Abstract:
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 compares 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 minimized (burst mode) and a
second in which they were maximized (spread mode). PRDLs were measured at base
rates of 100 pps 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 of 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.
