Pulse rate discrimination with deeply inserted electrode arrays.

Pulse rate difference limen (PRDL) and amplitude modulation difference limen (AMDL) were assessed as a function of base rate and cochlear electrode location in seven (three for AMDL) subjects implanted with the MED-EL COMBI 40+ implant. The MED-EL COMBI 40+ electrode array allows deep insertion of the electrode up to the apex of the cochlea to minimize the rate/place mismatch for pulse rates below 500 pps. A three interval, two alternative forced-choice procedure with feedback was used to measure the difference limen. The base rate was in the range between 200 and 800 pps. The carrier rate for the AMDL measurement was 5081 pps. The PRDL increased with increasing base pulse rate. At 200 pps the average PRDL measured at the apical electrode amounted to 48.7 pps, at 400 pps the average PRDL reached 206.6 pps. No significant difference between PRDL obtained from apical or basal electrodes could be observed. AMDL was higher than PRDL at all tested base rates. The ability to discriminate rate changes is limited to base rates up to about 283 pps. The results indicate that rate changes smaller than a major third do not elicit distinguishable auditory perceptions in electrical hearing. The absence of a difference between apical and basal electrode locations indicates that a reduction of the rate/place mismatch does not improve discrimination performance.