Changes in 2f1 - f2 acoustic distortion products in humans during quinine-induced cochlear dysfunction

Quinine is a suitable model substance for the study of otoacoustic emissions (OAEs) as it reversibly affects the outer hair cells, thus reducing sensitivity, frequency-selectivity and various forms of OAEs. The aim of this experiment was to study quinine-induced changes in the input/output (I/O) function of 2f1 - f2 distortion product OAE (DPOAE; f2/f1 = 1.22; 750-6,000 Hz). Six volunteers with normal hearing (26-39 years old) were intravenously infused to achieve pseudostable quinine plasma concentrations (approximately12 microM) inducing an average pure-tone threshold (PTT; 750-6,000 Hz) shift of 18 dB (5-30 dB) (frequency-independent and reversible). The mean quinine-induced DPOAE shift increased continuously with decreasing equal-level primary tones, e.g. from 1.0 dB at 70 dB sound pressure level (SPL) (n = 42) to 10.5 dB (n = 22) at 40 dB SPL (pooled data, no frequency dependence). According to recruitment, the mean slope of the DPOAE I/O function (at 30-60 dB SPL) increased from 0.86 to 1.35 dB/dB. The lack of correlation between shifts in DPOAE and PTT is in stark contrast to the excellent correlation reported between shifts in transient evoked OAE detection threshold and its corresponding psychoacoustic threshold. The highly vulnerable spontaneous OAEs, in combination with the less vulnerable DPOAEs, fit into a recently proposed taxonomic classification for OAEs.