Computational modelling of the suppression of optic nerve fibre
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Retinal neuroprostheses aim to restore vision in degenerated retina but face challenges with low selectivity, where electric current activates both intended and neighbouring neurons. It was hypothesized that optic nerve suppression could improve selectivity using frequency-induced neuromodulation (FIN) with sinusoidal waveforms to selectively block transmission based on optic nerve fibre diameter. To explore the electrical parameters for diameter-based selective suppression, computational models of retinal ganglion cells (RGCs) and the optic nerve were given sinusoidal currents between 25-10000 Hz. FIN was found to induce complete conduction block, with the highest probability of suppression at frequencies above 500 Hz. Additionally, axon fibre diameter influenced the frequency and amplitude of FIN that inhibited conduction, with the highest selectivity between ON and OFF RGC fibres occurring at a diameter of 0.5 µm. Nodal sodium channels were involved in inhibition at frequencies above 500 Hz, supporting findings from studies of peripheral nerve fibres. Overall, suppressing a subset of optic nerve fibres could potentially enhance the selectivity of a bionic eye system by filtering out confounding visual information. However, practical application may be hindered by variability in RGC morphology and biophysical properties, and the possibility of filtering intended stimuli.
Author summary
Retinal neuroprostheses deliver electrical stimulation to retinal cells to produce perceptions of vision called phosphenes. However, the lack of selectivity associated with electrical currents leads to unnatural cell activations, which in turn limits the usability of many visual prostheses. To improve selectivity, we proposed a novel method that involves suppressing the conduction of action potentials along a subset of axon fibres, which carry the information from the eye to the brain’s visual centre. We believe that this could remove superfluous information caused by electrical stimulations. Our findings demonstrated that sinusoidal electrical currents can be given to the optic nerve fibres to induce suppression, and that the effectiveness of suppression is influenced by axon fibre diameters. This suggests that modulating the response of the optic nerve can lead to selective suppression based on the axon fibre diameters, and consequently improving the performance of a retinal neuroprosthesis.
