Effects of contralateral acoustical stimulation on three measures of cochlear function in the guinea pig.

The magnitudes of suppression of the click-evoked compound action potential of the auditory nerve (CAP), transient click-evoked otoacoustic emissions (TEOAEs) and ensemble background activity of the auditory nerve (EBA), elicited by contralateral acoustical stimulation, were compared in awake or lightly sedated guinea pigs. The contralateral ear was stimulated either by continuous broad-band noise or by low-pass or high-pass noise (intensity 41-62 dB SPL) with cut-off frequencies of 2, 8 and 12 kHz. The maximal suppression of TEOAEs was achieved by contralateral noise containing mainly low frequencies, whereas for suppression of the CAP it was necessary for middle frequencies to be present in the contralateral noise (less than 8 kHz). In contrast to this, EBA was suppressed mainly by high-frequency noise (higher than 8 kHz) whereas low- and middle-frequency noise was ineffective in suppressing EBA. Evaluation of the root mean square voltage of the EBA (filtered in frequency range 0.75-1.25 kHz) enabled the evaluation of fast and slow components of olivocochlear activation. Both fast and slow effects were proportionally suppressed by individual types of contralateral stimulation. The mechanisms of TEOAEs and CAP generation has been confirmed in many earlier studies, but the origin of EBA has yet to be fully elucidated. The obtained data support the hypothesis that a large part of EBA is formed by spontaneous activity of high-frequency-tuned auditory nerve fibres. Suppression of the EBA magnitude during contralateral stimulation may be caused either by a reduced spontaneous firing rate or by a decrease in possible synchronised neuronal firing.