The Growth of Loudness Functions Measured in Cochlear Implant Listeners Using Absolute Magnitude Estimation and Compared Using Akaike's Information Criterion

The input/output function for acoustic hearing can be characterized by the growth of loudness with sound pressure level and generally follows a compressive power law. In contrast, in electric hearing, loudness reportedly is an expansive function of applied electrical current but the specific shape of the function has not been fully determined. Loudness growth models have implications for the implementation of cochlear implant speech processors. Having an appropriate loudness growth model is important to cochlear implant users because they have a small dynamic range of hearing compared to normal hearing listeners. To compensate for this, appropriate models of loudness are necessary for the design of cochlear implant speech processors. It is also necessary to understand how loudness is encoded and may affect the relative performance in speech recognition. Currently, there is no consensus on the actual shape of the loudness growth function, with power or exponential functions being suggested. In this study psychophysical loudness growth measures were obtained in twelve adult cochlear implant listeners, using the method of absolute magnitude estimation and production. Best-fit loudness growth functions as determined by Akaike's Information Criterion (AIC) method for finding the best-fit loudness model, seem to show a difference in the loudness growth functions across subjects and across electrode pairs within individual subjects. The range of functions observed is greater than previously reported and goes from linear to expansive, suggesting that individual variations in dynamic range should be incorporated in the design of cochlear implant sound processors.