In his time, Artemidorus Daldianus was a highly regarded man. He was a dream doctor, and in the second century A.D. his fellow Greeks considered dreams to be encoded messages from the gods. Deciphering them required an expert, with Artemidorus chief among them.
Artemidorus declared that all dreams were not created equal, however. If the nocturnal visions could be explained from past events in the sleeper's life, the good doctor wrote them off as meaningless constructions of the individual's experiences and mental orientation; these dreams were not secrets of the gods. Artemidorus himself would never have imagined that, with this idea, he had anticipated a core debate that would arise some 1,700 years later.
The physician who sparked that debate was none other than Sigmund Freud. According to his monumental 1899 work, The Interpretation of Dreams, our nighttime hallucinations are activated by subconscious wishes that can burst forth from behind the protective veil of sleep. Freud's contention was just that, however--a hypothesis, one that neurologists of the day could never prove despite a flurry of scientific investigation. Freud lacked the answer to the ancient question, What does the brain do when we enter the dreamworld? And it frustrated him. He openly wished for neurological evidence, worked at it himself and even said that such information would likely supersede his psychological theories about dreams. But he lacked the science and tools needed to find it.
Today we have better tools, and modern explanations of dreaming are being turned on their heads, in some cases leading back to age-old theories. But as scientists try to pin down what causes dreams and what they mean, if anything, one lesson has clearly emerged: dreams play a vital role in memory and learning, and it is too early to give up on the proposition that they provide a window into our true emotions as well.
The REM Revolution
As Freud's stature grew in the early 1900s, psychologists the world over strongly embraced his theory of dreams. It was not until the 1950s that we reached the next turning point in our understanding. Nathaniel Kleitman of the University of Chicago and a student assistant in his sleep laboratory, Eugene Aserinsky, began to record the eye movements of sleeping children. Kleitman hoped to find an indicator for when the wee ones would awaken. In 1953 the duo found that during overnight sleep, test subjects went through four to six periods of eye twitching, each lasting from 10 to 50 minutes. The pattern held in adults, too. The scientists named this phase rapid eye movement (REM) sleep.
Kleitman was even more amazed when he looked at the sleepers' brain waves, recorded by electroencephalograms (EEGs). The brain was extremely active during the REM phase; neurons fired about as much as they did when the subjects were awake. Yet their muscles were practically flaccid during REM sleep. Kleitman and Aserinsky wondered what all the activity was about. So they began waking their subjects during the high point of REM sleep and asking them if they had been dreaming. From 80 to 95 percent said yes. If the same people were woken during other sleep phases, however, only 5 to 10 percent reported dreams. Neurologists celebrated the discovery: REM sleep, the high-frequency pattern of brain waves and the reduced muscle tone were objective manifestations of the subjective experience of dreams. The excitement was so great that dream researchers dismissed the rest of the sleep cycle as meaningless non-REM, an assumption that would later prove premature.
A plethora of experiments about the biochemical mechanisms of REM sleep boosted scientific euphoria for two decades. Proof that REM sleep occurred in almost all mammals--mammals that in labs could be much more comprehensively investigated than humans--added fuel to the fire. In 1962 neurophysiologist Michel Jouvet of the University of Lyon in France discovered that in cats, a relatively small bundle of nerve cells in the brain stem known as the pons was always active when muscles were relaxed during sleep. If he disturbed the pons, muscles stiffened and quick eye movements did not occur.