The patients under hypnosis or without hypnosis were randomly told to see color or grayscale, regardless of what they were seeing, and the blood flow was shown -- to the various parts of the brain that relate to color processing were shown to increase when the subjects were asked under hypnosis to perceive color and decreased when they were told to see grayscale, regardless of what they were actually seeing. As David Spiegel likes to say, “believing is seeing.”

This is a PET scan illuminating blood flow in the region of the occipital cortex involved with color vision, and other neighboring regions are more primarily associated with black and white vision. So you can ask, what is the brain working on? Not just, what is the brain being asked to see. And this is the pattern with identical gray tones: color or black and white.