Why we evolved to forget our dreams

None of the other dream theories we have discussed so far on this site can satisfactorily explain why we forget almost all of our dreams. As we shall see, the expectation fulfilment theory of dreaming does provide a satisfying explanation for this widely observed phenomenon.

Evolution for expectation

Brains evolved to help animals make more accurate predictions about what behaviours would help them survive. But the type of expectations we have as humans, or that other animals have, for that matter, are infinitely more complex than those of a bee. When mammals evolved, they developed warm-bloodedness, which meant that they were no longer dependent on the sun's heat for mobility. But maintaining a constant warm body temperature required a greatly increased energy intake (estimated at up to a 500 per cent increase in calories needed). So, to meet this need, mammals had to become much better at locating food supplies while also avoiding becoming food themselves for other warm-blooded predatory mammals — all of which required a much more sophisticated prediction system, to reduce the risks.

The cortex provided the answer. The evolution of the cortex, with its much increased processing capacity, enabled mammals not just to act purely on instinct — see a food source and go for it — but to weigh up the risks and benefits of an action — do I have time to make the kill and hide it or will I get eaten by another animal while I'm doing it? In more technical terms, it enabled the ancient dopamine prediction circuits of the limbic system to be subjected to a higher-order risk analysis, based on the additional computing power provided by the cortex.

However, that left another problem to be solved. The limbic system communicates with the cortex via behavioural impulses (emotions). If these are not acted upon (for instance, because the strategy is deemed too risky or because the cortex has set other priorities — such as deciding, in certain circumstances, that it is more important to protect young than to chase a possible food source) they don't go away. In the case of humans, this state of unfulfilled expectation can also occur when we think about something in the future or the past that causes emotional arousal in the present but which can't, by its very nature, be acted upon. These uncompleted emotional impulses — expectations — stay switched on, taking up processing capacity in the expectation system.

Reducing the processing capacity

So far, two strategies have evolved for dealing with this. The first, in the spiny anteater, is the development of a much bigger cortex to store all these expectations whilst retaining sufficient spare computing power for making new, ongoing risk assessments. This may also be the strategy evolved by dolphins, which have an exceptionally large cortex. The muscle paralysis that accompanies REM sleep places dolphins at risk of drowning, so they can have hardly any REM sleep.

The second and much more efficient method is dreaming. In dreaming, we act out the unrealised expectations from waking by pattern matching them to analogous sensory patterns — images and events stored in memory — as it is through pattern matching that the REM system works. I am often asked why the pattern match has to be analogical or metaphorical. Apart from the evidence I have published explaining this point, [see FAQ question 8] there is a sound physiological reason for why it must be so. An expectation is an imagined scenario, using images from memory.

In dreaming, we are asking memory to provide a scenario that matches a scenario that is already a part of memory — the event that aroused the expectation. So the matching scenario has to be the best fit that memory can provide. Think of it this way — if I hold up my left hand and ask my brain for a best-fit pattern match, it can't use my left hand because that is the one I want a match for — so it must use my right hand, as the best-fit pattern match for my left. (This does not happen in waking because we pattern match our expectations to whatever stimulates them in the environment, not to a memory. If we want an ice cream, the expectation is fulfilled when we are actually eating it.)

The dream, then, by fulfilling the expectation, completes the circuit and switches off the arousal. But that is not the end of the matter, for we have now converted an unrealised expectation into a factual memory of completing it. Ordinarily, the hippocampus, the conscious memory store, holds our memories of recent events and quickly deconstructs those memories and sends them to various parts of the cortex — the parts concerned with vision, hearing, touch, etc — for storage. It does that to facilitate efficient pattern matching. But, if the dream is allowed to be stored as a real memory, it will corrupt the memory store and greatly diminish our ability reliably to predict the outcome of similar experiences in the future. This is avoided by preventing the hippocampus from sending the dream information to the cortex for long-term storage.[2] As explained earlier, PET scans and other types of research have shown that, in dreaming, the prefrontal cortex is closed down.

So it is no accident that the prefrontal cortex is switched off during dreaming. It is no accident that the hippocampus doesn't de-construct information and send it all around the brain because what the hippocampus is doing in dreaming is getting rid of expectations that didn't pan out while we were awake. It is getting them out of the way, making them inaccessible, in effect, so as to allow us to build up a proper, intelligence prediction and expectation system, an accurate storage of knowledge. (This also explains the evidence for memory consolidation — if you take away all the false expectations, the memories that are consolidated are more accurate.)

The expectation fulfilment theory can therefore explain why dreams are about emotionally arousing events, particularly about emotionally arousing expectations. It explains why dreams are consistent over time. It explains the developmental aspects of dreaming. It can explain the other tests put down by Domhoff. But, more than that, it explains the cutting-edge evidence that the brain is ever malleable, by explaining how it can be so malleable.

The purpose of the brain is to predict, so that we can get our needs met. We need to have a system that can continually adapt itself, and the expectation fulfilment theory shows how the brain does that by cancelling out the expectations that didn't work. It enables us to have a bang-up-to-date register of what really does get needs met in our lives, so that we can more accurately predict what we need to do in the future. (But we can only work with the experiences we have had. If, as a child, a young woman experienced both abuse and love from her father, she may continually seek a relationship with abusive men, until eventually she can learn that love exists separately from abuse.)

But what about remembered dreams?

You might then wonder if recalling our dreams, as sometimes happens, is undoing the dreamwork. The answer is no, because the arousal is switched off once the expectation is acted out. When we are awake, the cortex is switched on, enabling us to compare dream content to what is really happening around us and, thus, to distinguish between dream and reality.

Occasionally the cortex is alerted by some incongruity in the dream experience, such as flying, and we become aware that we are dreaming. (this is known as lucid dreaming) But this risks undoing the dream work of deactivating the experiences — because we now know we are experiencing a fantasy. (In more primitive mammals, if an altering of the cortex were to happen, it is less likely their brains could make the distinction between dream and reality; the fantasy would be treated as real and would therefore corrupt the memory stores.)


References

1]. Griffin, J and Tyrrell, I (2004). Dreaming Reality: how dreaming keeps us sane or can drive us mad. HG Publishing, East Sussex.

2]. Busáki, G (1995). The hippocampo-neurocortical dialogue.
Cerebral Cortex, 6, 81—92.

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