A Japanese group has compiled images that correspond to those seen by the person in a dream enabling researchers to reconstruct the dreams. In short, this was accomplished in two passes: first recording the images that are relevant to people and then correlating it with the specific areas of brain stimulated, as seen through MRI. Next time, when the people are dreaming, the computer figures out which areas of the brain that were stimulated and then looking up images that were associated with the image in the first pass.
Thus, Horikawa’s group has figured out the images that are seen in the dream. They have published a paper in Science that involved confirming that the subjects were indeed watching/dreaming the images .