Channel Interaction During Infrared Light Stimulation in the Cochlea

Abstract

Background and Objectives The number of perceptually independent channels to encode acoustic information is limited in contemporary cochlear implants (CIs) because of the current spread in the tissue. It has been suggested that neighboring electrodes have to be separated in humans by a distance of more than 2 mm to eliminate significant overlap of the electric current fields and subsequent interaction between the channels. It has also been argued that an increase in the number of independent channels could improve CI user performance in challenging listening environments, such as speech in noise, tonal languages, or music perception. Optical stimulation has been suggested as an alternative modality for neural stimulation because it is spatially selective. This study reports the results of experiments designed to quantify the interaction between neighboring optical sources in the cochlea during stimulation with infrared radiation. Study Design/Materials and Methods In seven adult albino guinea pigs, a forward masking method was used to quantify the interaction between two neighboring optical sources during stimulation. Two optical fibers were placed through cochleostomies into the scala tympani of the basal cochlear turn. The radiation beams were directed towards different neuron populations along the spiral ganglion. Optically evoked compound action potentials were recorded for different radiant energies and distances between the optical fibers. The outcome measure was the radiant energy of a masker pulse delivered 3 milliseconds before a probe pulse to reduce the response evoked by the probe pulse by 3 dB. Results were compared for different distances between the fibers placed along the cochlea. Results The energy required to reduce the probe's response by 3 dB increased by 20.4 dB/mm and by 26.0 dB/octave. The inhibition was symmetrical for the masker placed basal to the probe (base‐to‐apex) and the masker placed apical to the probe (apex‐to‐base). Conclusion The interaction between neighboring optical sources during infrared laser stimulation is less than the interaction between neighboring electrical contacts during electrical stimulation. Previously published data for electrical stimulation reported an average current spread in human and cat cochleae of 2.8 dB/mm. With the increased number of independent channels for optical stimulation, it is anticipated that speech and music performance will improve. Lasers Surg. Med. © 2020 Wiley Periodicals LLC

Towards an optical cochlear implant

“…In other words, assuming the length over which the electrodes are distributed in contemporary CI electrodes, about 8 (48/5.9) independent channels are expected [56]. This correlates well with the values reported in the literature [57,58].…”

“…Power calculations are based on the long term behavioral study in cats [40] and the experiments on channel interaction during optical stimulation in guinea pigs [56]. During the behavioral study, responses were obtained with a 200 pulses/s repetition rate, a 100 µs pulse length, and 12 µJ radiant energy.…”

Targeted Optical Neural Stimulation: A New Era for Personalized Medicine

“…The Richter group has worked to exploit the specificity of INS for implantable cochlear prosthetics (Izzo and others 2006). Agarwal and others has reported that the number of optical channels can be twice that of electrical channels, providing promise for greater number of spectral bands and ability to recognize pitch (Agarwal and others 2021). In 2010, a patent (Walsh and others 2010) was approved for a set of implantable optical stimulators of the auditory nerve containing multiple light sources.…”

Infrared neuromodulation—a review

“…The advantage of using INM over electrical stimulation is that due to its improved spatial selectivity, the optical light delivery is not limited by the same minimum spacing as for electrodes before channel interaction sets in. This can enable a better acoustic resolution by stimulating more independent channels in parallel (associated with more spectral bands), thus improving the richness and clarity of the auditory experience, especially in noisy environments, and for music perception [167].…”

“…The multi-channel excitation requires close placement of the light sources, which determines the frequency resolution of the auditory response. For such a light delivery system, various options for the optrodes based on fiber, waveguides, arrays of light sources are currently considered to induce a 360 • coverage in a compact size [163,167]. Recently, for a practical implementation, an implantable optrode design based on side-emitting laser diodes was demonstrated [163].…”