Size constancy in visual impairment: a comparison with normally sighted individuals.

Size judgements are usually based on size constancy. However, as the number of visual cues decreases, such as by decreasing the field of view, size judgements gradually change from those based on size constancy to those based on the visual angle. The present study investigated whether visually impaired (VI) individuals, most of whom do not have measurable stereoacuity and have visual field defects, might therefore make judgements based on visual angle rather than size constancy. Size constancy was measured on 42 VI and 20 normally sighted subjects. Results indicated that size judgements are similar for both VI and normally sighted individuals at 0.50 m, 1 and 2 m. Statistically significant differences were found at 3 m. Despite this it would appear that VI individuals are able to make judgements based on size constancy rather than the visual angle.