Adaptation by normal listeners to upward spectral shifts of speech: Implications for cochlear implants

Multi-channel cochlear implants typically present spectral information to t he wrong ''place'' in the auditory nerve array, because electrodes can only be inserted partway into the cochlea. Although such spectral shifts are kn own to cause large immediate decrements in performance in simulations, the extent to which listeners can adapt to such shifts has yet to be investigat ed. Here, the effects of a four-channel implant in normal listeners have be en simulated, and performance tested with unshifted spectral information an d with the equivalent of a 6.5-mm basalward shift on the basilar membrane ( 1.3-2.9 octaves, depending on frequency). As expected, the unshifted simula tion led to relatively high levels of mean performance (e;g., 64% of words in sentences correctly identified) whereas the shifted simulation led to ve ry poor results (e.g., 1% of words). However, after just nine 20-min sessio ns of connected discourse tracking with the shifted simulation, performance improved significantly for the identification of intervocalic consonants, medial vowels in monosyllables, and words in sentences (30% of words). Also , listeners were able to track connected discourse of shifted signals witho ut lipreading at rates up to 40 words per minute. Although we do not know i f complete adaptation to the shifted signals is possible, it is clear that short-term experiments seriously exaggerate the long-term consequences of s uch spectral shifts. (C) 1999 Acoustical Society of America. [S0001-4966(99 )02012-3].