Prototype retinal prosthesis shows promise in normal volunteers

Testing of an experimental retinal prosthesis in normal volunteers suggests that about 500 phosphenes, arranged over a 10- by 7-degree retinal area, provide the minimal amount of visual information required to restore useful function, said Marco Pelizzone, ScD, of the Eye Clinic, University Hospital of Geneva, Switzerland.

Dr. Pelizzone, a member of a consortium attempting to develop a retinal implant meeting these requirements, noted that visual perception with any retinal prosthesis would be severely limited. In addition, these devices will have a finite number of stimulation contacts that will limit image resolution. He and his colleagues studied the effects of these limitations using simulations of artificial vision on young, healthy subjects.

They assessed the performance of everyday tasks such as full-page reading, visuomotor coordination, and whole body mobility in various environments. Mobility was tested in a laboratory maze, but investigators decided to discontinue attempts at street crossing after deciding it was too dangerous, Dr. Pelizzone said. Image resolution, image eccentricity, and the effective visual field were varied during their experiments.

For images projected on a central visual field, they found that about 500 pixels are necessary for full-page reading, while about 400 pixels encoding an effective visual field of 6 by 12 degrees are sufficient for efficient visuomotor coordination. Only about 200 pixels encoding an effective visual field of 33 by 23 degrees are necessary for mobility in familiar environments, but a much higher amount is needed for the volunteers to feel safe in unfamiliar environments.

"The real question now is: Can we develop a device where we can effectively transmit this kind of information at the electric interface?" Dr. Pelizzone said.