Research paper Suppression of the acoustically evoked auditory-nerve response by electrical stimulation in the cochlea of the guinea pig H. Christiaan Stronks, Huib Versnel, Vera F. Prijs, Sjaak F.L. Klis * Department of Otorhinolaryngology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, P.O. Box 85500, 3508 GA Utrecht, The Netherlands article info Article history: Received 28 November 2008 Received in revised form 27 September 2009 Accepted 13 October 2009 Available online 17 October 2009 Keywords: Electro-acoustical stimulation Electrocochleography Compound action potential Cochlear implant abstract There is increasing interest in the use of electro-acoustical stimulation in people with a cochlear implant that have residual low-frequency hearing in the implanted ear. This raises the issue of how electrical and acoustical stimulation interact in the cochlea. We have investigated the effect of electrical stimulation on the acoustically evoked compound action potential (CAP) in normal-hearing guinea pigs. CAPs were evoked by tone bursts, and electric stimuli were delivered at the base of the cochlea using extracochlear electrodes. CAPs could be suppressed by electrical stimulation under various conditions. The dependence of CAP suppression on several parameters was investigated, including frequency and level of the acoustic stimulus, current level of the electric stimulus and the interval between electric and acoustic stimulus (EAI). Most pronounced suppression was observed when CAPs were evoked with high-frequency tones of low level. Suppression increased with current level and at high currents low-frequency evoked CAPs could also be suppressed. Suppression was typically absent several milliseconds after the electric stimu- lus. Suppression mediated by direct neural responses and hair cell mediated (electrophonic) responses is discussed. We conclude that the high-frequency part of the cochlea can be stimulated electrically with little detrimental effects on CAPs evoked by low-frequency tones. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Cochlear implantation is currently the method of choice for treatment of severe to profound sensorineural hearing loss. Ongoing improvement of the performance of cochlear implants (CIs) has led to a relaxation of the clinical criteria for candidacy for implantation (Lorens et al., 2008). Nowadays, patients with considerable residual low-frequency hearing are implanted (Wil- son et al., 2003; Gantz et al., 2005). Clinical studies have indicated that it is possible to take advantage of residual hearing after implantation. While the use of a hearing aid in the non-implanted ear can be advantageous for speech and melody recognition, and for sound localization (Dooley et al., 1993; Kong et al., 2005; Ching et al., 2006; Mok et al., 2006; Novak et al., 2007), most research has focussed on electro-acoustical stimulation (EAS) by means of hy- brid implants. Hybrid implants combine a CI with a conventional hearing aid in the same ear. EAS can improve speech understand- ing in noise and can increase the aesthetic quality of sound (Von Ilberg et al., 1999; Fraysse et al., 2006; Gstoettner et al., 2008; Turner et al., 2008). To preserve residual hearing, surgical techniques have been adapted to minimize surgical trauma during implantation (Adunka et al., 2004; Gantz et al., 2005). In addition, short electrode arrays have been developed that do not penetrate the acoustically sensi- tive apical parts of the cochlea (Gantz and Turner, 2003; Gstoettner et al., 2004). Besides minimizing insertion trauma, short electrode arrays can decrease the interaction between electrical and acousti- cal stimulation by spatially segregating both stimulus modalities. It seems a reasonable assumption that the beneficial effect of residual hearing in patients using EAS is optimal when electrical stimulation does not interfere with the acoustically evoked responses. Electro-acoustic interaction might be a factor contribut- ing to the variability in performance of patients using EAS (Luetje et al., 2007; Turner et al., 2008) and may be a contributing factor to the occasionally observed detrimental effect of residual hearing on speech understanding (Gstoettner et al., 2008). Recent research on electro-acoustic interaction by Abbas and co-workers has focussed on the (suppressive) effects of acoustical stimulation on 0378-5955/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.heares.2009.10.004 Abbreviations: ABR, auditory brainstem response; CAP, acoustically evoked compound action potential; CI, cochlear implant; CM, cochlear microphonics; eABR, electrically evoked ABR; eCAP, electrically evoked compound action potential; EAI, electric-to-acoustic stimulus interval; EARI, electric-to-acoustic response interval; EAS, electro-acoustical stimulation; FFT, fast Fourier transform; N 1 , 1st negative peak; N 2 , 2nd negative peak; SP, summating potential; SPL, sound pressure level * Corresponding author. Address: Hearing Research Laboratories, Department of Otorhinolaryngology, Rudolf Magnus Institute of Neuroscience, University Medical Center Utrecht, Room G.02.531, P.O. Box 85500, 3508 GA Utrecht, The Netherlands. Tel.: +31 (0) 88 755 7724; fax: +31 (0) 30 254 1922. E-mail addresses: c.stronks@umcutrecht.nl (H.C. Stronks), h.versnel@ umcutrecht.nl (H. Versnel), v.prijs@umcutrecht.nl (V.F. Prijs), s.klis@umcutrecht.nl (S.F.L. Klis). Hearing Research 259 (2010) 64–74 Contents lists available at ScienceDirect Hearing Research journal homepage: www.elsevier.com/locate/heares