Eye movements are a basic visual function and play an important role in acquiring visual information.
6 7 Prosthetic vision with an implant location outside the fovea resembles eccentric viewing in patients with a central scotoma, caused by conditions in which foveal vision is damaged but some peripheral vision remains functional. These patients often develop a retinal area outside the scotoma, called preferred retinal locus (PRL), to be used for fixation.
8 9 10 A PRL usually resides next to the scotoma and can be thought of as a “high-end” retinal prosthesis that is placed eccentrically and has spatial resolution lower than foveal vision but higher than the prosthetic vision made possible by current technology. Studies have shown that, through practice, patients with central scotoma and normally sighted subjects with a simulated central scotoma could direct their PRL using eye movements.
11 12 13 PRLs have the following characteristics. First, fixation with the PRL is less stable than foveal fixation, and it decreases with increasing eccentricity, resulting in jerky saccades for refixation.
14 Second, visually guided saccades with a PRL, compared with normal saccades, have longer latency, longer duration, slower velocity, and poorer accuracy.
12 Those saccades are usually undershot in proportion to target eccentricity.
12 For pursuit eye movements, one study on simulated central scotoma showed that spatial accuracy decreased with the increase of scotoma size or PRL eccentricity.
11 Those findings suggest that extrafoveal viewing with a retinal prosthesis affect oculomotor dynamics temporally and spatially. It has been reported that PRL above or below the fovea might offer better control of horizontal eye movements.
15 Studies of simulated prosthetic vision, though few, have shown effects of implant location on functional vision. For example, Sommerhalder et al.
16 17 studied the effects of implant location on text reading and found that, in simulated prosthetic vision subtending a 10° × 3.5° visual field with 286 phosphenes, subjects’ reading accuracy was approximately 90% when the visual field was 5° below foveal fixation but was reduced to approximately 25% to 30% when the visual field was 15° below foveal fixation.