Reverse learningreverse learning theory, reverse learning
Reverse learning is a neurobiological theory of dreams1 In 1983, in a paper2 published in the science journal Nature, Crick and Mitchison's reverse learning model likened the process of dreaming to a computer in that it was "off-line" during dreaming or the REM phase of sleep During this phase, the brain sifts through information gathered throughout the day and throws out all unwanted material According to the model, we dream in order to forget and this involves a process of 'reverse learning' or 'unlearning'3 Reverse learning eliminates unwanted modes of neural network interaction acquired in the adult mammal's learning and also in the process of fetal brain growth; therefore, there is a possibility that abnormalities of reverse learning in the fetal brain might explain some aspects of the autistic syndromes or other neurodevelopmental disorders4
The cortex cannot cope with the vast amount of information received throughout the day without developing "parasitic" thoughts that would disrupt the efficient organisation of memory During REM sleep, these unwanted connections in cortical networks are wiped out or damped down by the Crick-Mitchison process making use of impulses bombarding the cortex from sub-cortical areas
The Crick-Mitchison theory is a variant upon Hobson and McCarley's activation-synthesis hypothesis,5 published in December 1977 Hobson and McCarley hypothesized that a brain stem neuronal mechanism sends pontine-geniculo-occipital or PGO waves that automatically activate the mammalian forebrain By comparing information generated in specific brain areas with information stored in memory, the forebrain synthesizes dreams during REM sleep
- 1 Crick verbatim on the function of REM sleep
- 2 Support for the theory of reverse learning
- 3 Objections to the theory of reverse learning
- 4 See also
- 5 References
- 6 External links
Crick verbatim on the function of REM sleepedit
Suppose one did not have REM, then one would mix things up That is not necessarily a bad thing — it is the basis of fantasy, imagination, and so forth Imagination means seeing a connection between two things that are different but which have something in common which you had not noticed before If one had too much REM, one would predict one would be a rather prosaic person without too much imagination But the process is not 100% efficient If one goes on too far, one begins to wipe out everything
Another way to look at it is to say "How could you prevent the brain being overloaded" One way would be to make it bigger, to have more neurons So perhaps the important thing to say is "The function of REM is to allow your brain or your cortex to be smaller"6
Support for the theory of reverse learningedit
In the echidna, a primitive egg-laying mammal that has no REM sleep, there is a very enlarged frontal cortex7 Crick and Mitchison argue that this excessive cortical development is necessary to store both adaptive memories and parasitic memories, which in more highly evolved animals are disposed of during REM sleep8
This theory solves the brain information storage problem, as our cortex would need to be much larger due to the inefficient storage of information It also explains why we forget dreams extremely easily
Objections to the theory of reverse learningedit
One problem for reverse-learning theory is that dreams are often organized into clear narratives stories It is unclear why dreams would be organized in a systematic way if they consisted only of disposable parasitic thoughts It is also unclear why babies sleep so much, because it seems they would have less to forget Additionally, the brain of the echidna has far less folding than the brains of other mammals, so has less surface area the location of the neo-cortex It may actually have less capacity for higher thought than that of other mammals, rather than more, as its greater mass suggests
In response to these objections, Crick and Mitchison proposed that the principal target for the unlearning process could be obsessive memories strong attractors and that the dream/REM purpose is to equalize the strength of memories
A computational model by Kinouchi and Kinouchi 2002 implementing a chaotic itinerancy dynamics in a Hopfield net shows that the Crick-Mitchison unlearning mechanism produces a trajectory of associated attractors "a narrative" where the strong "emotional", "obsessive" or "overplastic" memories have their dominance downplayed and an equalization between memory basins produces a better recovery of memories not recalled during the "dream"9
It is argued that fetuses and babies sleep so much to downgrade "unlearn" the force of synapses present in these developmental phases
Francis Crick: Neuroscience and other interests
- ^ Shepherd, Michael 1983 "Sleeping and dreaming" British Medical Journal Clinical research ed 287 6391: 512–513 doi:101136/bmj2876391512 PMC 1548456 PMID 6411198
- ^ Crick, Francis; Mitchison, Graeme 1983 "The function of dream sleep" Nature 304 5922: 111–4 doi:101038/304111a0 PMID 6866101
- ^ Hopfield, John J; Feinstein, David I; Palmer, Richard G 14 July 1983 " 'Unlearning' has a stabilizing effect in collective memories" Nature 304: 158–159 doi:101038/304158a0
- ^ Brown 1996 "Autism, Asperger's syndrome and the Crick-Mitchison theory of the biological function of REM sleep" Medical Hypotheses 47 5: 399–403 doi:101016/S0306-98779690220-0 PMID 8951804
- ^ Hobson; McCarley, RW 1977 "The brain as a dream state generator: an activation-synthesis hypothesis of the dream process" The American Journal of Psychiatry 134 12: 1335–48 doi:101176/ajp134121335 PMID 21570
- ^ Crick, Francis 1988 "Neural networks and REM sleep" Bioscience Reports 8 6: 531–5 doi:101007/BF01117331 PMID 3242640
- ^ Allison, T; Cicchetti, D 1976 "Sleep in mammals: ecological and constitutional correlates" Science 194 4266: 732–4 doi:101126/science982039 PMID 982039
- ^ Crick; Mitchison, G 1983 "The function of dream sleep" Nature 304 5922: 111–4 doi:101038/304111a0 PMID 6866101
- ^ Osame Kinouchi; Renato Rodrigues Kinouchi 2002 "Dreams, endocannabinoids and itinerant dynamics in neural networks: re elaborating Crick-Mitchison unlearning hypothesis" arXiv:cond-mat/0208590v2 cond-matdis-nn
- Crick; Mitchison, G 1995 "REM sleep and neural nets" Behavioural Brain Research 69 1–2: 147–55 doi:101016/0166-43289500006-F PMID 7546306
- Brown 1993 "Crick and Mitchison's theory of REM sleep and neural networks" Medical Hypotheses 40 6: 329–31 doi:101016/0306-98779390212-9 PMID 8377668
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