cochlear hair cells that is mediated by shearing interactions between the tectorial membrane and the organ proper.
Standard texts describe the human tympanic recess as a shallow depression. Khanna and Tonndorf (1975) made silicone rubber casts from the auditory meatus of cats and found the recess to be narrow and deep. Guided by this finding, casts were made from 13 different mammals. At the terminal zone of the auditory meatus, profound variations were found, the consequence of three changing parameters: (1) angle between roof of the canal and tympanic membrane; (2) shape of the isthmus; (3) shape of the recess. For all specimens, values were assigned to each of these variables and a composite graph plotted. A reasonably good correlation was found to exist: the larger the angle, the higher and narrower will be the isthmus and the more narrow and deep the recess. The acoustic implications of these findings will be assessed in a separate study.
In cat, a small tympanic membrane (TM) perforation produces a low-frequency loss--in terms of sound pressure changes (deltaSP) in front of the TM re a 10-micronV round window cochlear microphonic (RW CM)--that varies inversely with frequency at a rate of 12dB/octave with a surgically shortened external auditory meatus (EAM). Losses were determined at the outer opening (deltaSP00) and at the TM (deltaSPTM) of four artificial EAM's of various lengths, volumes, and leakiness. In the low-frequency region, a leaky EAM produced a flat loss. In the midfrequency region, the flatness of losses was attributable to (1) the length of the EAM and (2) the location at which the loss was determined. EAM volume was not related to the configuration of the loss. Losses, under all conditions, were always identical in shape and magnitude for the open and closed systems. Clinically, hearing losses due to TM perforations are essentially frequency independent, especially in the low frequencies. The relation between voltage changes (deltaV) across the transducer and losses with different EAM's suggests that the discrepency between audiometric results and CM losses--at least in the high and midfrequencies--may be due to the use of precalibrated SPL's in clinical audiometry.
1. Replacement of a non-functioning cochlea by a prosthetic device appears to be quite feasible. 2. In their current form, such prostheses provide only a rudimentary auditory perception, permitting discrimination of some simple noises, but providing substantial aid in lip reading. Speech perception is limited to a few words. 3. Further improvements of the prosthesis must take the patho-physiological situation into account: (a) It will be necessary to establish connections between individual electrodes and limited groups of cochlear nerve fibers so as to achieve a correct space/time/amplitude distribution of the applied signals; (b) signal components which are to be transmitted to selected nerve-fiber groups have to be encoded in such a way that they can be correctly processed by those fibers. 4. Only the future will show if the current prosthesis can still be improved. Even if one assumes that substantial improvements can be made, it is doubtful that "normal" speech perception can be restored.
Pathological changes on the stereocilia of cochlear hair cells observed by Bredberg et al. (1972) after noise exposure and in the early stages of antibiotic intoxication are suggested to cause a mechanical decoupling between the tectorial membrane and the hair cells. During acute episodes of endolymphatic hydrops a similar decoupling is suggested to occur, although for different reasons. Harris (1968) calculated the noise generated at the input to the hair cells on account of the Brownian motion of air particles in front of the tympanic membrane. For tight coupling, he obtained a level of (--)22 dB re auditory threshold. For loose coupling this noise was found to increase to +33 dB, i.e., for 55 dB. These two independent findings are used to account for the combination of hearing loss, recruitment, and tinnitus observed with the inner-ear pathologies mentioned. The proposed hypothesis is in good agreement with clinical observations.
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An explanation for the mechanical origin of the hearing loss in endolymphatic hydrops is presented that is based on studies in mechanical cochlear models. An elastic bias of the basilar membrane and/or a mass loading of the cochlear duct account for the low-frequency hearing loss, diplacusis, and even-harmonic distortion. In addition, the static shearing displacement between the tectorial membrane and the organ of Corti, caused by the displacement of the basilar membrane, may partially decouple the hair cells from the tectorial membrane, an event that would explain the tinnitus, recruitment, and perhaps even the disportional loss of speech intelligibility associated with endolymphatic hydrops.
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In 1961, Hallowell Davis proposed the occurrence of the following general sequence of interrelated events in sense organs, in response to specific sensory inputs: (1) the generation of receptor potentials in the sensory cells; (2) chemical transmission across the synapses between hair cells and afferent dendrites; and (3) the triggering of postsynaptic generator potentials capable of eliciting the actual nerve action potentials. Although at the time of his writing there was no evidence for this hypothesis in the auditory organ, there is now good support for the occurrence of all three events and for their general importance in the sensory transduction process.
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Explore the source record for details and available documents.
Explore the source record for details and available documents.