Authors: Francois Guerit1*; Iwan Roberts2; Chloe Swords2; Robert Carlyon1
1MRC Cognition and Brain Sciences Unit, University of Cambridge
2Department of Clinical Neurosciences, University of Cambridge
Background Cochlear Implant (CI) users struggle when listening to speech under noisy conditions. This has long been attributed to channel interactions: a given neuron not only responds to electrical stimulation from the closest electrode (“channel”), but is also influenced by electrodes located further away. Shaping the stimulation voltage into a very sharp profile (e.g. tripolar) has been proposed as a solution, with mixed outcomes. This reflects that the outputs of adjacent CI channels are anyway highly correlated, and that some neural spread of excitation is necessary for sufficient loudness. Here, we pilot a Hilltop approach, where the voltage is shaped by a Gaussian function to be broad near a given stimulation electrode, but reduced by several orders of magnitude more than 3-4 electrodes away.
Method In each participant (N = 7, Advanced Bionics), we measure the stimulation voltage for all possible combinations of stimulating and recording electrodes. Using a constrained least-squares optimization, we obtain the current amplitude and polarity to be injected at each electrode to produce the desired Hilltop voltage patterns (HT2, HT3 and HT4, see figure 1). Six participants loudness balanced these Hilltop stimuli to a monopolar (MP) stimulus, for three different centre electrodes. We also measured charge summation as a function of inter-channel spacing in five participants, again comparing MP to Hilltop. Ongoing experiments are investigating channel pitch ranking.
Results When loudness-balanced to monopolar stimulation, Hilltop produced large voltage differences compared to monopolar stimulation (figure 1A, more than 3 electrodes away from electrode 8). Charge summation results indicate a possible effective polarity reversal along the electrode array with Hilltop stimulation, which we are investigating with a 3D finite-element model as well as a physical model of the cochlea. We expect the Hilltop approach to provide insights into the optimisation of electrical stimulation with CIs.