Corresponding effects of hypoxia on the cochlear microphonic and the compound action potential

Mild hypoxia has an unexpected influence on the compound action potential (CAP) compared to its effect on the cochlear microphonic (CM). While the CM decreases in amplitude near threshold, the low-level CAP increases in amplitude by as much as 400% and decreases in latency and width. The magnitude of latency decrease is dependent on the center frequency of $\text{1}\text{3}$ octave band-filtered clicks used as stimuli. Below 13500 Hz the latency is increasingly shortened at a rate of 0.073 ms/octave. At high intensities moderate hypoxia has no effect on either the CM or the CAP. However, with more severe hypoxia the high-level CM also begins to deteriorate. Correspondingly both the high- and low-intensity CAP decrease in magnitude and increase in latency relative to their control values. Assuming that the low-level CM is generated by outer hair cells, these results suggest that one relationship between outer hair cell function and auditory nerve function near threshold is that of inhibition. The fact that N₁ latency is shortened suggests that this inhibition occurs in the basalward direction.