Cochlear threshold assessment using tone-derived action potentials

An evoked-potential technique has been evaluated which detects whether the cochlea responds to a continuous, low level tone. The technique involves recording the cochlear action potential (AP) response to a suprathreshold probe tone, first in the absence and then in the presence of a continuous masking tone at the same frequency. Subtraction of the masked AP waveform from the unmasked AP yields a derived potential, provided the continuous tone is above the threshold of cochlear sensitivity. Derived AP responses may be recorded with continuous masking tones over 10 dB below the threshold to the probe stimulus. In normally hearing guinea pigs, the mean best derived threshold using a 10-μV response criterion was 7.1 dB SPL, compared to 18.9 dB for conventional AP thresholds. The tone-derived response appears to provide a more sensitive and frequency-specific method for determining cochlear thresholds.