FEASIBILITY OF A VISUAL PROSTHESIS FOR THE BLIND BASED ON INTRACORTICAL MICROSTIMULATION OF THE VISUAL-CORTEX

The feasibility of producing a visual prosthesis for the blind using i ntracortical microstimulation (ICMS) of the visual cortex was studied in a 42-year-old woman who had been totally blind for 22 years seconda ry to glaucoma. Thirty-eight microelectrodes were implanted in the rig ht visual cortex, near the occipital pole, for a period of 4 months. P ercepts reported as small spots of light, called phosphenes, were prod uced with 34 of the 38 implanted microelectrodes. Threshold currents f or phosphene generation with trains of biphasic pulses were as low as 1.9 mu A, and most of the microelectrodes had thresholds below 25 mu A . Phosphene brightness could be modified with stimulus amplitude, freq uency and pulse duration. Repeated stimulation over a period of minute s produced a gradual decrease in phosphene brightness. Phosphenes did not flicker. The apparent size of phosphenes ranged from a 'pin-point' to a 'nickel' (20 mm diameter coin) held at arm's length. Phosphene s ize usually decreased as stimulation current was increased but increas ed slightly as the train length (TL) was increased. At levels of stimu lation near threshold, the phosphenes were often reported to have colo urs. As the stimulation level was increased, the phosphenes generally became white, greyish or yellowish. Individual phosphenes appeared at different distances from the subject. When two phosphenes were simulta neously generated, the apparent distances of the individual phosphenes sometimes changed to make them appear to be at about the same distanc e. When three or more phosphenes were simultaneously generated, they b ecame coplanar Except for rave occasions, phosphenes extinguished rapi dly at the termination of the stimulation train. When stimulation TLs were increased beyond 1 s, phosphenes usually disappeared before the e nd of the train. The duration of phosphene perception could be increas ed by interrupting a long stimulation train with brief pauses in stimu lation. Intracortical microelectrodes spaced 500 mu m apart generated separate phosphenes, but microelectrodes spaced 250 mu m typically did not. This two-point resolution was about Jive times closer than has t ypically been achieved with surface stimulation. With some individual microelectrodes, a second closely spaced phosphene was sometimes produ ced by increasing the stimulation current Phosphenes moved with eye mo vements. When up to six phosphenes were simultaneously elicited, they all moved with the same relative orientation during eye movements. All phosphenes were located in the left hemi-field with the majority abov e the horizontal meridian There was a clustering of most of the phosph enes within a relatively small area of visual space. The potentially g reater microelectrode density and lower power requirements of ICMS com pared with surface stimulation appears encouraging for a visual prosth esis. However, further studies with blind subjects are required to opt imize stimulation parameters and test complex image recognition before the feasibility of a visual prosthesis based on ICMS can be establish ed.