Emmert's law in the dark: active and passive proprioceptive effects on positive visual afterimages

The relationship between apparent size and apparent distance is given by Em mert's law, which states that a retinal image is proportional in size to th e distance of the surface it is projected upon. This principle also applies to retinal afterimages in that they, too, will change in apparent size if distance cues suggest that the location of the object projected onto the re tinal image has been altered. It has also been known for some time that non -retinal cues can produce quantitative and qualitative effects on an afteri mage when it is viewed in the dark. In the present two studies, positive af terimages of an observer's hand, as well as objects held by that hand, were used as targets to investigate the effects on size-constancy scaling of mo ving the hand to and fro along the line of sight for different distances in the dark. Results show that, when observers focus on a held object, the ch anges in size predicted by Emmert's law occur in response to both active an d passive proprioceptive or haptic cues. The most intriguing result consist ed of the finding that, when only the hand is the target, there appears to be a limit to the decrease in apparent hand size. It appears that the visua l system 'refuses' to size-scale the hand below a limit it accepts as repre sentative or acceptable of 'its' hand.