Pitch perception and signal processing in electric hearing

A study comprised of six hearing experiments was conducted in order
to investigate parameters to influence the pitch perception
elicited by direct electric stimulation of the auditory nerve. In
addition, a new stimulation strategy for the cochlear implant COMBI
40+ (MED-EL, Innsbruck, Austria) was developed and tested. The
results derived from a total number of 16 subjects reveal a
dominating influence of the place of stimulation in contrast to the
rate of stimulation on pitch perception. It was shown that the
electrode distance of 2.4 mm for this device is sufficient to allow
discriminable electrodes in pitch along the whole array. The
influence of stimulation rate on pitch is limited to pulse rates up
to about 300 pps. Within this range, the just noticeable change of
pitch elicited by pulse rate as well as modulation rate amounts to
about 25% of the base rate. In addition it was observed that the
sound quality increases with increasing pulse rate up to about 566
pps independent of electrode location. Subjects with residual
hearing at the non-implanted ear revealed that the pitch elicited
by the most apical electrode depends on the insertion depth of the
array and is linearly increasing with electrode location (40
Hz/mm). The results of the hearing experiments were implemented to
modify the well known CIS strategy. The new development (termed
RateCIS) was designed in order to increase the amount of
transmitted spectral information, thus the number of effective
channels. Six electrodes were selected to switch adaptively between
a high stimulation rate (1515 pps) and a low stimulation rate (252
pps). A test of the RateCIS strategy showed that results for speech
recognition are comparable to the CIS strategy. The RateCIS
strategy was subjectively preferred by some of the subjects
although the majority preferred the CIS strategy for speech
recognition and sound quality. Concerning the recognition and
appraisal of music however, the RateCIS strategy was preferred by
the majority of subjects. Regarding the fact, that the tests were
conducted during one day without time for adaptation to the new
signal processing, the RateCIS strategy could serve as an
interesting option especially for music appraisal.