The stimuli used in the current study are (top-to-bottom): an click, a click, a rising-frequency ‘A-Chirp’ (compensating for BM traveling wave delays calculated from tone burst-evoked ABR), and a rising-frequency ‘M-Chirp’ (compensating for BM traveling wave delays calculated from modeled data). Bold traces indicate the acoustic time course as measured in a 2 cc coupler. Lighter traces indicate the pre-transducer time course. The ‘reverse chirps’ of the current study are not shown, but are temporal reversals of the A-Chirp and M-Chirp.
The frequency spectra of the click and chirp stimuli were obtained using a 2 cc coupler and an Aurical sound level meter. All stimuli were presented at 40 dB HL, corresponding to 87 dB peSPL for the click and 80 dB peSPL for the chirp stimuli.
Mean wave V amplitudes (top) and mean adjusted wave V latencies (bottom) for our test populations. Latencies were adjusted for the ER-3A tubing delay and, for rising-frequency chirps, the duration of the chirp was subtracted to give latency relative to stimulus offset. Vertical lines on each bar indicate one standard deviation.
Grand average waveforms (in bold) were assembled from all waveforms at a threshold of 3.1, after first aligning wave V locations to the group average. Vertical axis is microvolts, horizontal axis is milliseconds following stimulus onset (corrected for ER-3A tubing delay). The time course of each evoking stimulus is shown in gray (not to scale).
Chirp-evoked ABR waveforms for three representative subjects. The left column are ABRs evoked by the A-Chirp, and the right column are ABRs evoked by the M-Chirp. Three repeat traces are shown to indicate repeatability. Morphology differences exist across the subjects shown. For the M-Chirp of subject 3, the ‘?’ indicates an uncertainty in identifying peak III and IV. Vertical axis is microvolts, horizontal axis is milliseconds following stimulus onset (corrected for ER-3A tubing delay). The time course of each evoking stimulus is shown in gray (not to scale).
Fast Fourier transforms demonstrate the differing frequency compositions of ABRs evoked by the stimuli in this study. The ranking of FFT magnitudes below 200 Hz is almost in tandem with the results for wave V amplitudes.
The component synchrony measure, calculated at each component frequency in the FFT, indicates the phase variance across multiple ABR sub-averages. A component frequency with a CSM close to 1 is highly synchronized with the stimulus onset, whereas a component frequency with a CSM close to 0 is unsynchronized—possibly as a result of neural jitter or interference from background physiological noise.
(Top) The overall SMs in this study were calculated to be the average of all CSMs up to our low pass filter cut off of 3 kHz (vertical line). The component synchrony measures for A- and M-Chirps at frequencies above 1000 Hz were lower than those of the other stimuli in this study, resulting in an impaired SM. (Bottom—note the change in ordinate scale.) Here we demonstrate the effect of limiting low frequency contributions to the SM calculation. The upper frequency limit is fixed at 3 kHz while the high-pass cut-off is varied from 0 to 3 kHz. The click-evoked ABRs exhibit greater synchrony measures irrespective of high-pass cut-off.
Article metrics loading...
Full text loading...