Aaron Thornton

Low‐frequency hearing loss: Perception of filtered speech, psychophysical tuning curves, and masking

Four subjects with low-frequency hearing loss were evaluated to determine whether their responses to low-frequency stimulation might be the result of stimulation of nerve fibers with higher characteristic frequencies. Two masking paradigms were employed to indirectly investigate the contribution of high-frequency nerve fibers to the detection of low-frequency stimuli: (1) masking of a low-level, fixed-frequency probe by a variable-frequency pure-tone masker (psychophysical tuning curve) and (2) masking of pure tones by a high-level, fixed-frequency pure-tone masker. Low-frequency remote masking by tones and displacements in the tips of tuning curves to higher frequencies were interpreted as evidence that low-frequency signals near threshold were being detected by high-frequency fibers in three of the subjects. Three subjects were also tested with high-pass, low-pass, and unfiltered speech both in quiet and in the presence of a high-pass noise masker. Results were interpreted as showing a relatively small contribution of high-frequency fibers to the perception of low-frequency speech.

The choice of stimuli for ABR measurements.

This tutorial describes some of the stimulus choices available to clinicians performing ABR evaluations. The motivation for choosing particular stimuli is described in relation to the clinical question being asked. Thus, emphasis is directed toward the rationales for choosing different stimuli. Data are reviewed only briefly to illustrate these rationales. Finally, some effort has been made to relate clinical ABR results for certain stimulus conditions to underlying physiological processes in the hope that these discussions will lead to a more complete understanding of the stimulus-response relation.

Ultrastructural Findings in a Case of Meniere's Disease

The temporal bones of an individual with documented unilateral Menieres disease were prepared for light and electron microscopy. a morphometric analysis was performed on hair cells, spiral ganglion cells, dendritic fibers in the osseous spiral lamina, afferent and efferent endings, and afferent synaptic contacts. In the ear with Menieres disease, we found hair cell damage, including disruption of the cuticular bodies and basalward displacement of some outer hair cells. There was no significant difference in the number of hair cells or spiral ganglion cells on the two sides. There was a significant decrease, however, in the number of afferent nerve endings and afferent synapses at the base of both inner and outer hair cells in the ear with Menieres disease as compared to the contralateral ear.

Evaluating intelligibility of speech reproduction and transmission across multiple listening conditions

A method of calculating a single number summarizing the performance of a device for transmitting, amplifying, or reproducing acoustic speech signals. The number can be used for evaluation and comparison of characteristics of devices for conveying speech, for instance to choose a hearing aid prescription. In the method, for each device of a plurality of acoustic devices, intelligibility measurements are obtained for speech signals transmitted from or reproduced by the device under multiple of listening conditions, and a weighted sum of the devices intelligibility measurements is formed. From among the plurality of devices, the one device best overall suited to the plurality of listening conditions is chosen by comparing the weighted sums and selecting the device with the largest corresponding weighted sum. The devices may be computer models of real acoustic devices; a plurality of the models are iteratively generated and the weighted sums corresponding to the computer models are evaluated, and modelled acoustic properties of successive ones of the computer models are altered to increase the weighted sum.

Low-frequency sensorineural loss: clinical evaluation and implications for hearing aid fitting

Spread of excitation in the cochlea places fundamental limits on the interpretation of audiometric pure-tone hearing loss as a simple map of dysfunction along the cochlear partition, and histologic evidence from human temporal bones will be presented to demonstrate the insensitivity of the audiogram to variations in pathology in the case of low-frequency hearing loss. This article will describe a clinical procedure using simultaneous pure-tone masking to improve upon the localization of cochlear disease, particularly for low-frequency hearing losses, and a model for using the Articulation Index (AI) to develop prognoses for hearing aid performance in these cases, which can then be tested. Fourteen patients with low-frequency hearing loss were divided into two groups based upon threshold shifts caused by a pure-tone masker: those that showed normal low-frequency threshold shifts and those that showed marked shifts at frequencies below the masker, indicating greater loss of function than shown by the unmasked audiogram. Hypothetical audiograms were then generated to model a complete loss of apical function for all patients. Measured speech recognition scores were then compared to AI predictions for the actual and hypothetical audiograms. Best agreement for the patients showing normal masking shifts was between the measured scores and the AI for the actual audiogram, whereas the best agreement for the patients showing marked shifts was with the AI for the hypothetical audiogram. The implications for hearing aid recommendation and fitting in these cases are discussed.

Failure to Clinically Predict NICU Hearing Loss

Neonatal intensive care unit (NICU) survivors demonstrate handicapping sensorineural hearing loss up to 50 times more frequently than normal newborns, yet little is known about the etiology of the hearing loss. Theoretically, accurate identification and triage of a particular infant based on a clinical profile would be useful. Forty NICU graduates of The Massachusetts General Hospital were selected for a detailed retrospective chart review evaluating prenatal, perinatal, and NICU medical conditions and treatment. Twenty-three patients identified with hearing loss and 17 infants with normal hearing were compared clinically. Univariate and multivariate analysis was performed on a subpopulation of patients (20 with hearing loss and 16 with normal hearing). A history of ventilation was associated with hearing loss (P=.0023), but this factor was not absolute. No other clinical parameters were convincingly linked to hearing loss. We conclude that reliance on risk factors is an inadequate clinical method to select a patient for a hearing test and that each NICU survivor deserves audiometric evaluation.

The articulation index in clinical diagnosis and hearing aid fitting

In cases of sensorineural hearing loss, speech intelligibility scores reflect a combination of the independent effects of sensitivity and information processing. The separate analysis of these two effects reveals a great deal that is useful in audio-logic diagnosis and hearing aid fitting. The theoretical ability to estimate all sensitivity effects using the articulation index has existed since the 1940s, but the mathematical complexity of calculating a normal reference for each unique audiogram has not been feasible until the recent application of computer technology. This review traces the development, validation, and emerging clinical use of the articulation index.

Magnetic stimulation of the facial nerve.

Intracranial activation of the facial nerve is now possible with the noninvasive techniques of magnetic stimulation. Brief magnetic pulses generated by a coil overlying the parietal scalp elicit compound muscle action potentials of similar shape and amplitude and greater latency than those produced by electroneurography. Mapping studies demonstrate the compound muscle action potentials to be of constant latency and varying amplitude with changing coil location. Maximum compound muscle action potential amplitudes are obtained with the coil center located in a rectangular area superior and posterior to the ear canal. A comparison of large and small diameter coils showed them to be equally effective for painless facial nerve stimulation; however, the smaller coil allowed for a more localized field of activation. Magnetic stimulation has the potential to provide cross‐the‐lesion testing of facial nerve function.

A New Design for Intraoperative Facial Nerve Monitoring

Electrom yograph y of the facial muscles has proved to be an effective method of intraoperative facial nerve monitoring. With surface electrodes on the patient’s face, the surgeon is immediately made aware of any facial movement through a loudspeaker system that is driven by an amplifier connected to the electrodes. We have assisted in the development of a facial nerve monitor that is simplified in its operation, while at the same time is sophisticated in its signal processing. The device integrates an amplifier, loudspeaker, display screen, and nerve stimulator into a single compact unit with a rechargeable battery. A minimum number of controls eliminates the need for highly trained personnel to operate the device. All incoming signals from the facial electrodes are analyzed so that only those that have characteristics of facial muscle action potentials will alarm the loudspeaker. This feature eliminates a common, annoying problem with EMG monitoring in the operating room-namely, background electrical noises, including the electrocautery and stimulus artifact that falsely trigger the loudspeaker. Activity is continuously monitored on a liquid crystal screen that can also display frozen images of individual muscle action potentials. To assist the surgeon with identification of the facial nerve, a synchronized, constant current stimulus of up to 3 milliamps (mA) can be delivered to either a monopolar or bipolar nervestimulating electrode. We have used this monitor during otologic surgery and other procedures performed in the

Standardized format for depicting hearing preservation results in the management of acoustic neuroma

The Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery recently published guidelines for reporting hearing preservation in the treatment of acoustic neuromas. These suggestions included pretreatment and posttreatment pure-tone hearing thresholds, word recognition scores, and hearing classification. We present a standardized reporting format that addresses the Committees recommendations and displays individual patient audiologic data as a simple, concise plot of posttreatment hearing results. To illustrate the use of the recommended format, preoperative and postoperative hearing data from our institution are reported. Such reporting criteria will facilitate comparative reviews of studies of hearing preservation after surgical or radiotherapeutic management of acoustic neuromas, while providing specific data for individual patient outcome analysis.