The Growth of Loudness in Cochlear Implant Listeners; Possible Developmental Effects?

Abstract

International Journal of Open Access Otolaryngology Open Access Research Article to decibels in acoustic hearing. Therefore, we measured loudness sensation in cochlear implant listening as a function of current level in microamperes. The growth of sensation magnitude with stimulus intensity is a fundamental property of sensory systems. Beginning with the classical studies by G.T. Fechner and S.S. Stevens, a wide variety of studies in several sensory systems have revealed that the growth of sensation, including loudness, is fundamentally related to the stimulus intensity by a power law. For a power law, y = a(x) b where in normal acoustic hearing y is perceived loudness, x is the sound pressure level (SPL), and b is an exponent approximately equal to 0.6, (i.e. the function is compressive). However, cochlear implants bypass the auditory periphery and stimulate the auditory nerve directly, and the growth of loudness magnitude with stimulus current for cochlear implant stimulation has been found to be very different from normal hearing individuals. The results typically conclude that loudness grows as an " expansive " nonlinear function of stimulus current. Some studies have shown that loudness growth functions are a power function of current when using magnitude estimation and are best-fit by power functions with an exponent of 2.72 to 3.5 [11,12,30,1,2,18]. Since the exponent range is much higher in electric hearing than acoustic hearing, the dynamic range is much smaller than for normal hearing subjects and hearing-impaired subjects with acoustic stimulation. In modeling the growth of loudness in cochlear implant listeners, there have also been several other proposed models of these expansive functions. The loudness growth functions have also been reported to grow exponentially with stimulus current so that y=ae bx , [30, 33]. Electrical loudness functions have also been described by combinations of both power and exponential functions [16], or by utilizing a forth-order polynomial, which follows the equation y=a 0 + a 1 x 1 + a 2 x 2 + a 3 x 3 + a 4 x 4 , [8]. The polynomial fit may be a convenient way to approximate individual variations in how loudness grows with current. While a higher-order polynomial may provide the best-fit approximation of individual variations in loudness functions in electric hearing, higher-order polynomial fits are more sensitive to perturbations or noise in the data. Previous studies have investigated the relationship between Abstract There have been limited psychophysical measurements on growth of loudness in early-deafened cochlear implant users as compared to postlingually-deafened cochlear implants users. In this study, the " shape " of the loudness growth function was investigated for prelingually-and postlingually-deafened subjects by measuring loudness growth within the electrical dynamic range on different bipolar electrode pairs using Absolute Magnitude Estimation (AME) and Absolute Magnitude Production (AMP). The shapes of the loudness functions were obtained and verified by comparing loudness growth models using the Akaike's Information Criterion (AIC) method to obtain the best-fit functions. Results suggest that adult cochlear implant listeners can exhibit different loudness growth functions, depending on their acoustical experience prior to implantation. Eight subjects with considerable acoustic experience prior to implantation and relatively late implantation demonstrated loudness functions that grew according to an expansive nonlinearity, in agreement with results from several previous studies. In contrast, the four early-deafened subjects with little or no useful acoustic experience prior to implantation, had loudness functions that grew in proportion to stimulus current amplitude. These results suggest that the shape of the loudness function is molded in part by the nature and functionality of input to the central auditory system during auditory development.