Acoustically and electrically evoked contralateral suppression of otoacoustic emissions in guinea pigs

It is generally accepted that stimulation of the efferent auditory system results in changes of cochlear activity. A simple method of activating the olivocochlear pathway by contralateral electrical stimulation of the round window (ES-RW) was used in this study with the aim of comparing the efficacy of acoustically and/or electrically evoked contralateral suppression. The suppression of transient evoked otoacoustic emissions (TEOAEs) and distortion product otoacoustic emissions (DPOAEs) was elicited by contralateral acoustic stimulation (AS) (61 dB SPL continuous white noise), and/or by electrical stimulation of an electrode implanted at the contralateral round window (monopolar rectangular pulses 0.1 ms, repetition rate 300 Hz, intensity 50-100 PA) in 12 guinea pigs. The average value of contralateral suppression of TEOAEs amounted to 1.04 +/- 0.48 dB for acoustic stimulation and 0.97 +/- 0.53 dB for round window electrical stimulation. The simultaneous presentation of both acoustic and electrical stimulation had only a slight additive effect and resulted in 1.27 +/- 0.79 dB diminution of TEOAEs. The suppression of DPOAEs during contralateral acoustic and electrical stimulation was evident mainly at low and middle frequencies (14 kHz). In two guinea pigs the maximum DPOAE suppression was present at high frequencies. The average values of contralateral suppression measured at individual f2 frequencies of DPOAEs were similar to those calculated from 1/4 octave power spectrum analysis of the TEOAEs in half of the animals. The results demonstrated that contralateral ES-RW had a similar suppressive effect on TEOAEs and DPOAEs as did contralateral AS and simultaneous AS+(ES-RW). The results of spectral analysis suggested that both modes of contralateral stimulation excited similar sensory cochlear elements and induce comparable suppression of both TEOAEs and DPOAEs.