Growth of loudness in cochlear implant listening

Abstract

Cochlear implants (CIs) roughly mimic the transformation from sound frequency to cochlear place that occurs in acoustic hearing. However, CIs are relatively less capable of creating the intensive transformations that normal peripheral auditory processing provides. This is partly because CIs have small operating ranges on the order of 10:1 in electric current, compared to the 1<th>000<th>000:1 operating range for sound-pressure level (SPL) in acoustic hearing. Furthermore, loudness in acoustic hearing grows as a compressive power function of SPL. In contrast, loudness reportedly grows as a more expansive function of current for CI users, i.e., a power law with a large exponent or an exponential function. Our results, obtained using the minimally biased method of magnitude balance without an arbitrary standard, reveal a previously unreported range of shapes of CI loudness functions, going from linear to power laws with exponents of 5 or more. The shapes seem related in part to the etiology of deafness preceding cochlear implantation, although the shapes can vary with stimulating conditions within a subject. Furthermore, differential sensitivity to changes in current appears to be related to the shape of the corresponding loudness function. Implications for sound processing in electric and acoustic hearing will be discussed.