Frequency-dependence of early auditory evoked responses in the guinea pig

Neural auditory responses in the guinea pig, monitored by surface electrodes (brain stem potentials and frequency-following responses) and by electrodes at the round window, were analyzed for evidence of frequency dependence in the range from 500 Hz to 15 kHz. The characteristics of the brain stem potentials and frequency-following responses with stimuli near threshold intensity for frequencies below 2 kHz indicate that this activity derives from the excitation of apical regions of the basilar membrane. The same interpretation applies to the potentials recorded at the round window. Comparison of the responses seen with surface electrodes and those appearing at the round window reveals that the broad potential PI of the brain stem response to low-frequency stimuli corresponds to the compound action potential, while the frequency-following responses correspond to phase-locked responses in the acoustic nerve.