Retinotopy and cortical magnification

The orderly mapping of visual-field position onto retinal and cortical location that any prosthesis must respect: neighboring electrodes evoke neighboring phosphenes only if placed according to this map. In cortex the map is grossly non-uniform; the cortical magnification factor devotes a disproportionate area to the fovea, so central vision is represented at high spatial gain and the periphery is compressed.

For a cortical implant this means a fixed-pitch electrode array samples the visual field very unevenly: an array over foveal cortex yields fine, closely spaced phosphenes covering a small central field, while the same array over peripheral cortex spans a large field coarsely. Array placement therefore trades field of view against resolution.