Simulation of thalamic prosthetic vision: reading accuracy, speed, and acuity in sighted humans

1. An array of radio receivers, connected to electrodes in contact with the occipital pole of the right cerebral hemisphere, has been implanted into a 52-year-old blind patient. By giving appropriate radio signals, the patient can be caused to experience sensations of light (;phosphenes') in the left half of the visual field.2. The sensation caused by stimulation through a single electrode is commonly a single very small spot of white light at a constant position in the visual field; but for some electrodes it is two or several such spots, or a small cloud.3. For weak stimuli the map of the visual field on the cortex agrees roughly with the classical maps of Holmes and others derived from war wounds. With stronger stimuli, additional phosphenes appear; these follow a map that is roughly the classical map inverted about the horizontal meridian.4. The phosphenes produced by stimulation through electrodes 2.4 mm apart can be easily distinguished. By stimulation through several electrodes simultaneously, the patient can be caused to see predictable simple patterns.5. The effects of the duration and frequency of stimulating pulses on the threshold have been explored.6. For cortical phosphenes there is no sharp flicker fusion frequency, and probably no flicker fusion frequency at all.7. During voluntary eye movements, the phosphenes move with the eyes. During vestibular reflex eye movements they remain fixed in space.8. Phosphenes ordinarily cease immediately when stimulation ceases, but after strong stimulation they sometimes persist for up to 2 min.9. Our findings strongly suggest that it will be possible, by improving our prototype, to make a useful prosthesis.

A retinal prosthesis was permanently implanted in the eye of a completely blind test subject. This report details the results from the first 10 weeks of testing with the implant subject. The implanted device included an extraocular case to hold electronics, an intraocular electrode array (platinum disks, 4 x 4 arrangement) designed to interface with the retina, and a cable to connect the electronics case to the electrode array. The subject was able to see perceptions of light (spots) on all 16 electrodes of the array. In addition, the subject was able to use a camera to detect the presence or absence of ambient light, to detect motion, and to recognize simple shapes.

The feasibility of producing a visual prosthesis for the blind using intracortical microstimulation (ICMS) of the visual cortex was studied in a 42-year-old woman who had been totally blind for 22 years secondary to glaucoma. Thirty-eight microelectrodes were implanted in the right visual cortex, near the occipital pole, for a period of 4 months. Percepts reported as small spots of light, called phosphenes, were produced with 34 of the 38 implanted microelectrodes. Threshold currents for phosphene generation with trains of biphasic pulses were as low as 1.9 microA, and most of the microelectrodes had thresholds below 25 microA. Phosphene brightness could be modified with stimulus amplitude, frequency and pulse duration. Repeated stimulation over a period of minutes 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 size usually decreased as stimulation current was increased but increased slightly as the train length (TL) was increased. At levels of stimulation near threshold, the phosphenes were often reported to have colours. 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 simultaneously generated, the apparent distances of the individual phosphenes sometimes changed to make them appear to be at about the same distance. When three or more phosphenes were simultaneously generated, they became coplanar. Except for rare occasions, phosphenes extinguished rapidly at the termination of the stimulation train. When stimulation TLs were increased beyond 1 s, phosphenes usually disappeared before the end of the train. The duration of phosphene perception could be increased by interrupting a long stimulation train with brief pauses in stimulation. Intracortical microelectrodes spaced 500 microns apart generated separate phosphenes, but microelectrodes spaced 250 microns typically did not. This two-point resolution was about five times closer than has typically been achieved with surface stimulation. With some individual microelectrodes, a second closely spaced phosphene was sometimes produced by increasing the stimulation current. Phosphenes moved with eye movements. 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 above the horizontal meridian. There was a clustering of most of the phosphenes within a relatively small area of visual space. The potentially greater microelectrode density and lower power requirements of ICMS compared with surface stimulation appears encouraging for a visual prosthesis. However, further studies with blind subjects are required to optimize stimulation parameters and test complex image recognition before the feasibility of a visual prosthesis based on ICMS can be established.