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.