Auditory evoked responses to pure tones between 1 and 8 kHz: Effects of notched noise

Auditory evoked potentials (AEP) elicited by pure tones between 1 and 8 kHz show a frequency-dependent amplitude gradient, with 1 kHz eliciting the highest amplitude response and 8 kHz eliciting the lowest. Why high frequencies elicit a lower amplitude remains unclear. We tested the hypothesis that the response gradient is caused by increased spread of excitation along the tonotopic axis at lower frequencies. Using a 32-channel Biosemi EEG system, AEPs were measured in response to 70-ms tones at four frequencies (1, 2, 4, and 8 kHz), presented in random order with 800- to 900-ms interstimulus intervals. The tones were presented at an equal loudness level of 70 phons either with or without background threshold-equalizing noise with spectral notches of 2/3 octaves around each tone frequency. Preliminary findings confirm the previous findings of greater AEPs, based on the N1-P2 amplitude, to lower-frequency tones presented in quiet, but also show that the differences between frequencies appear to be reduced in the presence of the noise. The results appear to be consistent with the hypothesis that AEPs are more consistent across frequency when spread-of-excitation cues are controlled. [Work supported by NIH grant R01 DC005216.]