Size constancy following long-term visual deprivation

Abstract

We can estimate the veridical size of nearby objects reasonably well irrespective of their viewing distance. This perceptual capability, termed size constancy, is accomplished by combining information about retinal image size together with the viewing distance, or using the relational information available in the scene, via direct perception [1]. A previous study [2] showed that children typically underestimate the size of a distant object. This underestimation is reduced with time, suggesting that years of visual experience may be essential for attaining true size constancy. But what if you have had very limited vision during the early years of life? We studied 23 Ethiopian children suffering from bilateral, early-onset cataract, who were surgically treated only years after birth. Surprisingly, most children were able to estimate object size reasonably well irrespective of distance; in fact, they usually tended to overestimate the far-object size. Closer examination indicated that, although before surgery the patients were diagnosed as having a full, mature bilateral cataract, they nevertheless had some residual form of vision, typically limited to very close range. Gandhi et al.[3] earlier reported immediate susceptibility to geometric visual illusions in a similar group of newly-sighted children, concluding that size constancy was probably innate. We suggest that their immediate ability to judge physical size irrespective of distance is more likely to result from their previous visual experience.