Comparison of the intrasubject repeatability of auditory brain stem and middle latency responses elicited in young children.

The auditory brain stem response (ABR) and middle latency response (MLR) were studied in 48 young children (96 ears). The responses were elicited using low intensity stimuli (30-dB nHL clicks) and simultaneously were recorded on a dual time base. Both the ABR and MLR were elicited in 70 ears. In 12 ears, just one response was recorded (ABR in eight ears and the MLR in four ears). In 14 ears, neither response was recorded. Test-retest analysis on the same subject demonstrated that the ABR was more repeatable and easier to identify than the MLR. The test-retest difference was determined for the amplitude and latency of the ABR and MLR waveforms. The test-retest latency difference for wave Pa was found to be 3.6 times larger than for wave V. The normalized test-retest amplitude difference for P phi-Na, Na-Pa, and Pa-Nb was found to be two to three times larger than for wave V. These data support the conclusion that the ABR, rather than the MLR, should be used to measure hearing in young children. The authors also advocate using minimal high pass (HP) filtering when recording the ABR in a sedated or sleeping child. Muscle artifact was not found to be a problem. The authors suggest the use of minimal HP filtering so that phase-shift distortion is minimized and a larger response amplitude can be recorded.

Digital filtering and spectral analysis of the low intensity ABR.

Spectral analysis along with zero and standard-phase shift digital filtering were performed on evoked potentials recorded from 12 normal hearing subjects. The results indicated a progressive shifting of the mean spectral content of the ABR toward the low frequencies as the stimulus intensity was lowered. Despite this, the effects of zero-phase shift high-pass digital filtering at 100 Hz (36 dB/oct) did not significantly differ between waveforms elicited by a 75 dB nHL, 55 dB nHL, and 35 dB nHL stimulus. The major response frequency of the ABR is related to the distance between the peak (IV/V) and the following major trough (approximates one-half the response period). In waveforms where the major trough occurred before 10 msec, the use of 100 Hz, 36 dB/oct, zero-phase shift high-pass filters produced only a small reduction in response amplitude, even at low stimulus intensity levels. Waveforms which had a major trough (Na1) between 10 to 15 msec were reduced in amplitude by 100 Hz, 36 dB/oct, zero-phase shift high-pass filters (the longer period of the response energy in these waveforms corresponds to a lower energy frequency). However, this trough has a latency that prevents it from being recorded on a 10 msec time base or defined as an ABR. Based on these results, the use of zero-phase shift high-pass filters with a high-pass cutoff frequency that is equal to or less than the resolution of the time base (1/time base) appears to be a desirable method of reducing muscle artifact and other electrical contamination of the ABR.(ABSTRACT TRUNCATED AT 250 WORDS)

High pass digital and analog filtering of the middle latency response.

Digital filtration with zero and standard-phase shift characteristics was performed on unfiltered auditory evoked potentials recorded from 10 adult subjects. Standard-phase shift filters were seen to distort the response. Mild phase shift distortion often augmented the IV/V-Na1 amplitude (auditory brain stem response). When recording the IV/V-Na1 amplitude on a narrow timebase (20 msec), we recommend using a phase shift filter that has a high-pass cutoff frequency approximating 15 Hz and a slope of 12 to 24 dB/octave, in order to take advantage of the observed phase shift augmentation. The augmentation of the IV/V-Na1 amplitude is at the expense of the IV/V-Na2 amplitude. Thus, this phase shift distortion is not desirable if a long timebase (50 msec) is used. If a high-pass standard-phase shift filter of 100 Hz is used to eliminate muscle artifact, a reduction in the IV/V-Na1 amplitude from phase shift distortion is seen compared to the zero-phase shift control. In site-of-lesion testing, a nondistorted waveform produced by digital zero-phase shift filtration is also desirable over a distorted analog response. It is recommended that the slow wave activity of the response be recorded with a timebase of at least 40 to 50 msec and with a filter that produces minimal or no phase shifting. The major recorded response will be the Na trough and Pa peak. Evoked response units with steep filters will phase shift the response and produce a reduction in the IV/V-Na amplitude and a concomitant augmentation of the latter middle latency response waveforms.(ABSTRACT TRUNCATED AT 250 WORDS)

Analog and zero phase-shift digital filtering of the auditory brain stem response waveform.

Auditory brain stem response waveforms, derived from normal ears, were analyzed with a computer-based digital filtering program as a means of systematically evaluating the effects of phase-shift distortion. Off-line analysis involved the stimulated responses of a Butterworth-type filter at five high-pass and four low-pass frequencies, three filter slopes, and standard versus zero phase-shifting. Phase-shift, as seen with analog filters, proved to be a major cause of waveform distortion including significant latency and amplitude changes. A discussion on the need and use of digital filtering as well as suggested analog filter parameter settings is included.

High-pass digital filtration of the 40 Hz response and its relationship to the spectral content of the middle latency and 40 Hz responses.

This study analyzed the effects of high-pass filtration on the 40 Hz response using zero and standard phase shift digital filtration. Filtration effects were compared to the spectral content of the middle latency response (MLR) and 40 Hz response. The spectral composition of the MLR was found to contain major response energy at 10 and 40 Hz. The major energy of the 40 Hz response was at 40 Hz with minor peaks every 40 Hz above the fundamental frequency. The elimination of the 10 Hz response energy in the 40 Hz response was due to cancellation caused by phase differences which occurred in the production of a steady state potential. The reduction was confirmed by the minor amplitude effects created by zero-phase-shift high-phase filtration (cutoff frequency 30 Hz with 36 dB/oct slopes) on the 40 Hz response as compared to the MLR. The only beneficial effect of high-pass filtration was the elimination of the mild undulation of the unfiltered waveform. The major effect of phase-shift high-pass filtration on the 40 Hz response was the displacement of the high frequency ABR from the peak to the descending slope and finally to the trough of the MLR. Shifting of the ABR from the peak of the MLR resulted in amplitude loss which may affect response identification at threshold. The authors recommend minimal high-pass filtration (i.e., less than 12 dB/oct slope and a cutoff frequency less than or equal to 15 Hz) when recording the 40 Hz response. The spectral analysis of the evoked response may be able to predict the optimal rate for eliciting a steady state response in divergent subject populations.

Bekesy-tracked aural harmonic distortion thresholds and uncomfortable loudness levels.

Sweep-frequency Bekesy audiometry was used to determine the uncomfortable loudness and threshold levels for the perception of an aurally generated harmonic distortion phenomenon in normal and sensorineural hearing impaired listeners. A continuous-interrupted tone crossover uncomfortable loudness level tracking pattern is described which appears to vary with the tracked aural harmonic distortion levels. Hearing impairment was seen to elevate the intensity required to achieve overload and distortion while reducing the range from audiometric threshold to the onset of aural harmonic distortion. Consideration of these findings to suprathreshold audiometric evaluation is discussed.

Suprathreshold Bekesy excursion widths in normal and sensorineural impaired listeners.

Normal-hearing (N:20) and sensorineural adults (N:15) performed sweep-frequency Bekesy audiometry from .1-8 kc/s at threshold and at MCL and UCL with both continuous (C) and interrupted (I) tones. Excursion width was measured in 1-db steps. Normals exhibited no C-I differences in excursion width at any level, and differences between the two groups of Ss were negligible for I tones, but the sensorineural Ss yielded significantly smaller widths at 3, 4 and 6 kc/s for C tones at both suprathreshold levels. SInce a number of sensorineural Ss did not yield tracings at UCL, it was recommended that MCL be traced as one feature of site-of-lesion testing. Of the sensorineurals, 75% yielded widths of 5 db or less for C tones at MCL, and this criterion in a sweep-frequency Bekesy is suggested as of clinical value.