Size scaling: retinal or environmental frame of reference?

Previous studies have reported that when subjects are presented with two forms in a same-different task, their reaction times increase with the size ratio. This suggests a "mental scaling" transformation analogous to the "mental rotation" used to compensate for differently oriented forms in similar tasks. However, since the stimuli were presented in isolation, retinal and environmental size ratios were confounded. The present study varied both retinal and environmental size ratios in a same-different simultaneous matching task. In one experiment, random forms were placed at different distances along a textured hallway. A second experiment varied the monocular size information: In one condition the forms were displayed in a textured hallway; a second condition added cast shadows; and a third condition displayed the forms against a frontal wall of indeterminate distance. The results suggest that environmental size is determined prior to mental scaling and form matching.