Studies with electroencephalogram recordings reveal that hypnotized subjects show random alpha waves (mental and physical relaxation) with more prominent beta waves (a state of arousal to outside stimuli)(Hirai, 1974). Conversely, during the imaginal experience, there is predominance of theta brain activity. In this state the brain waves are of low frequency, which means that a person is minimally aroused by outside stimuli. And, according to Brown, during the mental imagery process a person is "..more likely than in hypnosis and meditation to lose the sense that he/she is actually creating the experience" (Brown, Forte, Rich, Epstein, 1982). That is, during the imaginal activity a person is inactive in relation to the outside world, but hyper-active and hyper-attentive in relation to his/her subjective inner experience. We see here two quite different phenomena. One (hypnosis) is of inner passivity and hyper-receptiveness to the outside reality, another (imagery) is of outer passivity and hyper-receptiveness to one's inner reality.
Out of this phenomenological difference of the two processes comes the difference in their therapeutic application.