Reverse learning
Encyclopedia
Reverse learning is a neurobiological theory of dream
s. In 1983, in a paper published in the famous 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 supposedly 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'. 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 disorders.
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 supposedly 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
, 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.
Jouvet
and many other prominent sleep researchers have their own their theories of REM sleep. Is the Crick-Mitchison theory of REM sleep an error, a partially correct insight, or a vague first step toward an important unifying in the theory of REM sleep and mammalian brain function?
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".
, a primitive egg-laying mammal that has no REM sleep, there is a very enlarged frontal cortex. 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 sleep.
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.
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".
It is argued that fetuses and babies sleep so much to downgrade ("unlearn") the force of synapses present in these developmental phases.
There is not much evidence to suggest that our brain is actively forgetting during sleep. In fact, it seems to be the exact opposite, where sleep helps consolidate memories instead.
Dream
Dreams are successions of images, ideas, emotions, and sensations that occur involuntarily in the mind during certain stages of sleep. The content and purpose of dreams are not definitively understood, though they have been a topic of scientific speculation, philosophical intrigue and religious...
s. In 1983, in a paper published in the famous science journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
, Crick
Francis Crick
Francis Harry Compton Crick OM FRS was an English molecular biologist, biophysicist, and neuroscientist, and most noted for being one of two co-discoverers of the structure of the DNA molecule in 1953, together with James D. Watson...
and Mitchison's reverse learning model likened the process of dreaming to a computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
in that it was "off-line" during dreaming or the REM phase of sleep. During this phase, the brain supposedly 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'. 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 disorders.
The cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
cannot cope with the vast amount of information received throughout the day without developing "parasitic" thoughts that would disrupt the efficient organisation of memory
Memory
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
. During REM sleep, these unwanted connections in cortical networks are supposedly 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
Allan Hobson
John Allan Hobson, M.D. is an American psychiatrist and dream researcher.He is known for his research on Rapid eye movement sleep. He is Professor of Psychiatry, Emeritus, Harvard Medical School,...
and McCarley's
Robert McCarley
Robert W. McCarley, MD, is Chair and Professor of Psychiatry at Harvard Medical School and the VA Boston Healthcare System. He is also Director of the Laboratory of Neuroscience located at the Brockton VA Medical Center and the McLean Hospital...
activation-synthesis hypothesis
Activation-synthesis hypothesis
The activation-synthesis hypothesis, proposed by Harvard University psychiatrists John Allan Hobson and Robert McCarley, is a neurobiological theory of dreams first published in the American Journal of Psychiatry in December 1977...
, published in December 1977. Hobson and McCarley hypothesized that a brain stem neuronal mechanism sends pontine-geniculo-occipital (or PGO) waves
PGO waves
Ponto-geniculo-occipital waves or PGO waves are phasic field potentials. These waves can be recorded from the pons, the lateral geniculate nucleus , and the occipital cortex regions of the brain, where these waveforms originate...
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.
Jouvet
Michel Jouvet
Michel Valentin Marcel Jouvet is Emeritus Professor of Experimental Medicine at the University of Lyon. He spent one year in the laboratory of the Horace Magoun in Long Beach, California in 1955...
and many other prominent sleep researchers have their own their theories of REM sleep. Is the Crick-Mitchison theory of REM sleep an error, a partially correct insight, or a vague first step toward an important unifying in the theory of REM sleep and mammalian brain function?
Crick verbatim on the function of REM sleep
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".
Support for the theory of reverse learning
In the echidnaEchidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
, a primitive egg-laying mammal that has no REM sleep, there is a very enlarged frontal cortex. 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 sleep.
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 learning
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 systematicSystematic
Systematic is an American hard rock band from Oakland, California. They were one of the first signings to Metallica drummer Lars Ulrich's record label, The Music Company, via Elektra Records. The band released two studio albums before disbanding in 2004....
way if they consisted only of disposable parasitic thoughts. It is also unclear why babies sleep
Sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...
so much, because it seems they would have less to forget. Additionally, the brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
of the echidna
Echidna
Echidnas , also known as spiny anteaters, belong to the family Tachyglossidae in the monotreme order of egg-laying mammals. There are four extant species, which, together with the platypus, are the only surviving members of that order and are the only extant mammals that lay eggs...
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
Hopfield net
A Hopfield network is a form of recurrent artificial neural network invented by John Hopfield. Hopfield nets serve as content-addressable memory systems with binary threshold units. They are guaranteed to converge to a local minimum, but convergence to one of the stored patterns is not guaranteed...
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".
It is argued that fetuses and babies sleep so much to downgrade ("unlearn") the force of synapses present in these developmental phases.
There is not much evidence to suggest that our brain is actively forgetting during sleep. In fact, it seems to be the exact opposite, where sleep helps consolidate memories instead.