Simultaneous and non-simultaneous dual electrode stimulation in cochlear implants: evidence for two neural response modalities

Conclusion: There are two modalities of dual electrode stimulation: a shifting, continuous excitation, which is the desired effect, and a split excitation with considerable variation in loudness. The first one most likely occurs in the basal turn, with adjacent contacts, stimulated simultaneously rather than sequentially. Objectives: This study examines the effects on place pitch and loudness of simultaneous current steering and sequential stimulation. These can give cochlear implant patients access to more perceptual channels than physical contacts in the electrode array. Materials and methods: For both lateral wall and perimodiolar electrodes, simultaneous current steering as well as sequential stimulation, place pitch and loudness of the percept were predicted with a computational model of the implanted human cochlea. The loudness predictions were validated with psychophysical loudness balancing experiments. Results: Simultaneous stimulation with adjacent electrode contacts in the basal end of the cochlea was generally able to produce a single, gradually shifting intermediate pitch percept. Simultaneous stimulation beyond the first cochlear turn, sequential stimulation and simultaneous stimulation with non-adjacent electrode contacts often produced two regions of excitation. In the case of sequential stimulation the total amount of current to reach most comfortable loudness was raised, both in the model and in the patients.