Mars mission experiment (part one)
The images on the left (below) are some of several million that were captured by the Mars Reconnaissance Orbiter Mission. The ones on the right are the same images rotated through 180 degrees. I believe this is how they should really look - some change quite dramatically, others not so. The main differences are the transformations from elevations to depressions and vice versa.
A few people have told me, that they can see both positive and negative images in just one given image but have little control which one they see. I have also been experimenting with negating images - see Mars 2.