A novel computational framework to model retinal electrical stimulation

Electrical stimulation of the retina can elicit visual perceptions in patients affected by certain types of retinal degeneration by activating some of the surviving neural cells. Even though retinal prostheses are now commercially available, many conceptual and technological issues remain to be clarified. Computational modelling has contributed in the past to optimize peripheral nerve, spinal cord, and deep brain electrical stimulation, and has the potential to provide a better understanding of how the retina responds to electrical stimulation, to improve the operation of existing implants, or to propose alternative designs. In this thesis, we developed a new computational framework to study retinal electrical stimulation. The electrical potential generated by the electrode current injection is computed through finite elements modelling (FEM) and used as input to a retinal network model where cells of different populations are modelled as point-neurons. The network model is used to compute a synaptic current, which can be applied, together with the extracellular field due to electrode current injection, to a retinal ganglion cell (RGC) morphological neuron model. Finally, the spiking activity of these RGC models is the information that reaches the visual cortex. Our model has shown the possibility to modulate smoothly the RGCs firing rate applying different stimulation currents. It replicates known phenomena, such as the production of elongated phosphenes, but also predicts that the effect of the different populations converging to RGCs cannot be ignored when modelling the response to retinal stimulation. Finally, we show how it is possible to model the response to electrical stimulation applied at different levels in the retinal tissue, and the effect that these choices have on the retinal network activation and on the retinal output in terms of RGC firing patterns. While our results need careful experimental validation, they pave the way for a better understanding of the interplay of the different cellular components implicated in retinal stimulation and thus to improve existing stimulation devices and techniques.