Psychophysical Measurements of Temporal Integration Effects in Cochlear Implant Users

In this study, the effect of high stimulation rates on temporal integration in cochlear implant hearing was investigated. We measured threshold amplitudes, maximal acceptable levels and a line of equal loudness in 11 cochlear implant users (all with implants from MED-EL). The measurements were done (a) with a clinically used single channel stimulation rate of 1500 pulses per second (pps) and (b) at a high stimulation rate of 18000 pps, both for an apical electrode and a basal electrode. The duration of the stimulation pulse trains varied from 1 pulse to 5400 pulses. Additionally, we collected data on the perception of the different stimulation rates and stimulation electrodes using a questionnaire. A power-law like function was used to fit the threshold amplitudes of individual subjects with a high accuracy (R² = 0.96 +/- 0.05). The change of the stimulation electrode did not cause any systematic effects regarding threshold amplitudes or the slope of the temporal integration (TI) curve. We found lower thresholds for the high rate and slopes of 3.48 dB and 5.55 dB per tenfold increase in duration for the low and high rate, respectively. The DR was increased with the high rate by 6.58 +/- 3.79 dB. Since the increased DRs at the high rate were accompanied by higher variability of the given answers, the same number of audible loudness steps in the given DR is expected for both rates. Some of the subjects perceived the change in stimulation rate with a change in pitch, whereas most participants were only sensitive to a variation of the stimulation electrode. It is noteworthy, that besides pitch, the perception of other characteristics like sharpness are affected by changes in stimulation rate and stimulation electrode