Designing sensory neural prostheses - lessons to learn from auditory brainstem implants

Abstract

After proving that stimulation threshold was lower with penetrating than with surface electrodes in animal testing we systematically investigated size, shape, surface depth and spatial orientation of the human cochlear nucleus in order to determine biological constraints for neuroeletronic interfaces to the auditory brainstem. We found that the terminal of auditory nerve fibres has a more complex outline and a wider inter-individual variability than previously estimated. It is boot-shaped from a lateral view and wedge-shaped from above. Slanted into the depth of the brainstem above the facial nerve entrance its rostral boundary is located more 7mm off the surface of the brainstem on average. Maximum extensions of the whole complex were 8.01×1.53×3.76 mm, the volume ratio between the ventral and dorsal part was about 1:5. In the absence of appropriate surface landmarks and imaging studies, to gain tonotopic access to the human cochlear nucleus with surface and depth electrode is a major challenge.