StarTrek illusion--general object constancy phenomenon?

We report a new powerful type of contrast and size illusion caused by apparent motion in depth, which we called the StarTrek illusion. We found that an optic flow pattern consistent with objects moving in depth strongly modulates their apparent contrast. Disks that appeared to move away from the observer appeared to grow higher in contrast and larger, while their retinal size and contrast remained constant. We explain this illusion of contrast by the term contrast constancy: Normally, objects lose their contrast when viewed from far away, but when this expected loss does not happen, the brain infers that the physical contrast of the object increases as the object moves away. This is perceived as the illusory increase of the object's contrast. The contrast constancy is largely analogous to the well-known size constancy phenomenon. We discovered that the two phenomena are related. By adjusting the size of the disks during the optic flow motion, the illusory contrast increase could be easily canceled or even reversed. On the other hand, the illusory size increase could not be manipulated the same way by contrast modulation. Our results suggest that the brain may use the same scaling factor to account for the size and contrast change with distance and that the estimated object size affects the contrast calculation.