WHEN THE visual cortex in the occipital lobe is electrically stimulated, human subjects see circumscribed and often punctate sensations of light, called phosphenes. Several groups have investigated the possibility of using stimulation with electrodes placed on the surface of the cortex to develop a prosthesis for the blind that might be of value in reading and mobility (BRINDLEY and LEWIN, 1968; BRINDLEY, 1973; 1982; DOBELLE and MLADEJOVSKY, 1974; DOBELLE et al., 1976). Problems with surface stimulation include high currents for eliciting phosphenes, interactions between phosphenes generated by simultaneously stimulated electrodes, and occasional persistence of phosphenes following cessation of stimulation. Because non-human primates have been able to detect significantly lower currents with intracortical stimulation of the visual cortex (BARTLETT and DOTY, 1980), we wanted to validate these findings in awake humans and to determine whether the percepts so evoked are suitable for a functional prosthesis. We found that electrical current thresholds for producing phosphenes by intracortical microstimulation are 10-100 times lower than those produced by stimulation with nonpenetrating cortical surface electrodes, that phosphenes can be resolved with simultaneously stimulated electrodes as close as 700# m, and that the phosphenes have simple forms that are stable and predictable. These results are encouraging for the possibility of a visual prosthesis based on intracortical microstimulation.