Hypoxia-Induced Changes of Cochlear Operating Points Assessed by Low-Frequency Biased Otoacoustic Emissions and Round Window Cochlear Potentials

Low-frequency biasing tones have been extensively used in the past to estimate the operating point of the active cochlea. Here, using anaesthetized gerbils, we synchronously observed the modulation of DPOAE, SFOAE, CM, and CAP by a 60-Hz biasing tone (85 dB SPL) aiming to analyze operating point shifts induced by 30 seconds of hypoxia. The time course of the perturbed blood oxygenation (SpO2) was monitored with an oximeter. Pronounced changes in the modulation pattern and overall levels of OAEs as well intermodulation products in the CM signal have been observed. However, the CAP modulation pattern remained unchanged. The overall amplitude of CAP, CM, SFOAE and the 2f1-f2 DPOAE component followed the SpO2 readings, all fully recovering after ~1 minute of resuming breathing fresh air. Their modulation patterns, however, and most notably, the overall level of the f2-f1 component did not return to their pre-hypoxia states for a further 2 minutes. Two repetitions of the experiment over a three-hour period showed almost identical traces. While DP changes in OAE and CM can be explained by changes in the operating point (OP) along a sigmoidal nonlinearity, SFOAE and CAP modulation patterns were not useful for monitoring OP changes.