The amount of visual information required for the restoration of elementary reading abilities, using a retinal implant in patients with external retinal dystrophy. Psychophysical study

Definition of the problem: Our goal is to determine the minimum of informat ion necessary for elementary reading, using a retinal implant. This concern s particularly the fragmentation (pixellisation) of the presented image and its position in the visual field. Fragmentation corresponds to the number of electrodes available, the position of the image in the visual field is e quivalent to the site of the implant on the retina. Material and methods: 10 degrees x 10 degrees windows, containing isolated words or letters, were presented to six healthy subjects on a computer scre en. A coupling between the computer and an eye traclter stabilizes these im ages in an area of the visual field. This coupling constantly corrects the position of the image on the screen according to the direction of gaze. Results: 1) A rapid decrease of the performance is observed at a certain th reshold of pixellisation, dependent on the eccentricity of presentation of the images. 2) In central vision, about 400 pixels are sufficient to recogn ize 80% of the four-letters words. At 100 of eccentricity, about 1225 pixel s are needed. 3) An acceptable comprehension of a text (identification of f our words out of five), is impossible at eccentricities higher than 100 4) About 50 pixels are sufficient for a satisfactory recognition of isolated l etters, independently of their eccentricity. Conclusion: These data validate the method of investigation and provide val uable indications regarding minimal visual requirements in prosthetic visio n.