Eye movements of low-vision individuals impose limitation in assessment

Our investigations have yielded several characteristics of eye movement control in individuals with maculopathies which differ substantially from those found in normally sighted individuals. While ~70% of our subjects adopted one preferred viewing locus for fixation, 30% developed two or more eccentric loci. In addition, the variability of the eccentric fixation in these individuals increased with increasing scotoma size. For subjects with central scotomata ranging from 2° to 30° in diameter, 1–3° of uncertainty of retinal position would be common with standard visual field testing. In subjects with larger scotomata, shifts in fixation angle of >20° have been recorded. While mean drift velocity in these individuals (31 min of arc/s; range: 13-186 min of arc/s) was slightly larger than that recorded in normally sighted subjects with artificial scotomata (mean: 25 min of arc/s; range: 13-55 min of arc/s), more important was the relatively high drift velocities in particular individuals and/or drift direction which may position the scotoma in the path of the desired image. Our results reveal that accurate psychophysical testing, such as higher resolution visual fields or spatial contrast sensitivity testing of specific retinal loci, on individuals with maculopathies, necessitates image stabilization techniques.