Dynamic stimulation of visual cortex produces form vision in sighted and blind humans

A visual cortical prosthesis(VCP) has long been proposed as astrategy for restoringuseful vision to the blind, under the assumptionthat visual percepts of small spots of lightproduced withelectrical stimulation of visual cortex (phosphenes) will combine into coherent percepts of visual forms, like pixels on a video screen.We tested analternative strategy in which shapes were traced on the surface of visual cortex by stimulating electrodes in dynamic sequence. In both sighted and blind participants, dynamic stimulation enabled accurate recognition of letter shapespredicted by the brain’s spatial map ofthe visual world.Forms could be presented and recognized rapidlyby blind participants, up to 86 forms per minute.These findings demonstrate that a brainprosthetic can produce coherent perceptsof visualforms.

Methods
We investigated the use of new electrical stimulation paradigms to produce form perception in sighted and blind subjects. Traditional approaches to electrical stimulation of visual cortex have been restricted both due to the small number of electrodes implanted, and the limited way in which groups of electrodes were activated simultaneously in an attempt to produce form vision. In an attempt to overcome these restrictions and produce more effective form vision, we used two new techniques: 1) dynamic stimulation, in which a series of electrodes defining a form are rapidly activated in sequence rather than simultaneously, and 2) current steering, in which two electrodes located nearby are activated simultaneously at various current ratios to produce ‘virtual electrodes’ in between the physical electrodes.

Sighted subjects were epilepsy patients undergoing invasive monitoring in the epilepsy monitoring unit (EMU). In those subjects, we recorded receptive fields for each electrode, and screened each electrode to see which electrodes produced phosphenes when electrical stimulation was delivered. For each electrode that produced a phosphene, we determined the approximate threshold current required to produce a phosphene, and current values slightly above threshold were used when investigating dynamic pattern stimulation. Once electrodes producing phosphenes and threshold currents were established we picked various sets of electrodes to stimulate in dynamic stimulation patterns in an attempt to convey particular forms to the subject. We had the subjects give a verbal report of what they saw and make drawings on a touchscreen placed in front of them. In some subjects, we additionally had them perform alternative forced choice discrimination tasks in which we presented different dynamic stimulation sequences, corresponding to different perceived characters or letters, in random order, and the subject reported which character they perceived on each trial.

The blind subjects used in this study were one subject (BAA) who was implanted with a Neuropace stimulation device with 8 electrodes over their occipital lobe as part of an early feasibility assessment prior to the Orion Visual Cortical Prosthesis Clinical Trial sponsored by Second Sight Medical Inc. The other subject (03281) was one of six subjects enrolled in the actual Orion clinical trial, and this subject had an array of 60 electrodes implanted over their occipital lobe. In blind subjects, we tested each electrode to see which produced phosphenes when stimulated in isolation and at what current magnitudes. We then selected dynamic stimulation sequences to produce the percept of lines in visual space or of particular forms or characters. As with sighted subjects, subjects verbally described what they saw and we recorded drawings they made on a touchscreen in response to the various stimulation sequences. In addition, we conducted forced choice tasks in which the subjects discriminated between forms. In subject BAA, we also conducted current steering experiments in which nearby electrodes on the cortex were stimulated simultaneously at various current ratios to create ‘virtual electrodes’ in between the two physical electrodes. Finally, in subject BAA we also examined how quickly we could deliver dynamic electrical stimulation sequences to the subject to allow rapid discrimination between various forms.

A more detailed description of all methods is included in a separate file titled ‘FullMethods.pdf’.