No abstract

Auditory brainstem responses (ABR) were obtained from graduates of an intensive care nursery (ICN) when those babies were in stable physiological states and ready for hospital discharge. Intensity ranged from ABR threshold to 80 dB nHL, and all recordings were made in a sound-isolated chamber. The data reviewed here are from 585 babies having presumably normal hearing, based upon bilateral ABR thresholds of 30 dB nHL or less. To insure that estimates of population statistics were not biased by high correlations between ears, only the data from the left ears were used in most analyses. Larger correlations were observed between conceptional age (CA) and ABR latencies than between either gestational age (GA) or chronological age (CHA) and the same latencies. Data were grouped into six CA groups for further analyses. Distributions of all response-component latencies were similar in shape and depended upon CA, showing orderly decreases in latency with increasing age. None of these distributions differed significantly from normal, and they were well fitted by normal ogives. Thus, accurate estimates of percentiles can be obtained from the means and standard deviations. The results indicate that it is important to take CA into account when evaluating ABR latencies.

ABR and behavioral thresholds were estimated as a function of stimulus duration for three normal and two hearing-impaired subjects. Stimuli were 2000-Hz tone bursts with 0.5-ms rise/ fall times and durations ranging from 1 to 256 or 512 ms. For both groups of subjects, ABR thresholds were independent of stimulus duration. Normal subjects showed greater improvement in behavioral thresholds as a function of duration than did subjects with hearing losses. Thus, it appeared that ABR and behavioral thresholds were affected differently by changes in stimulus duration and that the magnitude of these differences could depend upon the presence of hearing loss. These data indicate that temporal integration may be one factor which makes comparisons between ABR and behavioral thresholds complicated. In the present study, the magnitude of hearing loss, measured by the ABR, would have been underestimated if normal behavioral thresholds for short-duration stimuli were used as the reference.

This paper describes preliminary data on the use of click-evoked ABRs in the hearing aid selection process. Four normal-hearing and 4 hearing-impaired subjects were tested with a hearing aid set at three different frequency response settings. Estimates of gain were calculated using shifts in Wave V thresholds, shifts in Wave V latency-level functions, acoustic-reflex measurements, coupler gain measurements, and measurements of functional gain. Results suggest that the click-evoked ABR does not distinguish between differing amounts of low-frequency gain, although reasonable estimates of high-frequency gain appear possible. Also discussed are technical factors that must be considered when using the ABR in the hearing aid evaluation process.