Retinal translocation of PRL under different lighting conditions in patients with central scotomas

Purpose. We noticed that some patients with central scotomas consistently and reliably used two different PRLs (preferred retinal loci) when target brightness was changed. This study investigates the characteristics of this adaptation to central scotoma phenomena. Methods. 17 patients (20 eyes) were investigated including 13 AMD patients (16 eyes), 2 POHS patients (2 eyes), and 2 macular hole patients (2 eyes). Most patients had subfoveal neovascular membranes. Dense and threshold macular scotoma borders were mapped using scanning laser ophthalmoscope (SLO) macular perimetry. PRL retinal locations and visual task abilities with changing target illuminance were tested with the SLO including fixation stability, pursuit ability, saccade ability. Threshold light sensitivity and acuity for both PRLs were assessed with the SLO. Results. In all eyes, a bright target exhibited a PRL close to the dense scotoma with typical low vision abilities in visual tasks (PRLhigh). When the target brightness was decreased, the PRL retinal location would shift suddenly close to the largest relative scotoma (PRLlow). In all eyes, PRLhigh had better visual acuity capability than PRLlow while PRLlow had better light sensitivity. Both PRLhigh and PRLlow exhibited similar abilities for all other visual tasks in 19 eyes while one patient had similar abilities between PRLhigh and PRLlow except for fixation stability. Conclusions. The retinal location of the PRL is critically dependent on the brightness of objects used in visual tasks. The visual system's adaptation to central scotomas has the ability (plasticity) to develop two well defined PRLs. The visual system prefers to use the PRL with better acuity (and possibly other better capabilities) when possible. However, when the stimulus light level is below the PRLhigh threshold, the system will shift to (not gradually slide to) the second PRL it must use for the lowest light level. Rehabilitation and treatment strategies need to consider the possible existence of multiple PRLs.