IT IS A SMALL, unlovely creature, native to Australia, with a long conical snout, spiny coat and sticky tongue well-suited to lapping up ants and termites. It is called an echidna and it belongs to a rare species of egg-laying mammals known as the monotremes.

The echidna intrigues neuroscientists. For one thing, the part of its brain called the prefrontal cortex is larger, compared to the rest of its body, than that of any other mammal including man. The prefrontal cortex is where such higher functions as planning, interpreting and strategy forming are thought to reside. Yet in these traits the echidna is hardly blessed. Why, then, its outsized brain?

The echidna can boast one additional distinction: Alone among mammals, it does not dream.

In 1953, Eugene Aserinsky and Nathaniel Kleitman, then at the University of Chicago, observed that sleeping human subjects showed, periodically through the night, a characteristic pattern of rapid eye movements, or REMs; if immediately awakened, they would report vivid dreams. The sure sign of this dream-laden REM sleep -- detected by electrodes attached to the brain -- was an unmistakable, low-amplitude, high-frequency brain wave.

By now much more is known of REM sleep. Sensory input to the brain shuts off, yet the brain is in a state of high arousal. Adults engage in REM dreaming for perhaps 100 minutes a night, spread across four or five increasingly lengthy periods. Dreams from early segments bear the stamp of recent waking experiences, while later ones show complex integration of the recent past with older memories.

The three-quarters of the night spent in non-REM sleep also reveals brain activity, but of quite a different sort. Wakened subjects report fewer "dreamlike" episodes, but those they recall make more "sense" and are less vivid and hallucinatory.

Both REM and non-REM sleep are necessary. Fail to get one or the other, and the next night you tend to compensate with more of that kind.

Mammals other than man also dream, or at least engage in REM sleep. Mice, dogs, monkeys, chimpanzees and elephants all show the low-amplitude brain wave pattern that, in humans, corresponds to dreaming.

All except the echidna. Among dream researchers, that slim fact makes the echidna special -- a mystery to be solved by any who would explain just why all other mammals do dream.

Perchance to Spark

The question "What does it mean?" has been asked of dreams since humans first evolved. Now, though, scientists are asking questions that go beyond the meaning of any single dream: Why do we dream at all? What functions -- biological, psychological or any other -- do dream serve?

Once, answers seemed more certain. Four thousand years ago in Egypt the gods made known their will through dreams; dreams were thought to warn of danger, urge repentance of sin, supply solutions to life's problems. Ancient Hindu and Chinese medicine looked to dreams for help in the diagnosis of illness. In ancient Greece, gods came to men in dreams. Aristotle saw dreams as sensory impressions left behind from wakefulness.

For most of this century, through the vision, brilliance and strength of personality of a Viennese genius, it came to seem at last that dreams served a purpose satifactory to science, that their interpretation might itself be reduced to a science. The year 1900 saw publication of Sigmund Freud's "The Interpretation of Dreams."

Dreams are neither message nor fantasies, said Freud, but they're profoundly meaningful. Beneath their bizarre surface they express repressed wishes, protecting sleep from the often painful knowledge of the unconscious mind. Though superficially built up from events of the day, they've actually been twisted and reshaped by a hidden censor into safer form. The psychoanalyst, encouraging techniques of free association, helps his patient root out the real meaning from behind the dream's facade.

Among Freud's proteges was Carl Jung, whom he groomed to succeed himself as leader of the world psychoanalytical movement. Jung later broke with his mentor. Jung shared Freud's sense of the primacy of the unconsious mind, and he, too, saw dreams as a window into it. But dreams, he maintained, did not individiously hide their true meaning through psychic devices, as Freud said. They were a product of nature, rooted in fantasy, legend and myth, that spoke to the dreamer through symbols.

Paul Jordan-Smith recently wrote in the journal Parabola that "from today's bookstands one could amass a considerable collection of works purporting to unravel the apparent messages of dreams. While biblical dreams had but one source of authority, today our heads are filled with notions of meaning derived from civil education, Sunday School, Freud and the Freudians, Jung and the Jungians, whimsical books of popular psychology, religion and occultism, fiction and fantasy and politics."

And yet could this millennia-old search for meaning in dreams be merely an exercise in human conceit? Could it be that dreaming itself is devoid of meaning, no more than a side effect of certain physiological processes, perhaps the product of random discharges of electrical energy?

In 1975, Harvard neurophysiologists J. Allan Hobson and Robert M. McCarley, proposed that "the primary motivating force for dreaming is not psychological but physiological." Extending research by the French investigator Michel Jouvet, they described a "dream-state generator" in a primitive part of the brain called the pons which, every 90 minutes, issues signals to the higher brain centers to launch the process of dreaming. Say what you will about dream content, argues Hobson today, dreaming is the natural physiological response of the forebrain to stimulation from the pons.

We see in dreams, says Hobson, "not because we are invaded by spirits, but because the visual system has been activated. Period." What we do see is often bizarre because brain systems which are rarely active simultaneously during wakefulness are stimulated together during REM sleep. And scene shifts, time compression, symbol formation -- how account for them? "The forebrain," Hobson and Mccarley write, "may be making the best of a bad job in producing even partially coherent dream imagery from the relatively noisy signals sent up to it from the brain stem."

But whatever the neurophysiological foundation of dreams, Hobson and McCarley noted in their original paper, it doesn't mean "that dreams are not also psychological events; nor does it imply that they are without psychological meaning or function. . . ." Saying otherwise would have run roughshod across human experience.

To men and women, almost universally, dreams mean. Something in dreaming pulls on the imagination and the intellect, leaving us sure that something important, meaningful, central to our lives is going on.

In 1963 the late psychologist and dream theorist Christopher Evans came to a similar conclusion. He was walking along a deserted beach when he spied a large black bird perched on a single leg, head tucked beneath a wing, asleep. So oblivious was it that Evans could get near enough to hear its breath and examine its feathers. After a while he tapped the bird and said "Hello" -- whereupon the bird awoke in fright and wildly flapped away.

"In biological terms," Evans wrote in his posthumously published "Landscapes of the Night," completed by Peter Evans, "sleep was an immensely dangerous exercise -- perhaps the most dangerous single thing an animal could do -- and yet all animals indulged in it. What fundamental process could it possibly serve?"

Purging Our 'Parasitic Modes'

There's no want of theories. But no model has won universal acceptance or even provided a common frame of reference. Scientists understand dreams as one might a grapefruit sliced open to see what it looks like inside: Depending on where you cut, you get a geometric array of segments or a mess of pits, pulp and rind.

One of the more controverial theories was developed in part by Francis Crick who, with James Watson, unraveled the structure of DNA in 1953 and ushered in a biological revolution. Having later turned to the study of mind and brain, he and a young British scientist, Graeme Mitchison, propounded a theory of dreams: We dream not to consolidate our memories, as some would have it, but to rid our brains of useless ones; we dream not to remember but to forget.

According to one line of evidence, the brain stores information not in individual brain cells but within close-woven networks of cells . Both theory and computer modeling predict that such networks can overload when too many patterns are stored on top of one another, resulting in "pathological" behavior that in humans might correspond to fantasies, obessions or hallucinations. In a properly functioning system, such "parasitic modes" must be regularly purged. And that, Crick and his colleague suggest, is what dreams do -- through "a special mechanism which operates during REM sleep and . . . which is, loosely speaking, the opposite of learning. We call this 'reverse learning' or 'unlearning.'"

Mitchison says response to their theory has been highly negative. One criticism, he says, is "that our theory fails to account for the rich, subjective quality of dreams, that it seems impoverished and mechanistic." For example, the theory seems to hold that dreams should ooze with meaningless material. Yet dreams more often pack a powerful emotional punch.

But while faulting it, Ernest Hartmann, director of Sleep Research at Lemuel Shattuck Hospital in Boston, points to important common ground between the Crick-Mitchison model and others. For its "unlearning" actually serves what he calls "the interests of learning." And in one guise or another it's learning -- processing information, consolidating memory, assimilating experience, sifting through strategies -- that is common to most theories of dreaming. Dreams help "tie up loose ends" in the brain, says Hartmann; they integrate recent experience with past knowledge. To the extent there's consensus about dreams, that's it.

David Cohen, of the University of Texas at Austin, notes that "a mammal requires a lot of learning to be a mammal. Everything isn't there on the genes." We have to learn language and how to interact with others. To do that, "you need experiences, exposures. It's all got to be integrated. Some of this learning," the evidence convinces him, "goes on in sleep. Awake, we accommodate to the world around us. Then sleep cuts off the outside world and, through dreams, we assimilate the day's experiences.

Rosalind Cartwright also sees dreaming as an active process, performing important emotional "work" -- as her research into the dream life of women going through divorce tends to confirm. Cartwright, the chairman of psychology and social sciences at Rush-Presbyterian-St. Luke's Medical Center in Chicago, places herself "among those folks who say dreams have memory-storage function, that they take the day's affective material and process it into long-term storage by finding a match for it in earlier-stored memory." Dreams, in her view, help work through the emotionally charged material of the dreamer's life.

In a study of divorced women aged 30 to 55, the happily wed women she used as a control proved to have "extremely dull dreams": taking the kids to the pool, selecting vegetables at the supermarket, and such workaday matters. They had little work for their dreams to do. The divorcees, on the other hand, had "long, complicated dreams, loaded in terms of emotional content." They were undergoing a "critical life event that forced them to reorganize who they were."

"Dreaming," summarizes Cartwright, "works to renew and rehearse, reaching backward and forward in our lives, on emotionally important issues that we have no time to cope with in wakefulness." By providing "an interval of off-line work, it gives us the opportunity to update the file on who we are and how to cope in the world."

"Off-line"? "File"?

Ubiquitous among today's dream reseachers are concepts and terms borrowed from the world of computers. Edmond Dewan in 1969 first likened dreams to "off-line processing," when a computer closes itself off to new data and manipulates what's already in storage. Christopher Evans writes, "The great software files of the brain become open and available for revision in the light of changes that have taken place as the result of the horde of new experiences which occur every day.

"What are dreams?" he asks. "Well, they are the programs being run."

Which programs? Social adaptation programs, for one. In an insight he credits to Cambridge University psychologist Nicholas Humphrey, Evans pictures dreams as "dress rehearsals for events we can expect, hope for or fear in everyday life. Situations present themselves in which the dreamer is an actor, playing a part, coping with the often strange twists of the plot, keeping abreast of the unfolding drama."

A Brain in a Wheelbarrow

In perhaps the most ambitious synthesis of disparate strands of evidence yet, Jonathan Winson outlines a theory of dreaming that seeks to reconcile neurobiological research with information processing models and the insights of Freud. His view, outlined in the book "Brain and Psyche" and based in part on research in his own laboratory at Rockefeller University in New York, sees a sausage-shaped piece of the brain known as the hippocampus as a kind of neural gatekeeper for information from the world outside. Closed during non-REM sleep, the gate opens during REM sleep and wakefulness, both periods of high arousal. For Winson, here is the physiological expression of "off-line processing."

That processing has immense survival value, helping to integrate experience and consolidate memory. "In man," Winson writes, "dreams are a window on the neural process whereby, from early childhood on, strategies for behavior are being set down, modified or consulted."

In support of his theory, Winson turns to a stunning variety of evidence -- in which the echidna again figures. To him, the echidna is nature's last attempt at doing without dreaming. The complex task of associating memories with new experience and formulating new behavior -- which more "modern" mammals do through dreams -- the echidna does while awake and busy with the world. That's what demands its enormous prefrontal cortex. "Should the organization of man's brain have been similar to the echidna's," Winson observes, "he might have needed a wheelbarrow to carry it around."

Unlike many neurobiologists, Winson does not wholly reject Freud, but rather demands of his own theory that it explain such Freudian staples as dream distortion, repression and and transference. For him, the Freudian unconscious is real. Dreams, he writes, "are the statements, wishes, hopes and fears of the unconscious personality" of the dreamer. ". . . The content of dreams day by day forms a cohesive pattern and reflects each individual's unconscious strategy for survival."

There are still rock-solid Freudians to be found, and they can offer a spirited and impressive defense of the master's thinking. But their ranks are thinning. "It's in great decline," says David Cohen of Freudian thinking, "embarrassed by poor results in the lab and in therapy." It's plain, as Graeme Mitchison says, that "the mainstream of psychology no longer holds rigidly" to Freudian orthodoxy.

The old, comforting certainty is gone. Until new insights or new experimental paradigms come along, anything goes. We are back in the state of befuddlement and uncertainty that existed on the eve of "The Interpretation of Dreams." Gleams of Escape Robert Kanigel

Whatever their neurobiological function, dreams evoke a sense of freedom from the grey constraints of everyday reality, suggest to us secret knowledge transmitted, mysteries revealed, a suspension of the ordinary rules of daily life.

Into the nighttime freedom, artists and thinkers have sunk deep wells of creativity. Writer Gloria Naylor ("The Women of Brewster Place" and "Linden Hills") reports that the titles of her first four books all came to her in dreams. A dream inspired Robert Louis Stevenson's "Dr. Jekyll and Mr. Hyde." A dream of a snake consuming its own tail brought the 19th-century chemist Friedrich August Kekule the long-sought chemical structure of benzene -- six carbons arranged in the form of a ring.

Dreams so proclaim freedom from dreary reason and waking constraint that the word has come to mean, figuratively, a wild fancy, ambition, or vision of the beautifully unlikely. Wrote Shelley: "Some say that gleams of a remoter world/Visit the soul in sleep."

A dream is an opening in the universe, a momentary freedom from the fixed and immutable laws of causality.

Robert Kaniegel is a Baltimore writer and author of "Apprentice to Genius: The Making of a Scientific Dynasty." This article was adapted from Notre Dame Magazine.