Donald E. Morgan

Auditory Brain Stem Evoked Response Characteristics in Developing Infants

The auditory brain stem response (ABR) of a single group of developing normal infants was examined longitudinally, from newborn through 6 months of age. A sufficiently broad range of stimulus variables was included to ensure that the auditory system was adequately sampled in order to demonstrate developmental principles. Findings indicate that there are no differences in wave V latency-intensity functions between infants and adults. For waves I, III, and V, absolute and interwave latency–repetition rate functions differ between infants and adults and undergo systematic changes throughout the first 6 months of life. The most dramatic ABR changes (between any two sequential test sessions in infants) occurred between the ages of newborn and 2 weeks, with less pronounced ABR changes beyond 2 weeks of age. The pattern of latency change for wave I was different from that for waves III and V. After 2 weeks of age, wave I latency was the same as the adult value at all repetition rates. In contrast, waves III and V were characterized by decreasing latency throughout the follow-up period. A curvilinear developmental model provided a satisfactory fit to ABR latency data.

Loudness discomfort level: selected methods and stimuli

Two investigations were conducted (i) to determine a reliable procedure for obtaining loudness discomfort levels (LDL) and (ii) to observe the effects of frequency on the LDL. Three psychophysical methods were compared in the measurement of the LDL—a programmed method of constant stimuli, the method of adjustment, and a tracking method. The method of constants provided the most reliable judgments over six trials for a 1000‐Hz stimulus. The LDL was then measured utilizing the method of constants for pure tones at octave frequencies from 125 to 4000 Hz, and for a wide and narrow band of noise. The LDLs (in dB SPL) were highest at low frequencies (through 500 Hz) and relatively constant for 1000, 2000, 4000 Hz, and the wide‐band noise. The relatively high LDLs in the low frequencies are explained on the basis of difference in the intensity level for low frequencies measured in a conventional 6 cm3 coupler and that observed in the ear canal when a supra‐aural earphone is fitted on the head. When a correction is made for this difference the LDL results across frequency compared closely with results from loudness‐contour studies.

Auditory Brain Stem Evoked Response Characteristics in the Full-Term Newborn Infant

The auditory brain stem response to click stimuli was investigated in a group of 50 full-term healthy newborns, as well as in a group of 20 older children and adults. The stimulus parameters of click level and click repetition rate were varied systematically to quantify the characteristics of the auditory brain stem response in the full-term newborn infant. The results reveal increased latencies for waves I, III, and V for all conditions among the newborns, relative to the older age group. The results suggest that the neurological system is the primary source of differences between newborns and older subjects, but do not rule out the possibility that external ear, middle ear, or cochlear mechanisms may also contribute to the differences observed.

Influence of middle−ear muscle contraction on pure−tone suprathreshold loudness judgments

Some effects of middle−ear muscle (MEM) contraction on TTS and suprathreshold loudness judgments have been assessed in three experiments. Experiment I was designed to demonstrate the effect of MEM contraction on TTS while in Experiments II and III the influence of MEM contraction on the loudness of suprathreshold pure−tone stimuli was measured. In Experiment I, MEM contraction decreased the effective level of a high−intensity TTS−inducing stimulus reaching the cochlea. In Experiments II and III signal paradigms were chosen to reduce or eliminate the effects of binaural loudness summation, and contralateral remote and backward masking effects, and to maximize the influence of MEM contraction. Results of the loudness judgments indicated no effect of binaural interaction, whereas the MEM contraction affected the loudness of pure tones only when the stimuli were low frequencye (250, 500 Hz) and high intensity (≳100 dB SPL). The results further imply that at high intensity levels there was an interaction betwe...

Temporal integration at the ’’threshold’’ of the acoustic reflex

Investigation of the temporal integration of the acoustic reflex may be complicated by the operating characteristics of the instrumentation used to record the reflex. Several experiments were conducted to determine the operating characteristics of two impedance measuring systems (Madsen, model Z0−70 and Zwislocki Bridge (Grason Stadler, model 3), and to measure selected characteristics of the acoustic reflex, including (1) ’’threshold’’, (2) reflex intensity‐growth functions, and (3) temporal integration functions at ’’threshold’’ for several stimulus frequencies. Results indicate (1) a smaller impedance change may be identified with a Madsen Z0−70 due primarily to the higher S/N (signal‐to‐noise) ratio in that instrument; (2) differences in temporal integration functions obtained between the two instruments were partially accounted for by differences in the operating characteristics of the two measuring systems; (3) the slope of the growth‐intensity function decreases as signal duration decreases; and (4...

Hearing Impairment among Orchestral Musicians

It would seem anomalous that an orchestral musician would have less than normal hearing in order to monitor an exacting musical output. However, some recent studies show that a proportion of symphony musicians do have hearing impairments of various pathologies of which noise exposure, including the music alone, is the dominant causal factor. Intense music exposures in symphony orchestras often exceed the intensity standards of hearing conversation. A basic procedure for industrial hearing conservation is the control of the sound at the source, but the very purpose of the orchestra prevents its use. Hearing protection which is used voluntarily by some musicians presents a different sound picture. The control of intensity of music exposures is necessary but is separate from the concern of the present study with industrial issues and the perceptual-motor performance of musicians. Relevant aspects which are briefly reviewed include the incidence, susceptibility, and severity of hearing impairment among musicians; musical performance of the hearing impaired and the effects of various pathologies on their performance; medical-legal rules of impairment, disability, and handicap; and the incidence of compensible losses among musicians of certain orchestras. Illustrative results are presented of a preliminary study of hearing among 13 volunteer members of The Los Angeles Philharmonic Orchestra. It is suggested that a three- phase study be done: (1) a comparative study of hearing among symphony orchestras; (2) development of comprehensive tests to determine hearing-related performance; (3) provision of a rational basis for hearing criteria in the case of musicians for dealing with their employment, transfer, retirement, disability, handicap, and award of compensation. A fourth issue requiring concurrent study is the conserving of the hearing of orchestral musicians.

Evaluation of the Speech Perception in Noise (Spin) Test

Several investigations were performed with normal hearing subjects to determine the effects of presentation level and signal-to-babble ratio on the speech perception in noise (SPIN) test. The SPIN test contains sentences that simulate a range of contextual situations encountered in everyday speech communication. Findings from several representative patients with sensorineural hearing loss demonstrate the possible clinical utility of the test to measure the effects of context on speech discrimination.

Loudness discomfort level under earphone and in the free field: the effects of calibration methods

Previous earphone LDL data from this laboratory were characterized by an intensity level increase in the low frequencies which we suggested resulted from the difference between the SPL measured in a standard coupler and that actually generated in the ear under a supra‐aural earphone. The experiments reported herein were conducted to determine if the frequency effect could be explained adequately on the basis of differences between conventional and real‐ear calibration methods and to extend the LDL measurement to free‐field conditions. Determination of the SPL generated by an earphone and by a loudspeaker in the free field was accomplished with a probe tube at the entrance to the ear canal under earphone and free‐field conditions and by conventional calibration methods. Psychophysical measurements of loudness discomfort (LDL) and loudness balance (ABLB) were conducted under earphone and in the free field. Corrections derived from differences between standard calibration procedures and probe tube measuremen...

Nonacoustic Stimulation of the Middle Ear Muscle Reflex

The purpose of these experiments was to determine the incidence of the middle ear reflex in response to several nonacoustic (tactile and air jet) stimuli among subjects with normal hearing who had an acoustic reflex and selected patients with severe hearing loss. The results demonstrate that the incidence of response to tactile stimulation increases as the facial area stimulated approaches the auricle. The response to an air jet stimulus directed toward the eye is high; however, the clinical utility of the air jet may be limited because it often results in a startle reaction and head movement, and the response appears to fatigue easily. In normal listeners the response to auricular air jet stimulation probably results from both acoustic and tactile stimulation. The presence of a reflex to tactile stimulation, together with normal tympanometry constitutes strong evidence of a normal middle ear; but the absence of a reflex to acoustic or tactile stimuli still leads to an ambiguous determination of potential stapedial muscle function.

Perstimulatory loudness adaptation

An experiment is described in which perstimulatory loudness adaptation was measured under heterophonic conditions using simultaneous dichotic loudness balance procedures on normal listeners. The adapting signals were 500 and 3000 Hz presented at 70 dB SPL for 12 min, the last 5 min of which involved perstimulatory loudness balances. A tracking technique and the method of constant stimuli were employed. The results indicated that continuous stimulation did not significantly decrease the loudness of the adapting signal. Thus, under test conditions that reduce or eliminate binaural interaction, suprathreshold loudness adaptation is essentially absent for pure‐tone signals of 500 and 3000 Hz at 70 dB SPL.