Simulation of thalamic prosthetic vision: reading accuracy, speed, and acuity in sighted humans

Abstract

The psychophysics of reading with artificial sight has received increasing attention as visual prostheses are becoming a real possibility to restore useful function to the blind through the coarse, pseudo-pixelized vision they generate. Studies to date have focused on simulating retinal and cortical prostheses; here we extend that work to report on thalamic designs. This study examined the reading performance of normally sighted human subjects using a simulation of three thalamic visual prostheses that varied in phosphene count, to help understand the level of functional ability afforded by thalamic designs in a task of daily living. Reading accuracy, reading speed, and reading acuity of 20 subjects were measured as a function of letter size, using a task based on the MNREAD chart. Results showed that fluid reading was feasible with appropriate combinations of letter size and phosphene count, and performance degraded smoothly as font size was decreased, with an approximate doubling of phosphene count resulting in an increase of 0.2 logMAR in acuity. Results here were consistent with previous results from our laboratory. Results were also consistent with those from the literature, despite using naive subjects who were not trained on the simulator, in contrast to other reports.

Perceptual learning in a non-human primate model of artificial vision

“…Phosphene patterns were constructed to have visual field sampling characteristics taken from the LGN, i.e. analogous relationships between size and eccentricity, and density and position in the visual field12. Patterns were denser toward the point of regard and tapered off toward the periphery, corresponding to the acuity profile across LGN.…”

“…Custom software was created for stimulus display in addition to task control, and data collection1112. The monitor had a display area of 40 by 30 cm that subtended 53 by 40 degrees in the visual field.…”

“…Overall, glyphs were rendered on a virtual surface centered on the screen and static in space; the phosphene pattern was relocated to track the instantaneous gaze location and thus revealed different parts of glyphs as the animal looked around the screen. Details of this method are given in previous reports1112 and are summarized here. Gaze was tracked at a 500 Hz sampling rate with a video-based infrared eye tracking device (SensoMotoric Instruments, GmbH, model Hi-Speed Primate; accuracy: 0.25–0.5 degree, precision: <0.01 degree) using a pupil minus corneal reflection method, without temporal averaging.…”

“…While perceptual grouping has often been studied with simple stimuli such as gratings or lines910, an artificial vision based paradigm presents an excellent opportunity to understand learning of perceptual grouping, with potentially important translational benefits. We previously created and tested a gaze-contingent thalamic visual prosthesis simulation in humans for both individual letter recognition11 and reading12. In these two studies we assessed behavioral letter recognition and reading performance as a function of font size and phosphene count in tasks that required well-developed skill in perceptual grouping.…”

“…many months). We previously found that humans are capable of performing letter-based perceptual grouping tasks with no training1112 and exhibited learning within a single session11. Others have observed perceptual learning in humans over a small number of days with trial counts on the order of thousands2225.…”