Response of Retinal Ganglion Cells to Electrical Stimulation: From Prosthesis to “Seeing”

Prathima Sundaram (1), Koichiro Yamashita (2), Paul Yoo (3), Willy Wong (1,3)

  1. Department of Electrical & Computer Engineering, University of Toronto

  2. Graduate School of Natural Science and Technology, Okayama University

  3. Institute of Biomaterials and Biomedical Engineering

One of the leading causes of blindness is age-related macular degeneration which results in blurred vision due to the death of photoreceptors in the retina. Advances in engineering and technology show encouraging results for retinal prosthetics. One promising method is stimulating the visual pathway through electrical stimulation. In sub-retinal implants, this involves replacing the photoreceptors – which capture and convert light into an electrical signal that can be interpreted by the visual system. While retinal implants have made significant progress, there is still much we do not know about the mechanism behind how these implants restore vision. 
Collaborators at Okayama University have developed a photosensitive dye coupled to a thin polyethylene film that can generate an electric potential when exposed to incident light called the OUReP system. This film can be surgically inserted into the eye through a minimally invasive procedure. Animal studies conducted by the Okayama team show signs of restored vision. 
To understand how the retinal tissue is affected by electrical stimulation, a computational model was built to solve for the electric field generated by the retinal prosthesis. Using Maxwell’s equations, the model was solved numerically and then used to calculate the activation of the bipolar and ganglion cells which are critical components for generating vision in the periphery. The Hodgkin-Huxley equations and the cable model were used to investigate the initiation and propagation of the signal from the bipolar to the ganglion cell pathway. 
IBBME is a world leader in theoretical approaches to visual perception. We have been developing a mathematical approach which allows us to understand and calculate the peripheral response given a visual input. Comparing the ganglion response obtained through neurostimulation with the response in a normal functioning eye will provide an evaluation of how the visual prosthetics works. The model can be used to investigate the response to adaptation from constant stimuli as well as the detection of visual boundaries and edges.
The modelling results are expected to determine: 
· the activation of the retinal cells when stimulated by the OUReP system, 
· the behaviour of these cells when excited, 
· what the person will “see” 
This work details a first approach to modelling the entire pathway from stimulation to seeing. To our knowledge, this is the first attempt of its kind anywhere in the world.