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 stimulating electrode pair was studied in four users of the Nucleus-22 cochlear implant. Two experiments were performed. In experiment 1, detection threshold for a 200-ms, 500-pulses/s biphasic pulse train was measured as the spatial separation between the active and return electrodes was increased. In experiment 2, loudness-growth functions were measured as the electrode separation was increased. The results of experiment 1 indicate that (a) threshold decreases monotonically toward an asymptote (monopolar threshold) as the electrode separation is increased and (b) the threshold versus electrode separation function is similar across subjects and electrode location. The results of experiment 2 indicate that loudness L is related to the current amplitude I by a simple exponential function: L = e beta I, where beta, the exponent of the loudness function, is, to a first approximation, linearly related to the separation between the electrodes of the stimulating pair. Thus, dynamic range and the shape of loudness growth can be directly linked to the spatial separation between active and return electrodes.