Improving cochlear implant performance with new pulse shapes: a multidisciplinary approach

The cochlear implant (CI) is a device implanted into a severely hearing-impaired person’s inner ear to allow sound perception by electrically stimulating the auditory nerve. Although this technology has helped more than 600,000 people worldwide, most CI users still struggle in complex acoustic environments. One limitation is that the intracochlear electrodes are not directly attached to the auditory nerve fibers. As a consequence, each electrode activates not just target nerve fibers nearby, but also nerve fibers further away, considered off-target. The aim of this Ph.D. project was to investigate if a novel biophysically-inspired stimulation paradigm using ramped electric pulse shapes could optimize theelectrode-neuron interface. The hypotheses that ramped pulse shapes provide more (1) efficient and (2) focused stimulation compared to rectangular pulse shapes, used in most common CIs, were tested using a multidisciplinary approach.
The first study provided a detailed protocol and video for acute deafening and the cochlear implantation of an electrode array in a mouse and the functional recording of electric stimulation with electrically-evoked auditory brainstem response (eABR).
The second study presented the first physiological data on CI-stimulation with a ramped pulse shape. Using the established animal model and eABR recordings, the study demonstrated that less charge, but higher maximum current, was needed to evoke responses with ramped pulse shapes similar in amplitude to responses obtained with rectangular pulse shapes.
The third study examined psychophysical responses to ramped pulse pulses in human CI listeners. Less charge, but higher maximum current, was needed with ramped pulse shapes at threshold and most comfortable levels, in accordance with the results inmice. The impact of less charge, but higher maximum current, at the physiological and perceptual level warrants further investigation. Additionally, ramped and rectangular pulse shapes seemed to produce similar spread of excitations measured using a threshold profiling task.
In summary, the present findings show the benefits of charge-efficiency, but not spatial selectivity, with ramped pulse shapes relative to rectangular pulses. The results lay the foundation for further research on a CI stimulation strategy with ramped pulse shapes, which may lead to improved performance with CI’s in the future.