Effects of stimulation mode on threshold and loudness growth in multielectrode cochlear implants

The effect of increasing the separation between the two members of a stimul ating electrode pair was studied in four users of the Nucleus-22 cochlear i mplant. Two experiments were performed. In experiment 1. detection threshol d for a 200-ms, 500-pulses/s biphasic pulse train was measured as the spati al separation between the active and return electrodes was increased. In ex periment 2, loudness-growth functions were measured as the electrode separa tion was increased. The results of experiment 1 indicate that (a) threshold decreases monotonically toward an asymptote (monopolar threshold) as the e lectrode separation is increased and (b) the threshold versus electrode sep aration function is similar across subjects and electrode location. The res ults of experiment 2 indicate that loudness L is related to the current amp litude I by a simple exponential function: L=e(beta I), where beta, the exp onent of the loudness function, is, to a first approximation, linearly rela ted to the separation between the electrodes of the stimulating pair. Thus, dynamic range and the shape of loudness growth can be directly linked to t he: Spatial separation between active and return electrodes. (C) 1999 Acous tic Society of America. [S0001-4966(99)00602-5].