Acoustic Overstimulation Reduces 2f1-f2 Cochlear Emissions at All Levels in the Cat
The cubic distortion product, 2f1 — f2 (DP), was recorded in the cat ear canal as the levels of two primary tones (f1=4.0, f2=5.2 kHz here) were varied from approximately 40 to 90dB SPL. The DP level grows as the cube of the primary level at low and high levels, and usually exhibits a dip near L1 = 65dB SPL, DP growth functions and N1 amplitude-level functions were compared before and after exposure to either 4 kHz pure tones or to 500 Hz octave bands of noise. Pure-tone exposure produced a dose-dependent shift in the N1 growth whereas the low-frequency noise exposure produced widely varying shifts in N1 functions. In either case, however, changes in DP were correlated with changes in the neural responses. The level of the DP was compared before and after exposure at 2 levels below and one above the dip at L1 = 65dB SPL. For all of these measures, the decrease in DP level was significantly correlated with the shift in the N1 growth function. At both low and high primary levels, decrease in DP grows approximately as the square of the N1 shift for shifts greater than 15dB.
KeywordsNoise Exposure Basilar Membrane Distortion Product Probe Tube Primary Tone
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