Is there a fundamental 300 Hz limit to pulse rate discrimination in cochlear implants?

Literature often refers to a 300 pps limit for cochlear implant (CI) electrical stimulation, above which pulse rate discrimination deteriorates or above which rate pitch is not perceived to increase. The present study investigated the effect on pulse rate difference limens (PRDLs) when using compound stimuli in which identical pulse trains were applied to multiple electrodes across the length of the electrode array and compared the results to those of single-electrode stimuli. PRDLs of seven CI users were determined in two stimulus pulse phase conditions, one in which the phase delays between pulses on different electrodes were minimised (burst mode) and a second in which they were maximised (spread mode). PRDLs were measured at base rates of 100 to 600 pps in 100 pps intervals, using compound stimuli on one, two, five, nine and 18 electrodes. As smaller PRDLs were expected to reflect improved rate pitch perception, 18-electrode spread mode stimuli were also included in a pitch ranking task. PRDLs improved markedly when multi-electrode compound stimuli were used, with average spread mode PRDLs across listeners between 6 and 8 % of the base rate in the whole range tested (i.e. up to 600 pps). PRDLs continued to improve as more electrodes were included, up to at least nine electrodes in the compound stimulus. Stimulus pulse phase had a significant influence on the results, with PRDLs being smaller in spread mode. Results indicate that pulse rate discrimination may be manipulated with stimulus parameter choice so that previously observed deterioration of PRDLs at 300 pps probably does not reflect a fundamental limitation to rate discrimination. However, rate pitch perception did not improve in the conditions that resulted in smaller PRDLs. This may indicate that listeners used cues other than pitch to perform the rate discrimination task or may reflect limitations in the electrically evoked neural excitation patterns presented to a rate pitch extraction mechanism.