The 3-channel Lissajous' trajectory of the auditory brain-stem response. V. Effects of stimulus intensity in the cat

Three-channel Lissajous' trajectories (3-CLTs) of the cat auditory brain-stem response (ABR) were recorded using click stimuli ranging from 10 to 70 dB impulse SPL and were analyzed using planar analysis. The number of planar segments increased from typically 4 at 10 dB to 12 at 70 dB but certain shape features of the 3-CLT (apices) were preserved across stimulus levels. As stimulus level was raised, size of individual planar segments increased. There were progressive decreases in apex latency as stimulus level was increased. The combined durations of groups of high intensity planar segments were similar to those of their low intensity forms. Shape, size and orientation of planar segments tended to change more across stimulus intensities below 40 dB than above and appear to relate to the number of planar segments at any given stimulus level. These results suggest that changes in latency seem to be primarily cochlear in origin, whereas the origin of other observed changes is uncertain. The 3-CLT ABR is sensitive to intensity, especially below 40 dB, and can thus detect changes in auditory system function in response to changes in stimulus intensity, regardless of electrode position.