A reaction time approach to size constancy and visual illusions

Size constancy is a property of the visual perceptural system that can keep relatively constant the perceived size of an object despite changes of the size of the retinal image with distance. The relationship between size and reaction time (RT) is well known: RT is faster in response to larger than smaller stimuli. Here i Wanted to verify whether this effect depends upon retinal or perceived size, by changing both stimulus size and the observer-object distance. I found that when the size of stimuli positioned at a different distance was adjusted so as to subtend the same retinal image, the larger stimuli were responded to faster than the smaller ones. This effect can be attributed to size constancy affecting RT. In keepeing with that, when size constancy cues were removed by using a pinhole for stimulus viewing, RT reflected stimulus retinal size only. In further experiments I employed visual illusion, such as Ponzo's and Ebbinghaus' to induce misperception of size. In accord with the size constancy result, stimuli perceived as larger were responded to faster than those perceived as smaller, despite retinal size was the same. In sum, all these results demonstrate that simple RT reflects perceiver rather than retinal stimulus size.