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Spatial Overlap of Combined Electroacoustic Stimulation Determines the Electrically Evoked Response in the Guinea Pig Cochlea

Hypothesis Limiting spatial overlap between electrical stimulation (ES) and acoustical stimulation (AS) in the cochlea reduces the effects of AS on electrically evoked auditory nerve activity. Background Some hybrid cochlear implant systems have a regular array, whereas others have short arrays that spatially segregate ES from AS. AS settings in hybrid implants may also affect electroacoustic interaction. Methods ES (900 &mgr;A) was delivered in the high-frequency part of the cochlea, and the electrically evoked compound action potential (eCAP) was recorded to assess auditory nerve activity. Maximal spatial overlap of ES and AS was tested by using normal-hearing animals (NH, n = 6), whereas minimal overlap was modeled by using animals with high-frequency hearing loss (HFHL, n = 6). AS consisted of broadband (BB) or low-frequency (LF) noise (0–100 dB SPL). Effects of AS on eCAP amplitude were statistically tested using 1-sample t tests (&agr; = 0.05). Results BB noise at 60 dB SPL significantly suppressed eCAP amplitude in NH animals but not in HFHL animals up to a 30 dB higher level. Suppression with LF noise at 60 dB SPL was not significant in either the NH or the HFHL group, but at 90 dB SPL, suppression was significant in both groups. Conclusion Minimizing spatial overlap between ES and AS reduces eCAP suppression when moderate sound levels are applied. Overlap can be reduced by applying ES in an acoustically insensitive part of the cochlea or by limiting the acoustic spectrum to low frequencies when ES is applied in acoustically sensitive areas.