What is perceived when two images are combined?

When two images are combined three perceptual outcomes are possible. One, they can be perceived as they are—separate and independent images. Two, they can become perceptually fused into a new image. Three, one image may dominate, or mask, the other. These possibilities are demonstrated with a variety of images and it is proposed that it is their spatial correlation rather than their spatial frequency similarities or differences which is critical in determining the particular outcome.