Effects of pulse rate and electrode on combined within-channel forward and backward masking in cochlear implant users

Cochlear implants restore hearing in profoundly hearing-impaired or deaf individuals by stimulating the auditory nerve with electric pulses. One remaining problem is a deficit in speech perception that occurs in noisy situations and is partially caused by masking originating in the cochlea. This work examined the time course of electrical masking of a single-pulse target by 300 ms maskers for two masker pulse rates (200 and 1000 pulses/s) and apical (el. 2) and basal (el. 8) electrodes. The psychoacoustic recovery function was sampled at four points, corresponding to time offsets of 1.5, 3.0, 6.0 and 100 ms after the forward masker and/or before the backward masker. Generally, there was more forward masking (recovery time constant ?1 = 336 ms) than backward masking (?2 = 206 ms) and no differences in the time course between the rates and electrodes were found. Pulse rate and electrode had significant overall effects (in terms of dynamic range), with more masking occurring for 200 pps and electrode 8. Finally, considerable individual differences were observed.