Toward the development of a color visual prosthesis.

All of the human prosthetic visual systems implanted so far have been achromatic. Schmidt et al. (1996) reported that at low stimulation intensities their subject reported that phosphenes usually had a specific hue, but when the stimulus intensity was increased, they desaturated to white. We speculate here that previous B/W prosthetic systems were unnecessarily over-stimulating the visual cortex to obtain white phosphenes, which may be why unexpected alterations in phosphenes and seizures were not an uncommon occurrence. A color prosthesis would have the advantage of being elicited by lower levels of stimulation, reducing the probability of causing epileptogenic responses. A "hybrid" mode of stimulation is suggested, involving a combination of B/W and color stimulation, which could provide color information without reducing spatial resolution. Software implementation strategies are discussed, as are the advantages and challenges for possible color prosthetic systems.