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This dissertation presents the results of a series of experiments designed to expand our understanding of the role that auditory cortex plays in threshold-level frequency discrimination. The unifying hypothesis of this dissertation is that auditory cortex is involved in discrimination of fine-grained frequency differences. Recording neural responses to auditory stimuli during performance of a difficult frequency discrimination task opens the door to investigating possible neural mechanisms which may encode frequency change detection. Additionally, simultaneous recording of neural and psychophysical data allows for insight into the rats\' perception of or lack of perception of a frequency change (as inferred from the behavioral response to the stimuli).

Small differences in the frequencies of a sound provide a significant amount of information about the identity, position, and motion of the source. Humans' auditory frequency discrimination ability also allows us to understand the subtle modulations that comprise spoken language. It should not be surprising, then, that deficits in auditory frequency discrimination ability have been often implicated in various language learning disorders. Understanding the neural basis for such deficits is difficult because the processes underlying auditory frequency discrimination haven't been fully identified. Recent evidence suggests that primary auditory cortex may be the ultimate arbiter of behavioral frequency resolution, but the frequency- and intensity-dependent properties of phasic-type responses most often seen in primary auditory cortex neurons do not correspond well with psychophysical trends. Tonic-type response components that occur after the phasic component in primary auditory cortex neurons may display frequency-selectivity more consistent with observed psychophysics, but are not yet well characterize in most animals. Studying the contribution of tonic responses to frequency discrimination behavior requires an approach that can examine both neural activity in primary auditory cortex and psychophysics in the same animals.

The Federal Highway Administration (FHWA) currently requires that all persons seeking a commercial driver's license for interstate commerce possess a certain minimal level of hearing. After an extensive literature review on topics related to hearing and driving, a human factors engineering approach was used to evaluate the appropriateness of this hearing requirement, the methods currently specified to test drivers' hearing, and the appropriate hearing levels required. Task analysis, audiometry, dosimetry, in-cab noise measurements, and analytical prediction of both speech intelligibility and masked thresholds were all used in performing the evaluation. One of the methods currently used to test truck driver hearing, the forced-whisper test, was also evaluated in a laboratory experiment in order to compare its effectiveness to that of standard pure-tone audiometry. Results indicated that there are truck driving tasks which require the use of hearing, that truck drivers may be suffering permanent hearing loss as a result of driving, that team drivers may be approaching a 100% OSHA noise dose over 24 hours, and that truck-cab noise severely compromises the intelligibility of live and CB speech, as well as the audibility of most internal and external warning signals. The forced whisper experiment demonstrated that there is significant variability in the sound pressure level of whispers produced using this technique (in the words, word types, and trials main effects). The test was found to be repeatable for a group of listeners with good hearing, but was found to have only a weak relationship to the results of pure-tone audiometry for a group of 21 subjects with hearing levels ranging from good to very poor. Several truck cab and warning signal design changes, as well as regulatory changes, were recommended based on the overall results of this evaluation.

Effectiveness of impedance audiometry in diagnosing serous otitis media in children was examined in this study. The impedance test battery was performed on seventy-six ears of pre-myringotomy children (Mean age: 4.6 years). The status of the middle ear was assessed by the operating physician during surgery, and impedance results were correlated with the operative findings. Middle ear effusion was the variable that most influenced impedance results. Acoustic reflex threshold and tympanometry were the most sensitive tests in predicting effusion. Multiple correlations between these tests and the presence of significant effusion indicated that both measures together are diagnostically more sensitive than either test alone. Also, reflex measurement at one test frequency is as accurate a predictor of effusion as reflex measurement at all frequencies.

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The ability to understand speech is, for all practical purposes, the most important aspect of hearing.¹ The most common method of assessing hearing is through pure tone audiometry. However, the great majority of pre- school children referred to an otologist or a hearing clinic for a suspected hearing difficulty, are unable to cooperate sufficiently to insure a reliable pure-tone audiometric test. Even after pure tone results have been established with children in this age group, the tester frequently may be in doubt as to the validity of the results. The reasons for failure to get reliable results with young children appear to be these:

Current methods of selection of amplification are not readily applicable to children, and relatively few alternatives have been explored. Consequently, the audiologist must make a number of rather arbitrary judgements with little concrete information to support his recommendation of a given instrument. He has limited means of clinically determining the sustainability of the instrument for the child. This study was designed to explore the relationship between several factors which may add to the existing body of data related to the problems of amplification in children. In brief, the problem was to determine the various electrical and acoustical characteristics of hearing aids worn by children under study. A comparison of this information with hearing loss data would be studied to aid in understanding what relationships (if any) exist between the two measures. If in fact there is a relationship between the factors of hearing loss and hearing aid characteristics, this information may provide a more comprehensive and precise means of examining a given hearing aid in view of the hearing loss specific to each child. While this information may be preliminary in nature, it has application in the development of ongoing programs of evaluation and maintenance of hearing aids. A brief comparison of the present performance of the instrument with both the original performance levels and the child’s current needs would appear to be an integral part of such a program.