Electrode pitch ranking was assessed as a function of cochlear region, electrode spacing, and stimulation rate in eight 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 and provides a wide electrode spacing of 2.4 mm.
The pitch of a reference electrode was compared with the pitch of two apical and two basal probe electrodes using a two-alternative forced-choice procedure. The reference electrodes were placed in the apical region, in the middle region, and in the basal region of the cochlea. Each condition was tested at pulse rates of 1515 pps, 500 pps, and 250 pps.
Electrode pitch ranking did not depend on electrode spacing, the position of the reference electrode in the cochlea, or stimulation rate.
These results indicate that a spacing of 2.4 mm seems sufficient to allow good electrode pitch ranking performance for the average user and that electrode pitch ranking is similar in the apical, middle, and basal region of the cochlea, irrespective of the stimulation rate. Most importantly, the results of this study indicate that residual neural structures in the apical region of the cochlea are sensitive to pitch changes provided by different places of electrical stimulation.