Development of Auditory Asymmetry in Transient Evoked Otoacoustic Emissions in Pre-Term Infants

We previously reported that transient evoked otoacoustic emissions (TEOAEs) continue to develop after the onset of cochlear function in pre-term infants from 34 to 39 weeks of conceptional age (Morlet et al., 1996). The time-course of development differed between genders. Reported here are findings of further analysis of data from the study cohort, comprising 1020 ears of 510 pre-term neonates (conceptional age ranging from 34 to 39 weeks), demonstrating developmental differences between right and left ears. The left ear of female pre-term infants showed enhancement of TEOAE amplitude at low and medium frequencies with age, whereas differences were less dramatic in the right ear. In male infants, TEOAE amplitude decreased in several frequency bands at high frequencies, above 4 kHz, between 34 and 39 weeks conceptional age; most developmental differences were found to be more dramatic in the right than in the left ear. It is tempting to speculate that these developmental features underlie well-known inter-aural asymmetries that have been demonstrated in the adult human.

An algorithm for analyzing linkages affected by heterozygous translocations: QuadMap

Heterozygous chromosome rearrangements such as reciprocal translocations are most accurately displayed as two-dimensional linkage maps. Standard linkage mapping software packages, such as MapMaker, generate only one-dimensional maps and so reciprocal translocations appear as clusters of markers, even though they originate from two nonhomologous chromosomes. To more accurately map these regions, researchers have developed statistical methods that use the variance in map distance to distinguish among the four segments (two translocation, two interstitial) of the translocation. In this study, we describe modifications to one of these protocols, that proposed by Livingstone et al. (2000). We also introduce QuadMap, a new software application for dissecting heterozygous translocation-affected linkage maps.

Lateralization of the effects of the benzodiazepine drug oxazepam on medial olivocochlear system activity in humans

Benzodiazepines (Bzd) are known to interact with GABAergic inhibitory neurotransmission. Previous research on their effect on human auditory efferent pathways--through evoked otoacoustic emissions suppression by contralateral acoustic stimulation (CAS)--indicated a decrease in medial olivocochlear (MOC) efferent system inhibitory activity, after oral intake of oxazepam--representative of the Bzd drug class. To date, this pharmacological effect was only assessed in the right ear. Since a leftward asymmetry of Bzd receptors localization in human auditory cortex has been described recently, we explored in this study the hypothesis of an asymmetrical action of Bzd on MOC efferent functioning. The results revealed a significant difference of Bzd effect probing the right ear versus the left ear, with CAS-induced suppression being less effective in the right than left ear after oxazepam intake. This finding raises the question of possible neurochemical left-right asymmetry in the descending auditory pathways. The potential localization of this asymmetry is discussed.

Effect of stimulus frequency and stimulation site on the N1m response of the human auditory cortex

The aim of the present study was to investigate the functional organization of the auditory cortex for pure tones of 1, 2, 4, 6, 8 and 12 kHz. Ten subjects were tested with a whole-head magnetometer (151 channels). The location, latency and amplitude of the generators of the N1m (the main component of the response, peaking approximately at 100 ms) were explored simultaneously in the right and left hemispheres under monaural stimulation. Our results revealed that tonotopy is a rather complex functional organization of the auditory cortex. From 1 to 12 kHz, tonotopic maps were found for contralateral as well as for ipsilateral stimulation: N1m generators were found to be tonotopically organized mainly in an anterior-posterior direction in both hemispheres, whatever the stimulated ear, but also in an inferior-superior direction in the right hemisphere. Furthermore, latencies were longer in the left than in the right hemisphere. Two different representations of spectral distribution were found in the right auditory cortex: one for ipsilateral and one for contralateral stimulation.

Development of auditory asymmetry in transient evoked otoacoustic emissions in pre-term infants

We previously reported that transient evoked otoacoustic emissions (TEOAEs) continue to develop after the onset of cochlear function in pre-term infants from 34 to 39 weeks of conceptional age (Morlet et al., 1996). The time-course of development differed between genders. Reported here are findings of further analysis of data from the study cohort, comprising 1020 ears of 510 pre-term neonates (conceptional age ranging from 34 to 39 weeks), demonstrating developmental differences between right and left ears. The left ear of female pre-term infants showed enhancement of TEOAE amplitude at low and medium frequencies with age, whereas differences were less dramatic in the right ear. In male infants, TEOAE amplitude decreased in several frequency bands at high frequencies, above 4 kHz, between 34 and 39 weeks conceptional age; most developmental differences were found to be more dramatic in the right than in the left ear. It is tempting to speculate that these developmental features underlie well-known inter-aural asymmetries that have been demonstrated in the adult human.

Frequency and temporal analysis of contralateral acoustic stimulation on evoked otoacoustic emissions in humans

Previous studies have shown that the effect of contralateral acoustic stimulation (CAS) on ipsilateral evoked otoacoustic emissions (EOAE) depends somewhat upon the spectrum of the eliciting stimulus. The latency of the EOAE, however, is itself frequency-dependent. Consequently, two general ways of analyzing the effects of CAS may be considered: by frequency band or by temporal segment. In this study, we analyzed the effects of CAS both ways in the same subjects, essentially simultaneously. The frequency analysis of the EOAE derived from the wavelet transform (WT). The WT is known to provide a robust approach to the analysis of non-stationary signals and was anticipated to avoid possible time-frequency confounds of the cochlear mechanical system. For comparison, a more basic analysis - using a temporal moving window - was employed. The results largely support earlier findings and confirm that in humans the greatest suppression of EOAEs by CAS is obtained for lower frequency and/or longer latency EOAE components. Despite expectations for the WT analysis, the more basic, temporal, analysis tended to yield the clearer results.

Somatosensory cuing of head-only rotational testing

Head-only rotational testing (HORT) is a method of assessing the vestibulo-ocular reflex (VOR) that can be performed at the bedside with inexpensive, transportable equipment. Previous studies of HORT using auditory cuing have suggested that test results are in large part independent of the method by which a subject's head is moved, that is, active versus passive. However, many subjects have difficulty moving their head at rotational frequencies of 3 Hz or greater. In an attempt to overcome this difficulty, and to learn more about HORT, we explored the possibility of providing subjects with a somatosensory, rather than an auditory cue for head movement. Somatosensory, cues were provided either by applying gentle pressure against a tightly-fitted headband or by lightly tapping on the shoulders. Our results with young, healthy subjects indicate that somatosensory cuing is an efficacious means of inducing periodic head rotation and that HORT results are unaffected by the type of cue provided. We conclude that somatosensory rather than auditory cuing may be the preferred method of performing HORT.

Accuracy of hearing aid use time as reported by experienced hearing aid wearers

OBJECTIVE

The purpose of this study was to investigate the relationship between the amount of time that a person reports wearing his or her hearing aid and the actual time that the hearing aid is worn. Although use time data are used in the analysis and interpretation of investigations concerned with auditory perceptual learning and with user satisfaction, the accuracy of self-reported use time has not been fully investigated.

DESIGN

The experimental and control group were fit with a hearing aid that has the capability of storing use time data for later analysis. The experimental group was told that the self-reported use time would be verified with a computer analysis of the hearing aid that provides the actual use time. The control group was not informed of the use time validation procedure. The agreement between self-reported and actual use time was compared statistically between groups (knowledge of validation versus no knowledge of validation).

RESULTS

The experimental group provided accurate self-reported use time whereas the control group showed a significant difference between actual use time and self-reported use time.

CONCLUSIONS

The results may assist in the interpretation of results of previous investigations that have depended on self-reported use time and in the design of future investigations. For the clinician, the results indicate that relying on a patient's self-reported hearing aid use time for documentation of satisfaction or signal processing preference may be misleading.

Head-only rotational testing in the elderly

Head-only rotational testing (HORT) is a noninvasive technique to assess the vestibuloocular reflex (VOR) that can be performed in a nonlaboratory environment with inexpensive, portable equipment. Little data are available regarding the applicability of this technique to the elderly in whom limitations of cervical range of motion might reduce the usefulness of the technique. This study was designed to apply HORT to a small group of normal elderly subjects to determine whether the technique could be used successfully in this population. Also, the VOR parameters derived from HORT were compared with the results obtained from a group of healthy young control subjects. HORT was performed both with and without visual fixation of an earth-fixed target, using both active head movement and passive head movement. Results indicated that the responses of the elderly were indistinguishable from those of the young except for a slightly higher gain at 1 Hz in the young subjects. Other findings, consistent across young and elderly, were that visual fixation increased gain and decreased phase lead at lower frequencies and that VOR parameters were unaffected by volition, that is, active and passive head rotation produced similar responses. These findings suggest that HORT can be applied to the elderly without difficulty. Future studies will assess the usefulness of HORT in elderly patients with VOR disorders.

Managing hearing loss in a patient with Alzheimer disease

This case study reports the management of hearing loss in a patient with Alzheimer disease (AD) living at home with a spouse care giver. The report highlights the interaction between symptoms associated with AD and hearing loss and the lack of data regarding remediation of hearing loss in this population. Specifically, the case illustrates the modifications in evaluation and verification of the hearing aid fitting that may be advisable when working with patients with AD. The data for this patient illustrate some novel measurement techniques that may assist the professional in documenting the impact of treatment in this population.

Are inner or outer hair cells the source of summating potentials recorded from the round window?

The relative contribution of inner hair cells (IHCs) and outer hair cells (OHCs) to the production of the summating potential (SP) is unresolved in the literature. Since OHCs in the base of the cochlea have been reported to produce little dc receptor potential except at very high sound pressure levels [I. J. Russell and P. M. Sellick, J. Physiol (London) 284, 261-290 (1983)], the IHCs appear to be the dominant source of the SP. However, results of intracochlear recordings are conflicting, although deriving from measurements in different turns of the cochlea [e.g., I. J. Russell and P. M. Sellick, J. Physiol. (London) 284, 261-290 (1983) versus P. Dallos and M. A. Cheatham, Sensory Transduction (1992)]. To determine which type of hair cells is the dominant source of the SP recorded at the round window, we used carboplatin to selectively destroy IHCs or a combination of IHCs and OHCs in the chinchilla. Related work, using measurements of distortion product otoacoustic emissions and cochlear potentials to assess the functional status of the OHCs served to validate this animal model [Trautwein et al., Hearing Res. 96(1-2), 71-82 (1996)]. The SP, cochlear microphonic (CM), and compound action potential (CAP) were recorded from the round window, and cochleograms were determined using well-established histological methods. The results were reasonably distinctive among three groups of ears--control (from untreated normal chinchillas), IHC-loss (extensive IHC loss with minor or no loss of OHCs), and IHC-OHC loss (total IHC loss plus extensive loss of OHCs over the basal half of the cochlea). Ears of chinchillas in the IHC loss group had a decrease of over 50% in SP output compared to control ears with the exact reduction depending somewhat upon the stimulus conditions. Ears in the IHC + OHC loss group, nevertheless, showed even further reduction in SP output which was clearly attributable to destruction of OHCs in the cochlear base. It was concluded that, although the IHCs are responsible for a greater contribution of dc-receptor potential to the SP recorded at the round window, a significant contribution is made by the OHCs, as well. The results suggest, specifically, that the round window "sees" SP output roughly in inverse proportion to the IHC:OHC. Lastly, the complexity of SP production, as recorded from the round window, precludes a completely straightforward interpretation of the SP:CAP in clinical ECochG.

Evaluation of the nonorganic hearing loss suspect

OBJECTIVE

To examine the utility of distortion-product otoacoustic emissions (DPOAEs) in the assessment of the nonorganic suspect because DPOAE analysis constitutes an objective test of hair-cell function that yields audiometriclike data.

STUDY DESIGN

Retrospective study of clinical findings.

SETTING

Audiology outpatient clinic of our university's medical center.

PATIENTS

The study cohort comprised 30 patients who presented with a profile of suspicion for nonorganic hearing loss. Most cases were found, with the aid of DPOAE testing, to be nonorganic or to have nonorganic overlays to organic hearing loss.

INTERVENTIONS

Interventions were diagnostic only.

MAIN OUTCOME MEASURES

Observed audiometric findings and changes thereof.

RESULTS

Statistically significant decreases in thresholds occurred in subgroups of those cases deemed to be truly nonorganic in origin.

CONCLUSIONS

Especially considering test efficiency, the results support the inclusion of DPOAE analysis in the diagnostic management of the suspect-nonorganic patient.

Surface dependence: a balance control strategy in panic disorder with agoraphobia

OBJECTIVE

Previous studies have reported vestibular dysfunction and impaired balance in patients with agoraphobia. Vestibular dysfunction may lead to an information processing strategy focusing on spatial stimuli from two nonvestibular sensory channels, vision and proprioception. This nonvestibular balance control strategy may in turn lead to discomfort in situations involving inadequate visual or proprioceptive spatial cues (space and motion discomfort). The objective of this study was to examine sensory integration of spatial information in agoraphobia. Because of previous findings that space and motion discomfort and vestibular dysfunction are common in agoraphobia, we hypothesized that agoraphobics would use a nonvestibular balance control strategy.

METHOD

Using computerized dynamic posturography, we examined balance performance in patients with panic disorder with agoraphobia, uncomplicated panic disorder, nonpanic anxiety disorders, and depression without anxiety, as well as healthy subjects for comparison. The posturography procedure included six sensory conditions in which visual and proprioceptive balance information was manipulated experimentally by permutations of sway-referencing the support surface or the visual surround or by having patients close their eyes.

RESULTS

The agoraphobics had impaired balance when proprioceptive balance information was minimized by sway-referencing the support surface (p < 0.02). This pattern, called surface dependence, tended to be more pronounced in agoraphobics who reported space and motion discomfort, including fear of heights or boats.

CONCLUSION

Agoraphobics rely on proprioceptive cues for maintenance of upright balance. This strategy may lead to intolerance of situations characterized by unstable support.

Development of cochlear active mechanisms in humans differs between gender

Despite onset of function early during the third term of gestation, the human auditory system demonstrates continued maturation, thought previously to occur primarily at the neural level. The electromotile properties of outer hair cells appear to contribute substantially to hearing sensitivity and frequency selectivity and lead to the generation of otoacoustic emissions (OAEs). This report demonstrates continued development of cochlear active mechanisms (i.e. end-organ level) after onset of cochlear function, as reflected by OAEs. Significant gender differences also are reported, corresponding to recently observed intersex differences in cochlear length and precursory to gender differences observed in the adult.

The efficacy of tympanic electrocochleography in the diagnosis of endolymphatic hydrops

Electrocochleography (ECoG), an objective electrophysiologic test, is useful in the clinical diagnosis of endolymphatic hydrops. The purpose of this study was further to define the role of ECoG in the diagnosis of this disease. A retrospective chart review of 100 patients undergoing tympanic ECoG was undertaken comparing symptoms, degree of hearing loss, duration of disease, and diagnosis with ECoG results. The apparent sensitivity and specificity of ECoG in the diagnosis of endolymphatic hydrops were determined to be 57% and 94%, respectively. Three of 30 positive ECoG results were falsely positive. Fluctuating hearing loss and the degree of hearing loss (< 40 dB) and duration of disease (< 48 months) were statistically significant in predicting positive ECoG results. We conclude that a positive ECoG result is helpful in objectively confirming the disease. However, a negative result does not rule out hydrops.

Localizing retrocochlear hearing loss

PURPOSE

The origin of acute/sudden hearing loss is multifactorial. The association of vestibular symptoms does not necessarily isolate the pathologic condition to the inner ear. The audiogram provides a screen for differentiating conductive from sensorineural loss but often fails to provide more localizing information.

METHODS

Three unusual patients with a variety of retrocochlear presentations of hearing loss are presented. Along with conventional auditory brainstem response (ABR) testing, newer auditory tests, including otoacoustic emissions and three-dimensional ABR analysis, can facilitate site-of-lesion testing. Magnetic resonance imaging (MRI) also provides graphic documentation for sources of retrocochlear hearing loss.

RESULTS

One patient had gamma-knife treatment of an arteriovenous malformation, incurring a localised lesion to the inferior colliculus contralateral to the side of hearing loss. This effectively eliminated wave V, as confirmed by three-dimensional ABR analysis. A second patient with human immunodeficiency virus developed sudden complete hearing loss with retained otoacoustic emissions, confirming a retrocochlear lesion. A third patient with acute otitis media with sudden hearing loss and vertigo had an abnormal ABR and "mass lesion" on MRI. Hearing subsequently returned to normal, as did a repeated scan.

CONCLUSIONS

The unique aspects of each case of retrocochlear hearing loss and the applied auditory electrophysiologic tests are reviewed.

Panic, agoraphobia, and vestibular dysfunction

OBJECTIVE

Otoneurological abnormalities have been reported in panic disorder. The purpose of this investigation was to determine the prevalence of such findings in panic disorder with and without agoraphobia and to discern whether vestibular dysfunction was associated with specific symptoms.

METHOD

Clinical audiological and vestibular tests were administered to 30 patients with uncomplicated panic disorder (without agoraphobia or with only mild agoraphobia), 29 patients with panic disorder with moderate to severe agoraphobia, 27 patients with anxiety but no history of panic attacks, 13 patients with depressive disorders but no history of anxiety or panic attacks, and 45 normal comparison subjects. Evaluators were blind to subjects' diagnostic group. Quantitative measures of subjects' discomfort with space and motion and of the frequency of certain symptoms between and during panic attacks were obtained. Anxiety state levels were measured during the vestibular tests.

RESULTS

Vestibular abnormalities were common in all the groups but most prevalent in the patients with panic disorder with moderate to severe agoraphobia. Vestibular dysfunction was associated with space and motion discomfort and with frequency of vestibular symptoms between, but not during, panic attacks. There were no major differences between the two panic groups in anxiety levels during vestibular testing. There were no significant differences between groups on the audiological component of the test battery. Exploratory data analysis indicated that the constellation of vestibular tests most specific for agoraphobia was one indicating compensated peripheral vestibular dysfunction.

CONCLUSIONS

Subclinical vestibular dysfunction, as identified by clinical tests, may contribute to the phenomenology of panic disorder, particularly to the development of agoraphobia in panic disorder patients.

Middle ear effects secondary to nasogastric intubation

A study was conducted of the status of the middle ear in neurologically impaired patients requiring extended nasogastric tube (NGT) feeding. Ninety percent of the study patients with NGTs demonstrated abnormal middle ear function via immittance testing; whereas, 8.5% and 0% of the clinical and normal control samples, respectively, had abnormal findings. These results support the need to conscientiously observe the middle ear status of patients requiring long-term NGT feedings. A plan for monitoring middle ear function in these patients is suggested.

Head-only rotational testing: influence of volition and vision

Head-only rotational testing has been suggested as a method of assessing the vestibuloocular reflex in patients with reduced vestibular function as a result of ototoxicity. The purpose of this study was to investigate the influence of vision and volition on horizontal vestibuloocular reflex responses during head-only rotation in normal subjects to better define the test parameters for future clinical application. Ten asymptomatic, healthy subjects were tested using various combinations of volitional and visual conditions during head-only rotation. Results indicated that visual fixation enhanced responses slightly and that response dynamics were altered somewhat by volition. These findings provide motivation for future studies to further define head-only rotation test parameters for patients with suspected vestibular abnormalities.

Comparison of earphone radiation recorded from hearing impaired subjects and a resistor network simulator

Electromagnetic radiation emitted by earphones is a potential source of contamination of auditory evoked potentials, particularly when long-duration stimuli are used to evoke the response. One practical solution to this problem would be to estimate the magnitude and characteristics of this earphone radiation so that an appropriate choice of earphone is made. With this aim, earphone radiation measures obtained from a resistor network simulator laid out over the dimensions of an average head were compared with those obtained from three hearing impaired subjects to determine the efficacy of the simulator in estimating the magnitude of this radiation. Results indicated a close agreement between these measures suggesting that the simulator measures are realistic. It was shown that the earphone radiation (i) increases linearly with stimulus intensity; (ii) varies systematically as a function of recording configuration; and (iii) recorded in the simulator can be used to subtract out earphone radiation contaminating evoked responses recorded from subjects using unshielded earphones. Based on these results it was concluded that the simulator provides a practical means to simulate real recordings, at least for the purposes of estimating stimulus artifact that may contaminate auditory evoked potentials to sustained stimuli.

3CLT ABR analyses in a human subject with unilateral extirpation of the inferior colliculus

Reported are the results of analyses of three-channel Lissajous trajectories (3CLTs) of the auditory brain stem responses (ABRs) in a human subject in whom a focal lesion of the brain stem was caused by stereotactic radiosurgery, the 'gamma knife'. The surgery caused total destruction of the right inferior colliculus. The results, using multiple measures for defining ABR components, confirm findings from more conventional 2-channel recordings which, in turn, suggested the presence of an intact wave IV but a negligible, if not totally absent, wave V with stimulation of the left (contralateral) ear. The results thus support theories that wave V is generated by crossed pathways and that wave IV is an independent wave generated by the lateral lemniscus. Since magnetic resonance imaging suggested no destruction of tissue below the inferior colliculus, the findings also support theories of wave V generation at or rostral to the inferior colliculus. In practical terms, the results demonstrate the value of multichannel recordings of the ABR in component identification and in interpreting ABR abnormalities.

Observations on temporal aspects of bone-conduction clicks: real head measurements

To further elucidate nuances of bone-conduction (BC) stimulation, particularly as they may pertain to BC-elicited auditory brainstem responses (ABRs), real head measurements were made of vibratory responses to BC clicks. A miniature accelerometer was held firmly against the skull near a bone vibrator applied to the mastoid, to the opposite mastoid, and to the forehead. Measurements also were made with the vibrator applied to the forehead and the accelerometer placed on a mastoid. The measured vibrations were compared among conditions within and across subjects. The results demonstrate that vibration picked up in the immediate vicinity of the vibrator, roughly mimics the temporal and spectral characteristics of the click observed on an artificial mastoid. However, probing at sites remote to the bone vibrator suggested the introduction of a propagation-like delay as well as low-pass filtering. These findings support observations of previous researchers who demonstrated phase differences between vibrations recorded at different points on the skull, further demonstrating the complexities of skull vibration. Latency differences of ABR responses obtained with forehead versus mastoid placements, for example, cannot be attributed merely to the difference in efficiency between sites of stimulation demonstrated audiometrically. While the real head observations do not permit determination of the actual vibratory lag at the cochlea, they do permit the deduction that a comparable onset delay occurs in BC as in air-conduction stimulation via supra-aural earphones.

Relative effective frequency response of bone versus air conduction stimulation examined via masking

An adaptation of the sensorineural acuity level procedure was employed to obtain thresholds under bone- (BC) vs. air-conducted (AC) white noise masking. For the BC masking condition, a Radioear B71 was placed on one mastoid. An Etymotic ER3 with a foam tip placed in the ear on the same side was used to deliver the pure-tone probe stimulus. This transducer was chosen to approximate the Telephonics TDH-39 earphone response characteristic while reducing occlusion effect. For the AC masking condition, the masker and probe were mixed electrically and delivered to the earphone. Masked threshold data, transformed into frequency response curves, demonstrated greater variance of BC vs. AC response across frequency but less high-frequency roll-off than expected from coupler measurements obtained using an artificial mastoid and 6-cm3 cavity, respectively.

Correlations of neuroanatomical measures to auditory brain stem response latencies

Magnetic resonance imaging (MRI) was used to obtain measures presumed to scale the dimensions of the lower auditory pathway in humans for the purpose of further defining the relationship between length of the auditory pathway and auditory brain stem response (ABR) latencies. Specifically, measurements of soft tissue structures, that is, the eighth nerve and brain stem, were made for comparison with skull dimensions and ABR latencies. It was hypothesized that the brain stem dimensions covary significantly with skull dimensions and that the ABR parameters covary with both skull and brain stem dimensions. In general, only weak correlations were obtained with coefficients failing to reach statistical significance for most comparisons. These findings suggest that variance in ABR latencies cannot be attributed completely to variance in brain stem dimensions and raise suspicion that skull dimensions do not directly reflect brain stem dimensions.

The frequency-following response and the onset response: evaluation of frequency specificity using a forward-masking paradigm

The onset and frequency-following response (FFR) components of the auditory brain stem response were investigated using a tone on tone forward-masking paradigm, as an alternate strategy to simultaneous masking, in an effort to further clarify the issue of place specificity of these components when elicited by a low-frequency stimulus (500 Hz tone burst). The results of this preliminary investigation suggest that both the onset and FFR components yield frequency-specific information, that is, they reflect activity along a limited apicalward region of the cochlea, but both are biased somewhat basalward to the place of the probe frequency. Also, of the two, the FFR is the relatively more place-specific response.

Changes in BAEP under hypoglycemia: temperature-related?

Latencies of the brain-stem auditory evoked potentials were observed to increase in subjects whose plasma glucose levels were reduced. These changes appeared to be attributable to reduced body temperature, rather than direct effects of hypoglycemia on the auditory nerve or the brain-stem. The results suggest the need for caution in interpreting evoked potential measurements under hypoglycemia.

On the origin of wave II of the auditory brain stem evoked response

The generators of the several components of the auditory brain stem evoked response (ABR) have yet to be completely identified. However, there is compelling evidence to suggest specific generators for waves I and II, namely the distal and proximal portions of the auditory nerve, respectively. Although there is no question concerning the origin of wave I, there remains some uncertainty as to whether or not wave II arises entirely from the auditory nerve. The purpose of this study was to examine the behavior of wave II, as recorded via vertical versus horizontal derivations, in an effort to determine if wave II in both derivations is equally affected by stimulus manipulations (i.e., as would be presumed from a single generator theory). ABR recordings were obtained from normal-hearing young adults using vertical (hairline-to-ipsilateral mastoid and hairline-to-contralateral mastoid) and horizontal (mastoid-to-mastoid) derivations of responses to clicks presented at a variety of stimulus levels (40-80 dB nHL) and rates (19.9-59.9/sec.). The results indicate a shorter latency for wave II recorded in the horizontal derivation (wave IIa), compared to the vertical derivation (wave IIb). Also, wave IIa was found to be more susceptible to increased stimulus rate and exhibited a different latency behavior than wave IIb. These findings seem difficult to reconcile on the basis of a single generator for waves IIa and IIb.

Evaluation of three strategies for fitting hearing aids binaurally

Three strategies for evaluating optimum frequency shaping and noise reduction in binaural digital hearing aids were compared in a repeated-measures design, using a new preference-based prescriptive fitting method. These strategies consisted of using preferred frequency shaping and noise reduction values binaurally: (1) based on monaural testing; (2) based on separate evaluations of each ear; and (3) based on evaluation of a second ear while subjects wore an aid programmed with the preferred values in the first ear. Individually preferred characteristics were programmed for 17 hearing-impaired subjects, most of whom exhibited symmetrical sensorineural hearing loss. Each subject was administered intelligibility estimation and midplane localization measurements in the laboratory, as well as a questionnaire survey based on situational listening in the real world. No statistically significant differences in preferences for either frequency shaping or noise reduction were found for the three fitting strategies, suggesting that monaural testing is sufficient in symmetrical cases to provide information for binaural fitting. Related to this finding, differences across binaural conditions were minimal for both intelligibility estimation and localization results. A significant improvement in localization performance under binaural conditions over monaural listening, however, was documented by both the laboratory and the real world data. A strong overall preference for binaural over monaural amplification was also documented under real world conditions.

Analog model of human click-elicited SP and effects of high-pass filtering

Recent advances in electrode design have helped to rekindle clinical interests in noninvasive electrocochleography--to enhance auditory brain stem response (ABR) recordings (namely, wave I) and to screen Meniere's disease. The salient feature of the response in suspected cases of endolymphatic hydrops is an unusually large summating potential (SP). Since the SP is a DC potential, conventional wisdom suggests the use of relatively low cutoff frequencies (i.e., long-time constants) for recording, but this may degrade the recording due to the presence of excessive low-frequency noise. However, the click-elicited SP also is a transient of relatively short duration, considering the characteristics of the click. This may permit more liberal high-pass filtering with acceptable wave-form distortion. This is a report of findings obtained using an analog model for SP generation and recordings from normal subjects using an electrode placed directly on the tympanic membrane. Responses were obtained using various low-frequency cutoffs. Cutoffs up to 30 Hz caused little distortion. Even in the face of considerable distortion at the highest cutoffs (100 and 300 Hz), the SP remained detectable and the SP:AP essentially unchanged. Therefore, higher filter settings may be used in recording the SP than commonly assumed, although the decreased amplitudes of the component potentials may not be tolerable under all clinical test conditions.

Extratympanic electrode support via vented earmold

An adaptation of the Stypulkowski extratympanic electrode design is described. The electrode is supported by a stock earmold which has been vented with a large enough bore to just accommodate the tubing used in this electrode design. The electrode tubing and wire assembly is merely advanced through the vent and released upon contact with the tympanic membrane; further manipulation is not required to maintain contact. The earmold also serves to couple the second transducer to the ear canal. This approach offers ease of electrode placement and consistency of positioning and, thereby, simplifies the extratympanic method of (noninvasive) electrocochleography.

Vestibular recruitment in Meniere's disease

The aim of this study was to search for vestibular recruitment in a sample of patients with Meniere's disease. Recruitment was defined as an abnormal growth of response with increasing stimulus intensity. Twenty-nine patients were tested with sinusoidal rotation of three different magnitudes at four different frequencies. We also searched for auditory recruitment in each patient via tests of auditory brain stem responses, acoustic reflexes, and loudness balance and discomfort level. Analysis of vestibular responses indicated, on average, a linear relationship between stimulus magnitude and response magnitude, ie, doubling the stimulus magnitude resulted in twice the response magnitude. Meniere's patients did not yield results significantly different (although they were more variable) from those of the normal subjects. The vestibular responses of patients with auditory recruitment did not differ systematically from those of patients without auditory recruitment. We conclude that vestibular recruitment, if it exists in patients with Meniere's disease, is not demonstrated by sinusoidal rotational testing.

Hearing loss--risk factor for cisplatin ototoxicity? Observations

Ototoxicity associated with cisplatin chemotherapy is well established, but opinion is split regarding the relative risk associated with preexisting hearing loss. The emerging consensus is that pretreatment loss does not increase the risk of cisplatin ototoxicity. Results from a preliminary study of a small sample from our patient population supports this consensus.

Correlation study of two-channel recordings of the brain stem auditory evoked potential

Two-channel recordings of the brain stem auditory evoked potential (BAEP), for example, forehead to ipsilateral mastoid and forehead to contralateral mastoid, presumably provide "views" of the generators from different, nearly orthogonal, directions. The two channels thus are expected to register somewhat different waveforms, as revealed by past research. Still, the absolute latencies of the peaks are expected to be strongly correlated between channels. To test this notion, correlation analyses were made of the latencies of positive and negative peaks of normal-appearing BAEPs. The results demonstrated the expected high correlations for brain stem components but less tight coupling between peaks of waves of more peripheral origins. The possible influence of residual noise also was considered. Implications of the findings for clinical and multichannel recording applications are discussed.

Gender, head size, and ABRs examined in large clinical sample

Intergender differences in auditory brain stem response (ABR) latencies have been attributed to intergender differences in head size. We examined ABR data obtained clinically and head dimensions measured from mastoid to mastoid and mastoid to vertex to derive horizontal and vertical vectors which might scale lengths of horizontally and vertically oriented pathways, respectively. Correlations with ABR latencies were all somewhat weak, and slopes of regressions were found to be too small to fully account for intergender latency differences. Head-size matched male and female samples also demonstrated significant differences in wave V latency, and, even in cases with relatively large heads, the use of a head-size correction could not be justified. The coupling between head dimensions and the I-V interpeak interval appears to be small enough to be ignored for clinical purposes. Nevertheless, to the extent to which relationships exist between head dimensions and ABR latency measures, this nonpathologic variable may be completely neutralized through the use of interpeak latency ratios (e.g., wave V latency divided by wave I latency).

Emergence of the brain-stem auditory evoked potential in the premature lamb

Brain-stem auditory evoked potentials (BAEPs) were elicited by bone conducted stimuli in the very immature lamb following delivery, wherein liquid ventilation techniques were utilized to control cardiopulmonary and acid-base conditions, independent of umbilical-placenta support. Unlike previous studies of the in utero fetal lamb in which the BAEP could not be elicited by earphone delivered stimuli until 117 days gestation, our results demonstrate that the BAEP emerges at least as early as 106 days gestation in the lamb and consists of a full complement of readily discernible and reproducible wave forms. In addition, the results demonstrate substantial maturation of the BAEP from 106 to 122 days gestation during which time there is a significant decrease in absolute and interpeak latencies with an increase in developmental age. It is concluded that the ability to elicit the BAEP utilizing bone conduction at this early stage of gestation is related to improved stimuli delivery. Furthermore, this study demonstrates the feasibility and flexibility afforded by liquid ventilation and bone conduction BAEP techniques to study brain-stem function at particularly vulnerable stages of development.

Eighth nerve signs in a case of multiple sclerosis

We evaluated a 25-year-old woman with multiple sclerosis who presented with the acute onset of a profound unilateral high-frequency, sensorineural hearing loss that resolved clinically within 10 days. Click-elicited brain stem-evoked responses were abnormal at the time of presentation and demonstrated only limited recovery over a follow-up period of 11 months. Magnetic resonance imaging disclosed a lesion in the eighth nerve root-entry zone and the cochlear nucleus. Our findings in this case support the hypothesis of eighth nerve root-entry zone involvement in sudden hearing loss in multiple sclerosis and reinforce the notion that click-elicited brain stem-evoked responses are useful primarily to evaluate the high-frequency regions of the auditory system.

Development of an educational module for surgical training: a case study

This paper describes the design of software that will be used as part of an interactive video programme for the training of otolaryngologists. Although the construction of the programme is still in progress, some insight is given into the process by which the ideas were first developed and the production and evaluation of the prototype software. The procedure chosen was stapedectomy.

Masking level difference at relatively high masker levels: preliminary report

The binaural masking level difference (MLD) was measured at multiple masking levels (i.e. 500 Hz narrow band noise presented at 50-100 dB overall SPL) in normal hearing subjects and clinical subjects with varying types and degrees of hearing loss. MLD versus noise level (MLD-NL) functions then were derived for each subject. Some growth in the MLD-NL was seen in nearly all cases of hearing loss up to 70 dB in the poorer ear. These findings suggest that the MLD-NL function may provide the basis for extending the diagnostic usefulness of the MLD test in cases of substantive hearing loss.

Long-latency rotational evoked potentials in subjects with and without bilateral vestibular loss

Human subjects with and without profound bilateral vestibular loss were evaluated using repetitive rotational stimulation about an earth-vertical axis and signal averaging of vertex-recorded potentials. The stimulus events to which averaging was synchronized were acceleration/deceleration pulses produced by abrupt reversals in direction of angular velocity. In control subjects the long-latency rotational evoked potentials elicited via this paradigm were robust and reproducible. The responses of subjects with vestibular loss did not differ remarkably from those of the control subjects. We conclude that the long-latency rotational evoked potential elicited using a conventional rotary chair is not primarily of vestibular origin.

Pattern-reversal auditory evoked potential

The late auditory evoked potential (AEP) was studied in response to an alternatingly frequency-modulated complex tone. This 'pattern-reversal' AEP was found to be a heartier response than the more conventional tone-burst evoked potential, albeit longer in latency.

Psychophysical tuning functions for brief stimuli: preliminary report

Observations from clinical and research applications of brain stem evoked response audiometry and electrocochleography have suggested that tone bursts of only a few milliseconds' duration can elicit frequency specific responses. Because such brief stimuli are not perceived as being total, the extent to which the auditory system can "tune" to such stimuli was questioned. In order to investigate this question, psychophysical tuning functions were determined for brief tone bursts (i.e., 1. to 2 msec. duration). The results suggest that in working near normal thresholds (and therefore at relatively low sound pressure levels), brief tone bursts do yield rather frequency specific information. However, the range of sound pressure levels of the stimulus for which the tuning is resonably sharp is limited. the tuning functions show signs of deterioration, particularly in the "tip region," with only modest increases in the sound pressure level above threshold (i.e., typically with 20 dB.). Differences also may be seen between tuning functions for these very brief tone bursts, which in turn are characterized by a so-called click pitch, and those of stimuli long enough to have tonal pitch (i.e., 20 msec.). Finally, tuning functions also were examined for click stimulation, which also revealed some degree of tuning at relatively low levels of stimulation.

Computer identification of waves in the auditory brain stem evoked potentials

The high reliability with which the waves I through V can be identified in auditory brain stem potential (BAEP) recordings suggests the possibility of using the computer to identify the peaks. This paper examines the application of a smoothing algorithm in which detection of zero crossings in the derivative array of the averaged potentials is used to identify waves I--V in clinical BAEP recordings. A comparison of computer wave identification with human evaluation is presented. The results suggest that computer evaluation cannot entirely replace 'manual' evaluation of the waves, but can expedite analysis and alert the clinician to potentially abnormal responses.

Biasing of the summating potentials

Summating potentials (SP) were recorded in the guinea pig cochlea, utilizing the differential-electrode technique. The effects of biasing the cochlear partition of the SP were observed under three different conditions: 1. electrical biasing--direct current polarization: 2. stimulus biasing--mechanical bias induced by low frequency sound stimulation, 3. COCB biasing--electrical stimulation of the crossed olivo-cochlear bundle. All three forms of biasing were found to have similar ellects on the SP which can be described in terms of the "absolute direction of change" in the potential. Under negative current (scala vestibuli negative; scala tympani positive), the negative phase of the stimulus bias (deflection of the cochlear partition towards scala vestibuli), or stimulation of the COCB, the SP is made more positive or less negative, as the case may be. In other words, the negative component of the differentially recorded SP is depressed, wheras the positive component is enhanced. The dominant influence os all three forms of biasing appears to be on mechanoelectric distortion arising in the hair-cell transduction process.

Remote extracochlear versus intracochlear recordings in the guinea pig

A comparison was made of the whole-nerve action potential recorded from a wire electrode placed on the tympanic membrane (TM) (remote extracochlear recording) and than recorded from intracochlear electrodes in the first and third turn scala tympani of the guinea pig. Cochlear microphonics and summating potentials were also studied. The expected decreased sensitivity at the TM site was observed in all responses, although the microphonic (CM) was diminished more that the action potential (AP) as compared to the responses monitored via the ST1 electrode. Nevertheless, the TM-recorded cochlear potentials largely mimicked the ST1-recorded potentials in a qualitative manner, i.e., similar configurations of the CM pseudo-threshold functions. It was consistently observed that the N2 of the AP is proportionally larger (relative to N1) in the TM recording than in the ST1 recording. These phenomena were attributed to the complex spread of the bioelectrical potentials in the nonhomogeneous volume conductor formed by the tissues of the temporal bone. It was suggested that the observed change in the N1/N2 ratio at the different sites of recording is due to the different contributions of second-order neuronal discharges to the compound AP. In effect, the TM electrode "sees" proportionally more activity from second-order fibers than does the ST1 electrode, and the ST3 electrode, less.

Cochlear inner and outer hair cells: functional differences

The cochlear microphonic response was measured with differential electrodes from the first and third cochlear turns of normal guinea pigs and those treated with the ototoxic drug kanamycin. Histological controls showed that the outer hair cells in treated animals were missing over the basal half of the damaged cochleas, while the inner hair cells were intact. Measurements are consistent with the hypothesis that the potentials produced by inner hair cells are proportional to the velocity of the basilar membrane, whereas potentials generated by outer hair cells (which dominate the response of normal cochleas) are proportional displacement of the basilar membrane.