Electrically evoked otoacoustic emissions from apical and basal perilymphatic electrode positions in the guinea pig cochlea

Stimulation of the cochlea with sinusoidal current results in the productio n of an otoacoustic emission at the primary frequency of the stimulus curre nt. In this study we test the hypothesis that the wide frequency response f rom round window (RW) stimulation is due to the involvement of a relatively large spatial segment of the organ of Corti. Tonotopically organized group delays would be evident from perilymphatic electrode locations that restri ct the spatial extent of hair cell stimulation. Monopolar and bipolar-paire d stimulus electrodes were placed in perilymphatic areas of the first or th ird cochlear turns and the electrically evoked otoacoustic emissions (EEOAE ) produced by these electrodes were compared to that from the RW monopolar electrode in the anesthetized guinea pig. Current stimuli of 35 muA RMS wer e swept across the frequency range between 60 Hz and 100 kHz. The EEOAE was measured using a microphone coupled to the ear canal. It was found that th e bandwidth of EEOAEs from RW stimulation extended to at least 40 kHz and w as a relatively insensitive to electrode location on the RW. The group dela y of the EEOAE from stimulation at the RW membrane (corrected to stapes mot ion) was about 53 mus. First and third turn stimulations from electrode pla cements in perilymph near the bony wall of cochlea yielded narrower band EE OAE magnitude spectra but which had the same short group delays as for RW s timulation. A confined current (from a bipolar electrode pair) applied clos e to the basilar membrane (BM) in the first turn produced the narrowest fre quency-band magnitude emissions and a mean corrected group delay of 176 mus for a location approximately 3 mm from the high frequency end of the BM (c orresponding to about the 18 kHz best frequency location). Bipolar electrod es in the third turn scala tympani produced low pass EEOAE magnitude functi ons with corrected group delays ranging between approximately 0.3 and I ms. The average phase slopes did not change with altered cochlear sensitivity and postmortem. These data indicate that the EEOAE from RW stimulation is t he summed response from a wide tonotopic distribution of outer hair cells. A preliminary model study indicates that short time delayed emissions are t he result of a large spatial distribution of current applied to perilymphat ic locations possibly giving rise to 'wave-fixed' emissions. (C) 2001 Elsev ier Science B.V. All rights reserved.