Over the past hundred years, there have been four major dream theories. Here we consider these one by one to find out where they stand now and we will then look at how the expectation fulfilment theory of dreaming explains the findings they have been based upon.
Video — Joe Griffin summarises other dream theories
The activation synthesis theory of dreaming
The theory that has dominated for the last 30 years is one put forward by Professor Allan Hobson of Harvard University and his colleague Robert McCarley. They called it the activation synthesis theory.
Laboratory studies of brain waves show that, just before we go into REM sleep and during it, powerful electrical signals pass through the brain like a wave. On electroencephalogram recordings (EEGs), they appear as sudden spikes. The signals arise from the pons (P) in the brainstem, from the neurons that move the eyes, and then travel up via a part of the midbrain called the geniculate body (G) to the occipital (O) cortex in the higher brain — so are known as PGO spikes. (They also constitute what is termed our orientation response, which, when we are awake, is what directs our attention to any sudden change in the environment, such as a sound or movement.)
Hobson and McCarley's theory was that these PGO spikes were sending a random barrage of stimulation through the brain every so often, activating the whole cortex as a result; the higher brain had to try and make some sense of this random barrage, and dreams were the result. Dreams, therefore, were an epiphenomenon: they had no intrinsic meaning. They were just the brain's efforts to synthesise some sense from random signals.
Evidence has accumulated over the last 30 years to disprove this theory. The first piece of evidence that disproved it emerged once PET scanning of the brain was developed. According to Hobson and McCarley's original theory, a barrage of random stimulation coming up periodically from the brainstem was synthesised by the prefrontal cortex into dreams. But scans of the brain in the REM state showed that the cortex was very selectively activated. The emotional brain (the limbic system) and the visual brain were highly activated but the prefrontal cortex was excluded from this stimulation (the very part supposed to be doing the synthesising). Indeed, Hobson himself, over the last few years, has been so drastically redrafting the theory that it is just a pale shadow of its original presentation. Even he now agrees with the evidence that, instead of global forebrain activation being responsible for dream synthesis, it is the emotional brain that is responsible for dream plot formation.
This evidence on its own disproves the theory. However, there is more. Research accumulated over the last 40 years, and universally accepted by dream researchers, shows that dreams are coherent and that they relate to previous waking experiences. There also tends to be continuity in the type of dream content over time and this could not be so if there were a random stimulus. Hobson and McCarley also theorised that REM sleep serves to 'rest' the cells in the brainstem which produce serotonin and noradrenalin, because in REM sleep these particular neurotransmitters are not used by the brain. Their idea was that these neuronal pathways were being rested so that we would wake up the next day, refreshed by REM sleep. Consequently, then, the more REM sleep people had, the more refreshed they should be. But researchers looking at the sleep patterns of depressed patients found that they had massive amounts of REM sleep in proportion to slow-wave sleep and yet, far from waking up refreshed, they were waking up exhausted! How did Hobson account for this? He just said, "It is a paradox."
Yet another problem with this theory, which Hobson admits in his latest book, is that it can't explain why certain dreams have positive emotions and some have negative emotions. But the final nail in the activation synthesis theory's coffin is the finding that deep brainstem lesions do not generally stop dreaming, whereas certain lesions in the cortex do, despite the existence of brainstem-initiated REM sleep.
Wish fulfilment: Who wants nightmares?
No we move on to Freud's wish fulfilment theory of dreams. There has recently been strong evidence to show that REM involves the expectation dopamine pathway. Professor Mark Solms, who holds the chair in neuropsychology at the University of Cape Town in South Africa, has been pre-eminent in synthesising this research, showing that when people go into the REM state, the motivation circuit in the brain — the expectation pathway — is activated. As Freud talked about motivation and emotion, and, as Hobson was clearly wrong, perhaps, says Solms, Freud was right after all. And, in a very broad sense, this activation is support for Freud. But what it leaves out of the picture is that, when you activate the expectation pathway, you are activating consciousness. You are not activating some subconscious conflict. So there is no real evidence there in support of Freud.
Secondly, Freud's theory has real difficulties explaining why people so often have anxiety dreams. Dreams also involve being angry a lot of the time. Freud said dreams were for fulfilling wishes. But who would want nightmares? Who would want to get beaten up or sexually assaulted in their dreams? So Freud's theory just didn't explain in any coherent fashion the fact that dreams involve far more than wishes and that only a minority of them can be characterised as wishes. And his claim that all dreams are sexually motivated is no longer given any credence.
Freud claimed that we dream to protect sleep, to prevent us being awakened by threatening, sub-conscious wishes. However, the REM state, in which most dreams occur, is a regularly occurring biological programme in humans and other mammals, and not something which arises to protect sleep.
To recap, expectation pathways activate conscious, not subconscious, experience. There is no evidence at all that dreams are sexually motivated and Freud can't plausibly explain why we would wish for anxiety dreams. The REM state occurs in all mammals, so it is not just a human activity, protecting sleep, as Freud suggested. A cat is unlikely to be dreaming about its Oedipus complex. So the attempt to revive Freud's theory seems to be based more on wishful thinking than on realistic considerations of its defects.
Strange parasitical connections – we dream to forget
Francis Crick and Graeme Mitchison's theory suggested that we dream to forget., Their idea came from studying work done on computer programs that simulated neural intelligence. An overload of incoming information could trigger "parasitical connections" between unrelated bits of information that interfered with memory, and an unlearning system had to be developed to knock these out of the computer systems.
Crick and Mitchison postulated that a complex associational network, such as the cortex, might become overloaded in the same way, and that the PGO spikes were an unlearning mechanism, in the form of random 'bangs' coming up from the brainstem every so often, to knock out these fairly weak parasitical neural links. As, at that time, most dreams were thought to be bizarre in content, this was taken as evidence for the existence of these parasitical connections.
Crick and Mitchison theorised that, if we didn't have dreaming, we would go on making more and more bizarre connections, which would imply that, if we block REM sleep, our memories should become more addled. If this theory is correct, then depressed people on antidepressants that block REM sleep (monoamine oxidase inhibitors — MAOIs) should suffer memory impairment they don't. If anything, depressed people on MAOIs report memory improvement rather than increased confusion in memory recall.
Yet another problem with the theory is that, over the last decade or so, there have been significant technical advances in the recording of what actually happens during dreaming. The overwhelming majority of dreams are, in fact, quite routine, everyday experiences. It is the tiny percentage of dreams that we recall that seem bizarre: dreams recorded in the sleep laboratory, when sleepers are woken as soon as they go into REM sleep, are mostly not bizarre at all. As a result of this discovery, Crick revised his theory to suggest that it might still, at least, explain those few dreams that do have a bizarre component to them. In other words, his theory has been so drastically modified that very little of it remains at all.
Finally, since Crick and Mitchison formulated this theory, not a shred of evidence has arisen to show that the human brain makes parasitical connections. That is something known only to occur with computer networks.
The memory consolidation theory of dreaming
The final theory on the table is the memory consolidation theory. Blocking REM sleep impairs the ability to perform procedural tasks tasks that involve the learning of a skill through a sequence of steps that involve making predictions. For example, rats trying to find their way around a complex maze to get a reward remember much better how to do the task if they have had REM sleep. Without REM sleep, the knowledge becomes damaged. So REM sleep would seem to facilitate this type of learning.
It has also been shown that, in complex learning, memory is improved if we have both slow-wave sleep and REM sleep, otherwise the knowledge doesn't seem to survive quite so well. So, clearly, there is some connection between memory and REM sleep.
But is there evidence that the REM state and dreaming exist fundamentally to carry out memory consolidation? The first piece of evidence that goes against this possibility is that taking MAOIs, the antidepressants that suppress REM sleep, does not lead to memory impairment.
So, although there is some evidence that certain types of learning do seem to be improved by REM sleep, most dreaming cannot be about retaining new learning. The second fact that goes strongly against the memory consolidation hypothesis is that almost nobody remembers their dreams! It is very rare to remember dreams, unless you've trained yourself to do so. We dream for about two hours a night. Dream researcher Allan Hobson, an expert trained to recall his dreams, has himself pointed out the fact that, when he looked at the number of dreams he had recorded against the number he had forgotten, it was something like 0.0002 per cent. So it seems hard to see how we could be dreaming to make our memories permanent if we forget the material we are processing as soon as we open our eyes. (Yet, as certain types of memory do seem to be facilitated by REM sleep, any state-of-the-art theory of dreaming must be able to account for it.)
"We need a new theory"
Writing in Behavioural and Brain Sciences, in a special issue devoted to the most widely promoted dream theories, Professor Domhoff of the University of California, recognised as one of the leading researchers in this field, commented on the evidence presented, concluding, "If the methodologically most sound descriptive empirical findings [ie the findings that are most solidly established to explain dreaming] were to be used as a starting point for future dream theorising, the picture would look like this:
1. Dreaming is a cognitive achievement that develops throughout childhood
2. There is a forebrain network for dream generation that is most often triggered by brainstem activation [the PGO spikes]
3. Much of dream content is coherent, consistent over time and continuous with past or present emotional concerns."
So any theory of dreaming would have to account for those three most solidly established findings. I personally would add to that the need to explain memory consolidation as well, if a theory were to explain the full picture.
Here is Domhoff's final conclusion: "None of the papers reviewed in this commentary puts forward a theory that encompasses all three of these well-grounded conclusions. This suggests the need for a new neurocognitive theory of dreaming." In other words, according to Professor Domhoff, theories that have dominated the field over the last 30 years do not explain why we dream, and there is need for a completely new one.
We suggest that that theory is already here.
Read on to discover how the expectation fulfilment theory of dreams explains the evidence these other theories cannot.