Effects of phase duration and electrode separation on loudness growth in cochlear implant listeners

Loudness estimates were obtained in a group of four adult subjects implanted with the Nucleus-22 multielectrode cochlear implant device, for a range of pulse amplitudes and different fixed phase durations and electrode separations. The stimulus was a 200-ms long train of biphasic pulses presented at 500 pulses/s. Subjects estimated loudness as a number from 0 ("don't hear it") to 100 (''uncomfortably loud''). Loudness was found to grow exponentially with pulse amplitude, at a rate that was dependent upon the phase duration as well as the electrode separation. An equation of the form L = e((lambda+gamma M)(D theta)I), where L is the estimated loudness, M is the separation between electrodes of a stimulating pair, D is the phase duration, I is current amplitude, and lambda, gamma, and theta are constants, appears to describe the observed data adequately. The findings are remarkably consistent across subjects. (C) 2000 Acoustical Society of America. [S0001-4966(00)01103-6].