Annals of Biomedical Engineering, VoL 20, pp. 439-449, 1992 0090-6964/92 $5.00 + .00 Printed in the USA. All rights reserved. Copyright 1992 Pergamon Press Ltd. Simulation of a Phosphene-Based Visual Field: Visual Acuity in a Pixelized Vision System Kichul Cha, K e n n e t h H o r c h , and R i c h a r d A. N o r m a n n Department of Bioengineering University of Utah Salt Lake City, UT (Received 1/4/91 Revised 6/27/91) A visual prosthesis for the blind using electrical stimulation o f the visual cortex will require the development o f an array o f electrodes. Passage o f current through these electrodes is expected to create a visual image made up o f a matrix o f discrete phosphenes. The quality o f the visual sense thus provided will be a function o f many parameters, particularly the number o f electrodes and their spacing. We are conduct- ing a series o f psychophysical experiments with a portable "phosphene" simulator to obtain estimates o f suitable values f o r electrode number and spacing. The simulator consists o f a small video camera and monitor worn by a normally sighted human sub- ject. To simulate a discrete phosphene field, the monitor is masked by an opaque per- forated film. The visual angle subtended by images from the masked monitor is 1.7 ~ or less, depending on the mask, and falls within the fovea of the subject. In the study presented here, we measured visual acuity as a function o f the number o f pixels and their spacing in the mask. Visual acuity was inversely proportional to pixel density, and trained subjects could achieve about 20/26 visual acuity with a 1024 pixel image. We conclude that 625 electrodes implanted in a I cm by I cm area near the foveal rep- resentation o f the visual cortex should produce a phosphene image with a visual acu- ity o f approximately 20/30. Such an acuity could provide useful restoration o f functional vision f o r the profoundly blind. Keywords-Phosphene simulation, Visual prosthesis, Visual acuity. I N T R O D U C T I O N Punctate electrical stimulation of the visual cortex evokes the sensation of a spot of light (phosphene) in the visual field. This suggests that a limited but functional vi- sual sense might be created by electrical stimulation of the visual cortex with an ar- ray of electrodes. Feasibility studies of this concept have been performed by Brindley and Lewis (6) and Dobelle and coworkers (11,13). They stimulated the surface of the human visual cortex with an array of electrodes and found that the resulting phos- phenes could produce useful visual patterns and provide a higher rate of information transfer than that obtained by tactile transmission. For example, a blind volunteer was able to read "braille" transmitted by cortical electrical stimulation much faster than Acknowledgments--This project was supported by the W.M. Keck Foundation. Address correspondence to Dr. Ken Horch, Department of Bioengineering, 2480 MEB, University of Utah, Salt Lake City, UT 84112. 439

440 K. Cha, K. Horch, and R.A. Normann he could read tactile braille (13), a significant observation because slow transmission rates have been a serious problem in visual aid devices which act via other sensory mo- dalities. However the electrodes used in these studies were positioned on the surface of the cortex and failed to produce images that were useful for functional restoration of vision because of the limited number of phosphenes and their irregular, noncon- tiguous distribution. Electrode spacing limits the total number of electrodes which can be implanted in a given area of cortex. The number and spacing of the electrodes determine the acu- ity and the scope of the resulting visual sense. Based on these considerations, a func- tionally useful visual prosthesis would require an array of numerous, closely spaced electrodes. Stimulation of the visual cortex with penetrating electrodes can evoke phosphenes with less current than is required with surface stimulation (1,3,10), producing more focal excitation of neurons and allowing electrodes to be placed closer together. We have been able to produce an array of closely spaced electrodes which penetrate the visual cortex as a multichannel, silicon-based microstructure (7,23,29). In order to develop a visual prosthesis based on penetrating cortical electrodes, it is essential to know how many electrodes are needed and how closely spaced they should be in order to achieve a given quality of visual sensation. We are addressing this issue by conducting psychophysical experiments with a phosphene simulator which optically creates visual sensations similar to those expected to be produced by intracortical stimulation of the foveal projection in the brain. This paper describes the apparatus and presents data from the first of our stud- ies: determining spatial resolution (visual acuity) as a function of pixel number and spacing. The results indicate that an array of 625 electrodes spaced approximately 400/~m apart could provide good acuity when implanted in foveal cortex. M E T H O D S Phosphene Simulator The device was designed to optically simulate the visual sensation of a field of punctate phosphenes similar to that produced by electrical stimulation of the visual cortex with an array of penetrating electrodes. Figure 1 is a diagram of the optics used / / TmrgeE C~mer~ PDosp.hen~ Simu/aLor F i e l d Eye Mosk L e n s e s Men 1 L o t FIGURE 1. Simulator optics. The subject views a small video monitor which projects an image encoded by a miniature video camera, A perforated mask placed over the monitor fragments the image into discrete pixels, which are sized below the limit of visual resolution for the subjects. Lenses adjust the size of the projected image so that it subtends an angle of 1.7 ~ with the largest mask. The acceptance angle of the camera can be adjusted by external lenses.