Phosphene mapping strategies for cortical visual prothesis recipients

Purpose. To develop strategies allowing phosphenes from implanted cortical electrodes to be mapped in visual space. Accurate, individualized phosphene maps are essential to derive the inverse maps needed to present recognizable images to visual prosthesis recipients. Methods. Flashing dots were presented to normally-sighted observers in 32-128 positions randomly chosen in the central 18 of one visual hemi-field. Gaze maintenance on a steady fixation point was monitored, and test dots blanked if the gaze deviated by more than 0.5 . Three different mapping techniques were investigated, using the same set of dots in each test: 1. Touchscreen method, in which the subject traced the index finger from the direction of gaze to the direction of the flashing dot; 2. Eye movement method, in which the flashing dot and fixation point disappeared, and the subject then moved his/her gaze to the remembered location of the flashing dot; 3. Triadic distance comparison method, in which the subject indicated which pairs among three sequentially presented dots were closest and farthest, and a multidimensional scaling technique was used to reconstruct relative dot locations. All tests were performed three times, thus far by 4 subjects. Results. Touchscreen and eye movement tests had better relative accuracy with increasing eccentricity, but their reproducibility was poor (up to 25% variability). Triadic comparison test performance was better on clusters of more closely spaced phosphenes than on distant clusters. A distortion metric was constructed by comparing distance estimation errors for all possible phosphene pairs across tests. Three subjects had distortions scores under 15% for all tests; combining maps across tests reduced the errors below 10%, enabling adequate image recognition to naive viewers. Conclusion. These procedures, especially in combination, permit construction of distortion maps with sufficient fidelity to enable image presentation to future prosthesis wearers.