Well, if you have a vision while still in possession of your body, I would imagine there would be brain waves in connection with it.
I thought this interesting: "Or, as one researcher later suggested, had the temporal lobe of her brain been briefly hyperstimulated? This, he told her, likely induced the illusion of an unseen presence." Does this mean that hyperstimulation of the temporal lobe always creates illusions? Given that an external stimulus and our mental reaction to it are entirely different from each other (see Russell Brain, Mind, Percception and Science), one might as well conclude that everything the brain tells us is an illusion.