Action Potentials Recorded from the Promontory and the Surface, Compared with Recordings from the Ear Canal in Man

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Abstract
With chlorided silver electrodes, a recording band width of 5–5k Hz, and a “2 kHz” half sinusoid rarefaction click stimulus, the early auditory potentials are recorded from different electrode positions. From the promontory and the ear canal almost identical waveforms of the action potentials are obtained, but the ratio between the amplitudes of the potentials is found to be intensity dependent. The background noise picked up is twice as high from the promontory as from the ear canal electrode. The signal-to-noise ratio of the promontory recordings is always superior to the ear canal recordings—near the threshold, appr. 3.5 times, Recordings from the ear canal/ear lobe, ear canal/vertex and ear lobe/vertex electrode placements demonstrate the interaction between the cochlear and the brain stem activity. In the far field ear lobe/vertex recordings this interaction is found to be minimum, and the latency of the Jewett V-wave shows that, irrespective of the intensity, this activity is generated 4.25 msec later than the compound cochlear action potential. This might indicate the same degree of frequency specificity of the Jewett V-wave as of the cochlear action potential. The background noise from the ear lobe/vertex pick up is found to be 6 times as high as from the ear canal/ear lobe—with a dominance of low frequency EEG components. In a special experiment, it is demonstrated that no brain stem activity (when elicited contralaterally) is present in the ear canal/ear lobe recordings. It is furthermore shown that increasing of the signal-to-noise ratio by high pass filtering should be handled with extreme care.