Late components in the compound action potentials (CAP) recorded from the intracranial portion of the human eighth nerve.

The compound action potential (CAP) that can be recorded from the exposed intracranial portion of the eighth nerve in man to stimulation with broadband clicks of about 100 dB Pe SPL normally has an initial (small) positivity followed by a sharp negative peak with a latency of about 3 to 3.5 ms. The negative peak is usually followed by another positive-negative deflection with a latency of about 4 ms. Usually, no stimulus-related potential can be discerned at latencies longer than 5 ms. However, in a few patients we found a series of waves that occurred between 4 and 12 ms after the stimulation. The polarity of these waves reversed precisely (180 degrees phase shift) when the polarity of the sound was reversed. Thus, these waves appeared clearly when the responses to clicks of opposite polarity were subtracted. These late waves were quasiperiodic with intervals between 1 and 2 ms. The waveform and duration differed between patients, but were remarkably constant in each patient. The timing of these late peaks was nearly independent of the stimulus intensity in the range studied (between 105 and 70 dB Pe SPL); in this respect the late waves differ fundamentally from the initial peaks, which showed a monotone decrease in latency with increasing stimulus intensity over this intensity range. Although the origin of these late waves is not known, the similarities between these waves and stimulated otoacoustic emissions indicate that the late waves may be the result of active cochlear processes similar to those that produce the cochlear echo.