The idea to study the properties of reading with simulated prosthetic vision is not new. Cha et al.,
13 simulating phosphene images that might be obtained from a visual prosthesis over the foveal projection in striate cortex, used a mesh of 1.62-arcmin openings covering a head-mounted video display, to determine the effects of dot number and spacing on reading speed for scrolled and stationary text. They used similar masks in a dual Purkinje eye tracker, to determine the effect of phosphene stabilization expected for a retinal or cortical prosthesis. They determined that a 1.7° mask of 32 × 32 dots provided essentially unimpaired reading, and that reading speed decreased when the number of dots was at or below 25 × 25. For scrolled text, there were additional impairing effects of dot stabilization and increased spacing with the number of dots at or below 16 × 16. More recently, Bagnoud et al.
14 simulated a retinal prosthesis by dividing a 10° × 10° field into abutting square dots and presented single letters and common four-letter words, stabilized on the retina at 0° to 20° eccentricity along the horizontal meridian. They also found that more than 400 (≤30 arcmin) and 1200 (≤18 arcmin) dots allow at least 80% correct word recognition at 0° and 10° eccentricity, respectively. At all eccentricities, letter recognition was accomplished with 6 to 7 dots/character width (charwidth). In a follow-up study, Sommerhalder et al.
15 studied letter and word recognition at the same eccentricities, in a rectangular (20° × 7° or 10° × 3.5°) area along the lower vertical meridian and reported a decrease in accuracy at and below 6 dots/charwidth, regardless of eccentricity, and a significant practice effect at 15° eccentricity. A further study by Sommerhalder et al.
16 demonstrated similar practice effects for full-page text reading at peripheral locations. The direction of gaze was used to scan a 10° × 7° reading window across the text, which consisted of a proportionally spaced font with 1.8° lowercase height, pixelized at approximately 5 pixels/char. In the latest study from the same laboratory,
17 the authors varied the dot profile, using circular Gaussian and square profiles, reasoning that the use of square pixels introduced edges with potentially high contrast, which would not occur between neighboring phosphenes. They also used a real-time filter, allowing the dots to move with the viewing window, but only did so for the square profile. They did not find a difference according to dot profile, but because they did not use the most natural condition (a Gaussian filter moving with the viewing window), they may have underestimated the benefit of smooth versus square profile dots. Most important, however, they presented text as black letters on a light background (i.e., as missing dots). Prosthesis designers (and presumably users) would prefer light text on a dark background, because this avoids potential problems of glare and stimulation overload, raises the effective contrast, and reduces implant power consumption.