Abstract

Purpose: Appropriate delivery of electrical stimulation to visual structures can evoke patterned sensations of light, called technically phosphenes. This pivotal finding settled the physiological basis for present efforts to develop a visual prosthesis for the blind. Here we aim to study the visual perceptions elicited by electrical stimulation of human visual cortex.

Methods: Electrical stimulation of occipital cortex was performed in patients with a diagnosis of intractable epilepsy that had to undergo a surgical resection. Electrical stimulation was applied through implanted subdural electrodes or using penetrating micro-electrodes. Electrical stimulation was applied using a biphasic neurostimulator. For mapping the visual perceptions we used a wireless system including an autofocus infrared (IR) camera and one IR projector. After each electrical stimulation, subjects were asked to make drawings of the perceptions with particular emphasis on the size, main features and localization within the visual field. A customized program allowed easy registration and analysis of collected data.

Results: All study subjects perceived phosphenes and tolerated the procedure without complications. The probability of detecting phosphenes or scotoma varied with the position of the electrodes. Most of the phosphenes were circular or dotted and appeared in the visual field contralateral to the cerebral hemisphere stimulated. Stimulation of early visual areas induced visual perceptions but stimulation of extrastriate occipital cortex was also able to induce phosphenes with retinotopic representation. Furthermore our procedure allowed an easy calculation of the position and area of the subjective perceptions from the coordinates of the drawings.

Conclusions: Electrical stimulation of visual areas in humans provides a unique opportunity to study the qualitative properties of induced perceptions, which can offer insights about the functional organization of human visual cortex and help to the development new rehabilitative strategies for profoundly blind based on multiple cortical microelectrodes.