Electrical stimulation of the cerebral visual system in man.

In patients undergoing thalamotomy for intractable pain, multi-contact platinum electrodes were stereotaxically implanted so that the striate cortex and geniculocalcarine tract could be stimulated to evaluate the feasibility of a visual prosthesis. The parameters of stimulation were 50–300 Hz, 0.1–1 msec pulse duration and 0.1–5 mA/phase. Phosphenes were evoked in 7 out of the 17 cases studied. The projection in space of the phosphene depended upon the site of stimulation. The configuration, color and brightness of the phosphene varied from a simple white or colored light to occasionally a multicolored complex pattern. Some phosphenes appeared to blink or flutter, some were stable in position, others appeared to move away radially from the fixation point. Histological studies on 6 of the brains suggested that the phosphenes were more often evoked from the striate cortex than from the geniculocalcarine fibers.