Is it the retina? So many people are blind because of retinal damage/degradation, but have perfectly healthy optic nerves and brain function. Is there a way of injecting light or some type of converted electrical signal later down the chain or does it then become impossible for electrical signals to turn into what the brain uses? I have seen a vest that turns vibrations into signals the brain can process for deaf people and am wondering if the same can be done for blind people, just not sure how or where to inject the alternate signal. Neuroplasticity may allow for some alternate paths to be recognized and integrated.
If you're talking about light, the signal is transduced from electro-magnetic wave quanta (photons) to electro-chemical quanta (neurotransmitter vesicle release, action potential generation) in the retina. Different wavelengths of photons activate different receptors in photoreceptor cells (rods, cones, retinal ganglion cells), and all signals downstream can be considered electro-chemical. Topography of the retina is reflected in topography of the downstream neural substrates of the geniculate nuclei and primary visual cortex. Theoretically, one could bypass defective tissue with prosthetic devices that project image data topographically on downstream substrates through direct electrical stimulation. If using light, the substrate neural tissue would need to contain photoreceptors (rhodopsins for example) in order to respond to light.
There is good evidence in the auditory system from patients and animals implanted with cochlear implants that the brain's plasticity is able to refine signals from the implant for perceptual processing. For example, immediately following the procedure the individuals are tested on a test of auditory function, and then again after several weeks. Individuals get better as time passes, suggesting that circuits in the central auditory system are adjusting to the novel input patterns delivered by the cochlear implant. It seems that similar principles could be at play in the visual system. The optic nerve would be the equivalent of the vestibulocochlear nerve, where cochlear implants are inserted.
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