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