Effects of stimulus level on electrode-place discrimination in human subjects with cochlear implants

Effects of stimulus level on discrimination of one stimulation site from an other were examined in 15 human subjects with Nucleus-22 cochlear implant s ystems. Bipolar stimulation was used in all cases with electrodes in the bi polar pair separated by 1.5 mm (center to center). Subjects were first test ed at a medium loudness level, using an adaptive tracking procedure, to det ermine the regions of the electrode array where electrode-place discriminat ion was best and the regions where it was poorest. Electrode-place discrimi nation was then tested at three regions distributed throughout the array, w hich included the regions of best and poorest discrimination. At each regio n, electrode-place discrimination was tested at three levels: 25%, 50%, and 75% of the dynamic range. For each of these nine conditions (3 sites x 3 l evels), the test-electrode pairs were loudness balanced with the reference- electrode pairs. A two-interval forced-choice same-different procedure was then used to determine discriminability of the reference-electrode pair fro m the nearest, apical, test-electrode pair. If P(C)(max) was < 0.707 at all three levels, additional testing was done using the next, more apical, ele ctrode pair as the test-electrode pair. A tendency toward better discrimina tion at more apical regions of the array was observed. Electrode pairs with poor discrimination typically had smaller dynamic ranges than those with g ood discrimination. There was a weak tendency toward better discrimination at higher levels of stimulation. However, effects of level on electrode-pla ce discrimination were less pronounced and less consistent than previously observed effects of level on temporal discriminations. These results sugges t interactions between current spread and the condition of the implanted co chlea as underlying mechanisms. (C) 1999 Elsevier Science B.V. Published by Elsevier Science B.V. All rights reserved.