Implications from cochlear implant insertion for cochlear mechanics

It is usually thought that the displacements of the two inner ear windows induced by sound stimuli lead to pressure differences across the basilar membrane and to a passive mechanical traveling wave progressing along the membrane. However, opening a hole in the sealed inner ear wall in experimental animals is surprisingly not accompanied by auditory threshold elevations. It has also been shown that even in patients undergoing cochlear implantation, elevation of threshold to low-frequency acoustic stimulation is often not seen accompanying the making of a hole in the wall of the cochlea for insertion of the implant. Such threshold elevations would be expected to result from opening the cochlea, reducing cochlear impedance, altering hydrodynamics. These considerations can be taken as additional evidence that it may not be the passive basilar membrane traveling wave which elicits hearing at low sound intensities, but rather factors connected with cochlear fluid pressures and fluid mechanics.     

An active process in cochlear mechanics

A model for cochlear mechanics is proposed to take account of its two systems, one passive and one active. The classical passive system stimulates the inner hair cells directly at levels above about 40 dB SL. At intensities below about 60 dB an active process, the 'cochlear amplifier' (CA), somehow provides additional energy that enhances the vibration of a narrow segment of the basilar membrane near the apical foot of the familiar, traveling wave envelope. The outer hair cells are essential for CA. The active system acts like a high-Q acoustic resonator, and it accounts for the great sensitivity and sharp tuning expressed by the 'tips' of neural tuning curves. The tips are selectively vulnerable to anoxia, noise exposure and other trauma. The CA model explains the detection of small differences in time as well as in frequency, the dual character of the electrocochleogram, recruitment of loudness in cochlear hearing impairment, the long latency of normal neural responses near threshold, acoustic emissions (both stimulated and spontaneous) and the locus of TTS in the frequency range above the exposure tone. Both the classical high-intensity system and the active low-level CA system are highly nonlinear and they combine to compress the great dynamic range of hearing into a much narrower range of mechanical movement of the cilia of the inner hair cells. The mechanism of CA is unknown, and the problem remains of how its action can be triggered by submolecular movements near threshold.     

Some current concepts of cochlear mechanics

The development of the current concepts is reviewed in historical perspective. Helmholtz's hypothesis of basilar-membrane resonance was partially confirmed and partially defeated by Békésy's experiments on models and postmortem cochlear preparations. He discovered that sound was propagated in the cochlea in the form of traveling waves which reached a flat maximum at a frequency-dependent location. Mathematical theory explained this type of sound propagation as a special case of surface waves. Johnstone and his coworkers discovered that the maximum of cochlear vibration in living animals was much sharper than postmortem, and more recently Khanna and Johnstone independently determined the maximum to be nearly as sharp as the tuning curves of the inner hair cells and the auditory-nerve fibers. These findings, together with the work at the Massachusetts Institute of Technology on alligator lizards, have led to new concepts of cochlear mechanics which include hypothesized micromechanical processes in the organ of Corti. These concepts deal not only with the sharpness of basilar-membrane tuning but also with the details of the basilar-membrane amplitude and phase characteristics, as well as with the hair cell and neural tuning curves and response phases. They suggest that some sharpening of the tuning curves occurs between the basilar membrane and hair-cell responses. Such sharpening has been demonstrated in lizards, but in the mammalian ear, the relation is less clear.     

Efferent neural control of cochlear mechanics? Olivocochlear bundle stimulation affects cochlear biomechanical nonlinearity

We confirm the report of Mountain (Mountain D.C. (1980): Science 210, 71–72) that stimulating the crossed olivocochlear bundle (COCB) can change the magnitude of the distortion product (?₂??₁) in the ear-canal sound pressure. Our results are extended to include (2?₁? ?₂) as well as (?₂??₁) from anesthetized chinchillas with both middle-ear muscles sectioned. In contrast to Mountain's report, the polarity of the change can be either positive, negative or absent, depending on the choice of two-tone stimulus frequencies. The influence of two-tone stimulus level is also complex, but we have not yet seen the polarity of the COCB effect change with stimulus level. The magnitude and polarity of the change in (2?₁? ?₂) are not simply related to those for (?₂??₁). The effect of COCB stimulation is abolished when scala tympani is perfused with artificial perilymph containing 10^?5 M d-tubocurarine.These results demonstrate that the COCB effect is postsynaptic, probably mediated by outer hair cells. We suggest that the normal cochlea contains an active biomechanical mechanism which reduces the damping of the cochlear-partition motion and is modulated by activating the èfferents. It is thus possible that the central nervous system may be able to control the dynamics of the motion of the cochlear partition.     

Another look at Daggett's 1948 tympanoplasty.

It is important that we look critically at the results of conservative tympanoplasty in the presence of cholesteatoma. Lessons and observations taught many years ago by earlier otologists may give us valuable leads. A method of conservative surgery for chronic suppurative ear disease devised by Dr William Daggett in London in the late 1940s is described and evaluated in the light of current technical expertise.     

Notes on the comparative mechanics of hearing. II. On cochlear shunts in birds

Birds differ in cochlear shunts of acoustic volume flow. One can distinguish three types of shunt: peri-, intra- and intercochlear. The functional significance of shunts is discussed.     

Experimental basis for lidocaine therapy in cochlear disorders.

In order to further our basic understanding of the effects of lidocaine hydrochloride in the inner ear, cochlear potentials and blood flow (CBF) were assessed after intravenous (i.v.), anterior inferior cerebellar artery (AICA), and local round window (RW) lidocaine administrations in guinea pigs and rats. Lidocaine RW applications produced a dose dependent decrease in compound action potentials (CAP) and cochlear microphonics (CM). The sensitivity changes were more pronounced at high frequencies. These findings suggest that lidocaine has specific pharmacological action in the inner ear other than simple anesthesia of the auditory nerve. The basal turn endocochlear potentials (EP) were not altered by topical lidocaine, implicating altered organ of Corti function following local application of lidocaine. RW applications of lidocaine had no effect on CBF or systemic blood pressure (BP). I.v. infusions caused substantial reductions in BP. In the case of systemic infusions the percent changes in CBF were equal to and accountable by the BP changes. The microinfusions (50 mg/ml, 100 nl/min) through AICA produced a 30%, long lasting increase in CBF. However, neither systemic lidocaine nor AICA infusions had an effect on CAP or CM. These findings indicate that systemically given lidocaine may not cross the blood-cochlear barrier and that the cochlear electrophysiological effects due to lidocaine when given locally are partly mediated by direct influence on cochlear hair cell function; they also suggest that lidocaine-induced interference with active ion transport in the lateral wall or an influence on CBF are not contributing factors.     

Experimental cochlear hydrops in cats

An experimental model of cochlear hydrops was created in cats. Ten cats underwent surgical procedures to obliterate the saccule, and their temporal bones were studied by light microscopy after sacrifice at 10 weeks. In one group the saccules were destroyed by maceration and aspiration. However, in these ears the saccular lumens were not obliterated and endolymphatic hydrops did not develop. Obliteration of the saccules was achieved in the second group after fascia was introduced into the area of the injured saccules. Cochlear endolymphatic hydrops was a consistent finding in these ears except when a fistula of the membranous labyrinth was present. However, in addition to fibrosis and new bone formation in the vestibules there were also degenerative changes in the hair cells, tectorial membranes, and striae vasculares of these cochleae. The results supported the longitudinal flow theory of endolymph and are consistent with the reported examples of cochlear endolymphatic hydrops in man.     

Sound-evoked efferent effects on cochlear mechanics of the mustached bat

The influence of the crossed medial efferent system on cochlear mechanics of the mustached bat was tested by measuring delayed evoked otoacoustic emissions (DEOAEs), cochlear microphonics, distortion product otoacoustic emissions (DPOAEs) and stimulus frequency otoacoustic emissions. Contralaterally delivered sinusoids, broadband noise and bat echolocation calls were used for acoustic stimulation of the efferent system. With all four measures we found a level-dependent suppression under stimulation with both broadband noise and echolocation calls. In addition, the sharply tuned cochlear resonance of the mustached bat which is involved in processing echolocation signals at 61 kHz shifted upward in frequency by several 100 Hz. Presentation of sinusoids did not have any significant effect. DEOAEs and DPOAEs were in some cases enhanced during contralateral presentation of the bat calls at moderate intensities. The most important function of the efferent system in the mustached bat might be the control of the extraordinarily fine-tuned resonator of this species, which is close to instability as evident from the very pronounced evoked otoacoustic emissions which sometimes convert into spontaneous otoacoustic emissions of high level.     

Another look at semantic relational categories and language impairment.

Earlier comparative studies of language-impaired and normal children involved semantic-relational analyses of broad categories (e.g., action, locative action, and so on) in which utterance types were not differentiated. In this study, locative action utterances were differentiated by the types of locative words used singly and in combination. The types of utterances used by a language-impaired child were tracked between 1 year 6 months and 3 years and compared with those of three age- and MLU-matched normal children. The results suggested that differences in the semantic properties of language-impaired and normal children's utterances may go undetected unless a fine-grained analysis is performed on the types of expressions used within a global relational category. The potential value of extending semantic-relational analyses by exploring word use in syntactic contexts is addressed.     

Validity of the Liouville-Green (or WKB) method for cochlear mechanics

This article presents a comparison of Liouville-Green (LG) calculations and exact solutions of 2- and 3-dimensional cochlea models. The agreement is in general quite good. For certain choices of the model parameter's, however, the 2- and 3-dimensional LG solutions show appreciable errors in the region just beyond the location of maximum amplitude of the basilar membrane response. The origin of these errors appears to be the non-uniqueness of the (complex) LG wave number k(x) in 2- and 3-dimensional models: the ‘eikonal equation’ from which k(x) has to be solved has multiple roots. To study this problem somewhat deeper, the properties of the locus of k = k(x) formed when x is varied, are investigated. Erratic behaviour of the LG solution is found to occur when this root locus approaches one of the saddle points of a complex function of k — called Q(k) — which plays the major role in the eikonal equation. Apart from this specific problem, the LG approximation is very well suited to unravel the mechanisms governing wave propagation and attenuation in the cochlea. The analysis shows clearly why and how the response of the basilar membrane builds up to a maximum and which factors cause a turnover and a rapid decrease to occur, in both the long-wave and the short-wave cases. A special discussion is dedicated to the relation between the LG approximation and the absence of wave reflection in cochlea models of the type studied.     

A look at ear canal changes with jaw motion.

Two procedures were used with one subject to examine changes in the ear canal with jaw motion. Silicone ear impressions were examined to measure the effect of jaw position. Magnetic resonance imaging was used to define the three-dimensional structure of the canal and visualize key underlying tissue. Results showed that inexpensive impressions can detail changes in the cartilaginous region of the canal. Approximately a 25% change in this subject's ear canal with different jaw positions was observed in the anterior/posterior plane with essentially no change in the superior/inferior plane. This is likely related to asymmetric stress from action of the mandibular condyle on neighboring cartilaginous tissue. Solutions to jaw motion-induced feedback problems are suggested.     

Continuous low level sound alters cochlear mechanics: an efferent effect?

A slow, but reversible, alteration is described in the level of acoustic distortion emitted during continuous two tone stimulation. The nature of this change and the factors which affect it indicate that it may be due to the influence of cochlear efferents on the mechanical response of the cochlea to sound.     

Unit responses at cochlear nucleus to electrical stimulation through a cochlear prosthesis

Afferent auditory fibers of the guinea pig cochlea were electrically stimulated with current introduced through electrodes in the scala tympani. Thresholds were determined for unit responses recorded in the ventral cochlear nuclei to a sinusoid of 98 Hz from response-rate growth functions versus stimulus intensity. Suprathreshold response rates for most units grew rapidly from threshold to saturation at 2-15 dB above threshold. Peristimulus time histograms were collected for responses to single sinusoids and combinations of two and five sinusoids ranging from 86 to 134 Hz. Spike occurrences were highly synchronous with individual cycles of the pure sinusoids, but responses to the more complex waveforms occurred primarily to the more intense peaks. The amplitude envelope was thus a major contributor to responses to multiple sinusoids. Destruction of cochlear structures with neomycin increased unit thresholds and produced some changes in waveform encoding.     

Multichannel cochlear implantation in children with cochlear ossification.

OBJECTIVE: To compare results of post-meningitic children who had cochlear implantation with partial or complete drill-out to those who had no drill-out. STUDY DESIGN: This study is a retrospective case review. SETTING: The Atlanta Cochlear Implant Group is a private, tertiary, outpatient clinic. PATIENTS: Eligibility included all our post-meningitic patients, 2-17 years, having a cochlear implant between June 1990 and July 1997. INTERVENTIONS: All subjects had a Nucleus 22 cochlear implant surgically implanted, speech processor programming and follow-up testing in our center, and aural rehabilitation in a variety of therapy settings. MAIN OUTCOME MEASURE: Open and closed set speech discrimination tests. RESULTS: Test performance for speech understanding was highest in the non-ossified group and lowest, but above chance, in the ossified group with complete drill-out. CONCLUSIONS: While children with non-ossified cochleas performed best, even children with extensive ossification requiring complete drill-out benefited from cochlear implantation.