Bilateral changes in cell density within the central auditory pathway upon chronic unilateral intra-cochlear stimulation

Cochlear implants have been applied successfully for the treatment of unilateral hearing loss with quite a surprising benefit. One reason for this successful treatment could be the occurrence of neuroplastic changes within the central auditory pathway upon a bimodal stimulation. Important parameters of the electro-stimulation which could trigger neuroplastic changes are largely unknown. The present study, therefore, investigated at a cellular level, the effect of different stimulation rates and intensities on key structures of the central auditory pathway. Normal-hearing guinea pigs were mechanically single-sided deafened through a standard HiFocus1j electrode array being inserted into the first turn of the cochlea. Four to five electrode contacts were available for stimulation. After eCAP-threshold based speech processor fitting, three experimental groups were stimulated 16 hours per day for 90 days. A HiRes ® -strategy, based on one of three stimulation rates, low-rate 275 pps/ch, mid-rate 1500 pps/ch, or high-rate 5000 pps/ch was used, with the animals living in a standardised free field auditory environment. Afterwards, the cell density was determined in key structures of the auditory pathway. Results were compared to those of unilateral implanted but not stimulated controls. A bilateral conservation of all the brain structures investigated was found in the low- and high-rate groups. A significant cell loss was observed in the mid-rate group. This group also showed the highest mean stimulation current. Unilateral intra-cochlear electrical-stimulation leads to bilateral central nervous changes which correlate with the stimulation current applied rather than the stimulation rate applied.