Can potentials from the visual cortex be elicited electrically despite severe retinal degeneration and a markedly reduced electroretinogram?

Outer retinal degenerations can cause severe visual handicap. Specific treatment is lacking. There is good histologic evidence that even in the face of total photoreceptor loss the ganglion cells remain viable. This study evaluates the possibility of eliciting an evoked potential by electrical stimulation of rabbit eyes with experimentally induced outer retinal degenerations. Electrical stimulation using a bipolar contact-lens electrode was performed in normal rabbits as well as in rabbits with experimentally induced outer retinal degenerations. Outer retinal degenerations were induced by injecting intravenously either monoiodoacetic acid (IAA) or sodium iodate (NaIO3). After administration of IAA or NaIO3, the electroretinogram was absent or markedly reduced and, histologically, the photoreceptor layer was severely damaged. However, the electrically evoked visual cortical response could nonetheless be elicited. We conclude that electrical stimulation of the globe can elicit evoked potentials from the visual cortex despite severe outer retinal damage. These results provide support for future efforts toward testing the feasibility of bypassing damaged outer retina and electrically stimulating the inner retina of patients with profound visual loss from retinitis pigmentosa.