science commentary


Freud called dreams the royal road to the unconscious. But the road still has many potholes.

by T.J. Nelson

science commentary

I have a dream.

I'm homogenizing alien dinosaur brains, but I'm having trouble because they're too cold and keep freezing. Then there's a riot and the cops show up and start singing to us from an Italian opera.

I have many more dreams like that one. Most are science fiction stories, complete with theme music, and the stories evolve and improve slowly each time they're repeated, as if my inner Ridley Scott was editing them, trying to build an alternative career while I sleep. The dinosaur dream is still evolving, but if there is such a thing as a mental cutting-room floor I suspect that's where this gem may end up.

Distorted MRI brain image

Which brings us to the eternal question: why do we dream? While we dream, parts of our prefrontal cortex, which controls executive function, are deactivated and unable to direct the flow of logic in our brains. But is the ensuing flood of hallucinatory chaos random, or is it somehow therapeutic? Or is it psychological wish fulfillment? Or are they, like these sci-fi dreams, pure entertainment?

Of course, entertainment, therapy, and wish fulfillment are not mutually exclusive: one might, for example, wish to be entertained, or entertainment might be therapeutic. But to the brain one function must be more fundamental than the others.

Sigmund Freud

What are dreams? We can't even ask the question without bringing up Sigmund Freud, the inventor of the Freudian slip, which (as the old joke goes) is when you mean to say “Pass the salt, please” but what comes out is “You ruined my life, you heartless bitch!”

Freud thought dreams were symbolic—usually, it seems today, having something to do with sex—but in reality he believed dreams were wish fulfillment. The wishes were usually repressed ones that could never be admitted to in public. He distinguished the dreams' ‘latent content’ or true meaning from their ‘manifest content’, which is the distorted form created by the psyche to conceal the dream's true meaning. If not for this censorship, he thought, we would be shocked at the images and wake up. We forget our dreams for the same reason: the superego represses them.

Freud's apparent obsession with sexuality and repression is easy to make fun of, and today his psychodynamic theories are regarded as speculative and unscientific. But the general concept of an unconscious mind is widely accepted, and his theories about dreams seem to be making a comeback (though with a bit less emphasis on sex).

Modern theories about dreams

Often dreams seem to be the brain's way of trying to solve problems. They're not expressed realistically, but are stripped of their outward appearance, leaving only the emotional content. For instance, if you dream your apartment is being burglarized, it might mean you feel that people are stealing things from you—taking advantage of you. Or, if you're dreaming it around April 15, it could mean your brain is telling you, in its own way, to pick up your homemade AR-15 and run out in the street yelling “Molon Labe!” The emotional content is that you're losing something of value and you need to do something about it. What that something may be, and what to do about it, depend on the individual.

According to this theory, the imagery in dreams is a way of getting around the intellectual barriers your conscious mind may have erected. The bizarre situations are designed to represent the emotional content of the problem in a different way, helping you see its true nature so you can solve it. Novels, jokes, and political cartoons work in much the same way, which is why they're so powerful. If you have a recurring dream, it's safe to say it means your brain is telling you there's some problem in your life that needs to be addressed.

Ernest Hartmann had similar ideas about the problem-solving function of dreams [1]. Hartmann was a follower of connectionist brain models. He believed that dreaming cross-connects disparate parts of the brain to contextualize an emotional concern, so the dream and its images could be understood as metaphors. Their purpose, he thought, was to integrate memories by relating them to broader ideas.

Dreams can also organize perceptions. There are many reports of patients having dreams warning them that they have breast cancer. It was found that up to 94% of these dreams induced a conviction of importance and 72% produced an emotional sense of dread or menace. So-called end-of-life dreams are common in dying people.

Other dreams are about bizarre or trivial problems, like the time I ate too many blueberries before going to bed. I dreamed I was in a corridor filled with blueberries five or six feet deep. Try as I might, I could not escape them. Another time I dreamed I ran out of Shredded Wheat and was forced to eat Kix cereal. That was a nightmare.

The activation-synthesis model

The most influential modern theory is from Hobson and colleagues, who developed the activation-synthesis model, which says that the brainstem and pontine nucleus (which sends signals to the cerebellar medulla, and induces generalized muscle atonia during REM sleep [2]) are sending random information to the forebrain, which turns it into a jumble of incoherent images. According to this theory, at least as originally conceived, dreams have little or no intrinsic meaning.

Silvio Scarone and colleagues in Milan [3,4] consider dreams as a possible model for psychosis. Claude Gottesmann and others pointed out that both dreams and schizophrenia involve a loss of reflective self consciousness, suggesting that there's some inhibition of the prefrontal cortex and disinhibition of the dopamine influence in the nucleus accumbens (the motivation / reward center). This is called aminergic demodulation, which means the catecholamine and acetylcholine neural pathways become ‘imbalanced.’

Patients with Lewy body dementia experience hallucinations that some scientists speculate may represent the intrusion of dreaming into the awake state. However, despite their superficial similarity, hallucinations are more stereotyped than dreams and may simply reflect disturbances in common brain regions [11].

Another long-standing theory is that dreams are the brain's way of encoding memories. Sue Llewellyn [5] suggested that dream imagery functions like a memorization trick that helps people remember things by creating bizarre associations.

All this is a roundabout way of saying we don't fully understand why we dream, and 100 years after Freud we still don't understand their content.

Dreams, LSD, and crazy psychiatrists

In the 1960s, the apparent similarity between dreams and hallucinations induced by psychedelic drugs produced a few theories about dreams that are what we today would consider highly idiosyncratic.

One example is a series of bizarre papers in the Journal of Asthma Research by H.A. Abramson, who used LSD to ‘analyze’ a variety of phenomena. One paper is titled “Reassociation of dreams. III. LSD analysis of a threatening male-female dog dream and its relation to fear of lesbianism.” The abstract for another, titled “Reassociation of dreams. II. An LSD study of sexual conflicts in eczema and asthma”, in which the psychiatrist tries to make associations between bee stings, the sex act, and bad toilet training, is so strange it raises the question: just how big a dose did this guy take?

LSD is a reasonably specific serotonergic receptor (5HT1A and 5HT2A) agonist (Ki=1.1 and 2.9 nM, respectively). Other serotonin agonists, such as 5-hydroxytryptophan (5HTP), are reported to cause vivid dreams and nightmares. This suggests a connection between dreams and the serotonergic Raphe nucleus. But SSRIs (selective serotonin reuptake inhibitors) decrease dream frequency, and it is now recognized that it's the cholinergic system that is most important for REM sleep. The raphe neurons are actually turned off during REM sleep, and activation of noradrenergic or serotonergic systems inhibits dreaming [10], while acetylcholinesterase inhibitors, which raise acetylcholine levels, can produce nightmares. L-DOPA, a treatment for Parkinson's disease, is well known to cause visual hallucinations (along with other undesirable side effects). These complicated results show that dreams are not a simple product of any single neurotransmitter, but a disturbance in the balance among cholinergic, serotonergic, and dopaminergic systems.

Miscellaneous facts about dreams

Sleep researcher A.S. Eiser of the University of Michigan wrote an interesting review paper [6] summarizing what we know about dreams. Some of the facts below are from that paper.

  1. Lucid dreams can be induced by low current electrical stimulation of gamma activity (25–40 Hz) in the frontal lobe [7]. This supports the idea that the frontotemporal region, which creates our concept of self [8], is not normally active during dreaming, but can be activated during sleep. If psychosis really is like a dream state, as some people hypothesize, this discovery may be very important.
  2. Patients with neurodegenerative diseases like Alzheimer's disease dream much less than healthy people the same age [9].
  3. Patients with lesions in the visual association cortex cannot see anything in their dreams.
  4. Lesions in the anterior limbic system lead to an inability to distinguish dreams from reality.
  5. Dreams are forgotten within 8 minutes [6], perhaps because they make no sense to the conscious mind and therefore cannot be integrated into the brain's worldview. Only if the sleeper creates a conscious narrative about the dream can it be remembered. Dreams seem to be mainly designed to affect us on an emotional, not an intellectual, level.
  6. Even at birth, babies spend 50% of their sleep time in REM sleep. Their dreams are mostly static pictures of animals and such [6].
  7. Dreaming also occurs in non-REM states, but they are much more boring than the familiar REM state dreams.
  8. The question of whether depriving people of REM sleep causes psychosis is still controversial.

Dreams and language

The Australian aborigines say their ancestors lived in a dream state, which is in agreement with theories that the invention of language was important in creating our conscious mind.

Maybe what my dream above was saying is that someday we'll all be spending our days homogenizing alien dinosaur brains while a gay baritone police sergeant sings the Di Provenza il Mar aria from La Traviata to keep the rampaging mobs away. Or in other words, just as in that famous Goya etching, when our brains collectively turn to mush, we forget the wisdom of the giants who came before us, and almost anything can happen.


[1]. Hartmann E. Outline for a theory on the nature and functions of dreaming. Dreaming 1996;6:147–170

[2] Adv Neurol. 1995;67:245-71. Physiology of REM sleep, cataplexy, and sleep paralysis. Hishikawa Y, Shimizu T.

[3] J Sleep Res. 2015 Apr 13. doi: 10.1111/jsr.12299. Right hemisphere neural activations in the recall of waking fantasies and of dreams. Benedetti F, Poletti S, Radaelli D, Ranieri R, Genduso V, Cavallotti S, Castelnovo A, Smeraldi E, Scarone S, D'Agostino A.

[4] Schizophr Bull. 2008 May;34(3):515-22. Epub 2007 Oct 17. The dream as a model for psychosis: an experimental approach using bizarreness as a cognitive marker. Scarone S, Manzone ML, Gambini O, Kantzas I, Limosani I, D'Agostino A, Hobson JA.

[5] Behav Brain Sci. 2013 Dec;36(6):589-607. doi: 10.1017/S0140525X12003135. Such stuff as dreams are made on? Elaborative encoding, the ancient art of memory, and the hippocampus. Llewellyn S.

[6] Semin Neurol. 2005 Mar;25(1):97-105. Physiology and psychology of dreams. Eiser AS.

[7] Nat Neurosci. 2014 Jun;17(6):810-2. doi: 10.1038/nn.3719. Epub 2014 May 11. Induction of self awareness in dreams through frontal low current stimulation of gamma activity. Voss U, Holzmann R, Hobson A, Paulus W, Koppehele-Gossel J, Klimke A, Nitsche MA.

[8] Brain Cogn. 2004 Jun; 55(1):69-83. The frontal lobes and theory of mind: developmental concepts from adult focal lesion research. Stuss DT, Anderson V.

[9] Psychol Neuropsychiatr Vieil. 2010 Jun;8(2):87-96. doi: 10.1684/pnv.2010.0209. [Dreams in normal and pathological aging]. [Article in French] Guénolé F, Marcaggi G, Baleyte JM, Garma L.

[10] Neuroreport. 1998 Feb 16;9(3):R1-14. The neuropsychology of REM sleep dreaming. Hobson JA, Stickgold R, Pace-Schott EF.

[11] Conscious Cogn. 2011 Dec;20(4):1016-20. doi: 10.1016/j.concog.2011.03.024. Epub 2011 Apr 30. Dreaming and hallucinations - continuity or discontinuity? Perspectives from dementia with Lewy bodies. Collerton D, Perry E.

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