The Parallel Auditory Brainstem Response

Abstract

The frequency-specific tone-evoked auditory brainstem response (ABR) is an indispensable tool in both the audiology clinic and research laboratory. Most frequently, the toneburst ABR is used to estimate hearing thresholds in infants, toddlers, and other patients for whom behavioral testing is not feasible. Therefore, results of the ABR exam form the basis for decisions regarding interventions and hearing habilitation with implications extending far into the child's future. Currently, responses are elicited by periodic sequences of toneburst stimuli presented serially to one ear at a time, which take a long time to measure multiple frequencies and intensities, and provide incomplete information if the infant wakes up early. Here, we describe a new method, the parallel ABR (pABR), which uses randomly timed toneburst stimuli to simultaneously acquire ABR waveforms to five frequencies in both ears. Here, we describe the pABR and quantify its effectiveness in addressing the greatest drawback of current methods: test duration. We show that in adults with normal hearing the pABR yields high-quality waveforms over a range of intensities, with similar morphology to the standard ABR in a fraction of the recording time. Furthermore, longer latencies and smaller amplitudes for low frequencies at a high intensity evoked by the pABR versus serial ABR suggest that responses may have better place specificity due to the masking provided by the other simultaneous toneburst sequences. Thus, the pABR has substantial potential for facilitating faster accumulation of more diagnostic information that is important for timely identification and treatment of hearing loss.

Keywords: assessment; auditory brainstem response; electroencephalography; evoked potentials; objective audiometry.

Figure 1.

pABR stimulus construction. (a) Individual toneburst stimuli for each frequency. (b) toneburst trains in each ear (colored lines) are summed to create a two-channel (left, right) stimulus epoch (black lines).

Figure 2.

The distribution of interstimulus intervals over all stimuli for λ = 40 stimuli / s (solid gray) compared with the predicted distribution given by P(t) = 40 e^−40t. There is a very close match indicating that the deviations from a true Poisson point process used in this experiment are negligible.

Figure 3.

The analysis chain, shown from stimulus creation and presentation to calculation of response waveforms. For clarity, only a 50 ms time period is shown. Dashed box: Zero-padding scheme shown for a single impulse train of a single epoch. Note that t_pre and t_post are not shown to scale. EEG = electroencephalography.

Figure 4.

Intensity series waveforms across frequencies and for the left and right ears. (a) Grand average of eight subjects. (b and c) Two example subjects’ responses. All responses are plotted over the interval 0 to 25 ms (see Supplementary Figure 1 for all subjects’ responses).

Figure 5.

Mean Wave V latency (a) and amplitude (b) as a function of intensity. Stimulus frequency is indicated on each line. Error bars (where large enough to be seen) indicate ± 1 SEM. Lines are given a slight horizontal offset in (b) to make error bars easier to see.

Figure 6.

Parallel versus series acquisition waveforms (right ear only). (a) Grand average. (b and c) Example subjects. pABR is shown in colored lines. Corresponding serial waveforms are shown in black. Vertical spacing is 0.3 µV/div (see Supplementary Figure 2 for all subjects’ waveforms).

Figure 7.

Comparison of pABR with serial Wave V latency (a) and amplitude (b) for all subjects (N = 9) at each frequency for both stimulus levels. Quantities shown are for the right ear. Stimulus frequency indicated by marker number and color.

Figure 8.

Comparison of time to reach 20 nV residual noise for all 10 waveforms between methods (pABR in black, serial in gray). Two intensities for nine subjects are shown, leading to 18 data points for each acquisition method. pABR = parallel auditory brainstem response.

Figure 9.

pABR shows faster acquisition and better SNR. (a) Real-time acquisition runs simulated from offline data for one subject at 45 dB peSPL. For serial ABR, unrecorded left ear runs were assumed to be equal to right ear. (b) Comparison recording time for nine subjects. Points below dotted unity line are cases where pABR is faster. Shaded regions indicate speedup ratios of 1–2 (light gray), 2–4 (medium gray), and > 4 (dark gray). (c) SNR of pABR runs upon serial acquisition completion (subjects colored points, median black lines), corresponding for one subject to the points on the red dashed vertical line in (a). pABR = parallel auditory brainstem response.

Figure 10.

Improved visual response detection. (a) 500 Hz response waveform alone. (b) Same response with other frequencies simultaneously acquired. Dotted gray box surrounds the waveform from A. (c) 500 Hz response with other frequencies present and analysis window extended 10 ms to the left and 20 ms to the right. Gray box as in (b).

Figure 11.

Parallel stimulation allows toneburst trains to also function as notched noise. (a) Cartoon representation of a single toneburst stimulus in the frequency domain (filled area), and the pattern of excitation it evokes in the cochlea (dashed line), showing spread of excitation toward the basal end. (b) Notched noise can be used to mask the off-frequency excitation, yielding a more place-specific response. (c) In pABR, each frequency band is masked by the others, which summed have a similar effect to a notched noise masker.