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), elic ited by contralateral acoustical stimulation, were compared in awake or lig htly sedated guinea pigs. The contralateral ear was stimulated either by co ntinuous broad-band noise or by low-pass or high-pass noise (intensity 41-6 2 dB SPL) with cut-off frequencies of 2, 8 and 12 kHz. The maximal suppress ion of TEOAEs was achieved by contralateral noise containing mainly low fre quencies, whereas for suppression of the CAP it was necessary for middle fr equencies to be present in the contralateral noise (less than 8 kHz). in co ntrast to this, EBA was suppressed mainly by high-frequency noise (higher t han 8 kHz) whereas low- and middle-frequency noise was ineffective in suppr essing 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 dow effects were prop ortionally 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 obtaine d data support the hypothesis that a large part of EBA is formed by spontan eous 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 synchronise d neuronal firing. (C) 2001 Elsevier Science B.V. All rights reserved.