p&mJRJz VOL.30 I4 JULY 1983 III COMMENTARY

The function of dream sleep Francis Crick* & Graemi Mitchison* Wepropose that thefunction of dreamsleep (more proper& rapid-&yemovemen tor REM sleep)LT to removecertain undesirable modes of interactionin networksof cells in the cerebralcortex. We postulatethat this is done in REMsleep by a revem learningmechan ism(see also p . 158), so that the trace in the brain of the unconscipusdream is weakenedrather, than strengthened,by the dream.

MANKIND has always been fascinated by is explained in more detail below. Without to go into large-amplitude instabilities5 . dreams. As might be expected, there have it we believe that the mammalian cortex heenmany attempts to assign a purpose Or could not perform so well. Neuronal networks significance to them. Although we dream We first describe our ideas about the cor- Now, if one asks what functions such richly for one or two hours every night, we do not tex followed by a brief account of neural interconnected assemblies of cells could remember most of our dreams. Earlier networks. Next we outline what is known serve, one attractive possibility is that they thinkers, such as Freud, did not know this. about REM sleep. (For general accounts, could store associations6-R. To see this, Modern theories (not reviewed here in see refs 1,2.) We then describe our suppose an ‘event’ is represented by the ac- detail) have usually proposed that sleep postulated mechanism and how it might be tivity of a subset of cells in a cell assembly. and dreams save energy or have various tested. Finally.we discuss various implica- If all the cells involved in that event form restorative functions, either to replenish tions of our ideas. . mutual synapses, then when part of that the brain biochemically in some way, or to event is encountered again these synapses reclassifyor reorder the information stored The cortex can causethe regeneration of the activity in in it. The cortex consists of two separate sheets the entire subset. Sleepis of several kinds. Dream sleep, or of neural tissue, one on each side of the Much exploratory theoretical work has rapid eye movement (REM) sleep, is head. The neocortex, which has a been done on such networks of cells (for an predominantly found in viviparous mam- characteristic layered structure, is found introduction seerefs 6-8). In these models, mals and birds. It seems to be associated only in mammals (see ref. 3 for recent information is stored in the strengths of the with homeothermy (a constant internal survey), although a somewhat analogous many synapsesand sometimes in the firing temperature) and the possession of an structure, the wulst, is found in birds. If thresholds of cells as well. Although the ex- appreciable neocortex or its equivalent. allowance is made for body weight, it is act behaviour naturally depends on the It is not unimportant because of the ap- larger in primates than in most other mam- details of the particular model, certain preciable amount of time we spend in this mals and larger in man than in other general properties can emerge even from Peculiar state. primates. It makes up a substantial fraction relatively simple models. The associations We propose here a new explanation for of the human brain. which are stored are not assigned specific the function of REM sleep. The basis of Different areas of the cortex perform locations for each item, as in a digital com- our theory is the assumption that in different functions, some being mainly puter. Instead the information is: (1) viviparous mammals the cortical system associated with vision, touch and so on, Distributed: this implies that a particular (the cerebral cortex and some of its’ while others appear to process more com- piece of information is distributed over associatedsubcortical structures) can be plex information not associated with a very many synapses. (2) Robust: this im- regarded as a network of interconnected single sensory mode. The exact function of plies that the information will not be totally Cellswhich can support a great variety of the neocortex is unknown but it appears to lost if a few synapsesare added or remov- modesof mutual excitation. Such a system be closely associatedwith higher mental ac- ed. (3) Superimposed: this implies that one is likely to be subject to unwanted or tivities. It seemslikely that it has evolved to synapse is involved in storing several ‘Parasitic’ modes of behaviour, which arise perform in a rather special way. distinct pieces of information. asit is disturbed either by the growth of the In examining the neuroanatomy of the A properly designed net can be trained brain or by the modifications produced by neocortex one is struck by the very large (meaning that the strengths of the synapses experience. We propose that such modes number of axon collaterals (this is not true, can be adjusted) so that given an input (a are detected and suppressed by a special for instance, of the thalamus). In any area pattern of axonal firings) it can produce the mechanism which operates during REM of the cortex the great majority of synapses appropriate output (another pattern of ax- sleepand has the character of an active pro- come from axons originating locally and onal firings). It is found that certain cesswhich is, loosely speaking, the op- running within it. There is also evidence general properties will often emerge. (1) posite of learning. We call this ‘reverse that the majority of the synapsesin the cor- Completion: given only part of the input learning* or ‘unlearning’. This mechanism, tex are excitatory in their action. This sug- (as a clue) it can produce fairly exactly the which is not the sameas normal forgetting, gests a capacity for self-excitatory modes whole of the appropriate output (examples of behaviour in the cortex. And indeed, in aregiven in ref. 7). In computer jargon, the *TheSalkInstitute 10010NorthTorreyPinesRoad, La JOlkX California 92037. USA. Present address (GM.): various conditions, such as epilepsy, memory is ‘content addressable’. (2) MRC Laboratory of Molecular Biology and the migraine and certain kinds of drug-induced Classification: given an input which is KmWh Craik Laboratory, Cambridge CBZ 3EG. UK. hallucination*, parts of the cortex appear related toseveral of its associations, it may NATURE VOL304 14 JUI Y I%3 112 COMMENTARY produce an output which combines many REM periods may of the muscles of the Jouvet “, have postulated a ‘dream state of the common features of its normal out- sleeping animal, especially its head and generator’ which lies mainly in the pontine puts. neck muscles, are more relaxed than in reticular formation (the question of which A major difficulty with all nets of this non-REM sleep. Its cortex, as judged by exact cell groups are involved is controver. general type is that they become overload- the electroencephalogram (EEG) and by sial). It produces the so-called PGO waves. ed if an attempt is made to store the rapid movement of the eyes beneath They propose that the activity of such cells simuhaneously too many. different pat- closed lids, appears to be very active and in is the cause of both rapid eye movements terns or associations of patterns, or if the a state similar to the waking state. On the and the periodic intrusion of new subject stored patterns have too large an overlap. other hand, the monoamine neurones in matter into hallucinoid dreams. Our pro- This is becauseof the superimposed nature the brain stem, especially those in the locus posals are based on this idea. of the storage. How the net will behave coeruleus, raphe and peribranchial nuclei, In summary, the evidence suggests that when overloaded depends on the exact reduce their firing rates in REM sleep to on- in REM sleep the brain is isolated from its structure of the net, but certain patterns of ly a few per cent of the corresponding rate normal input and output channels and that behaviour are likely to emerge: (1) The net in the waking state9. it is very active, this activity being pro- may produce many far-fetched or bizarre Another major difference between REM moted by rather nonspecific signals from associations (‘fantasy’). (2) The net may and non-REM sleep lies in the dreams the brain stem and reflected in the uncon- tend to produce the same state, or one of a associated with them. For most people the scious equivalent of dreaming, which only small set of states, whatever the input few dreams found in non-REM sleep tend reachesnormal consciousnessif the sleeper (‘obsession’). (3) Certain kinds of nets, to -have a rather thought-like character. awakes. particularly those which feed back on During REM sleep, on the other hand, themselves, may respond to inappropriate dreams occur more frequently and usually The postulated mechanism input signals which would normally evoke have a perceptual vividness and the illogical We need a mechanism which will tune the no response from the net.(‘hallucination’). episodic character with which we are all cortical system, in the sense of removing It is against this background of rather familiar. A human adult usually spends a parasitic modes which arise after the tentative and idealized theory that our pro- total of 1% to 2 hours each night in REM system has been disturbed either by growth posals must be judged. sleep, spread over several periods. The of the brain (when new connections are If the cortex were hard-wired during em- evidence suggeststhat most of the dreams constantly being made) or by the modifica- bryogenesis to an exactly predetermined during these REM periods do not reach tions produced by experience. The pattern of synaptic connections, the normal consciousness, dreams being mechanism we propose is based on the burden of eliminating parasitic modes in remembered only if the sleeper awakes more or less random stimulation of the cortical nets would have to be undertaken whiledreaming. Even then the memory of a forebrain by the brain stem that will tend to by the genesalone. Although there is con- dream is usually very transient, fading excite the inappropriate modes of brain ac- siderable evidence for specificity in the cor- quickly if no effort is made to remember it tivity referred to earlier, and especially tical wiring, it is likely that many of the by rehearsing its content. those which are too prone to be set off by details of the synaptic connections - their A most remarkable finding is that random noise rather than by highly struc- exact locations and their strength - are newborn humans may have as much as 8 tured specific signals. We further postulate made in a semirandom manner and refined hours of REM sleep per day lo. There is also a reverse learning mechanism which will by experience. This is almost a necessity in evidence to suggest that in the womb, modify the cortex (for example, by altering an organism which is capable of learning especially in the third trimester, REM sleep the strengths of individual synapses) in very large amounts of novel information. occurs even more frequently. This large such a way that this particular activity is Thus it seemslikely that both during cor- amount of REM sleep before and after less likely in the future. For example, if a tical growth (when we may say that certain birth is also found in other mammals. synapse needs to be strengthened in order broadly predetermined ‘associations’ are AI1 viviparous mammals examined, in- to remember something, then in reverse layed down), and also in facing the ex- cluding primitive marsupials such as the learning it would be weakened. Put more periencesof adult life, such parasitic modes. opposum, show periods of REM sleep”*‘*. loosely, we suggest that in REM sleep we will be unavoidably generated. Even an animal like the mole, which can unlearn our unconscious dreams. “We How would one attempt to eliminate hardly move its eyes, shows the dream in order to forget.” these modes? We suggest the following. characteristic EEG of REM sleep. Birds After this paper had been initially sub- The major inputs and outputs of the system have REM sleep, although often only-a mitted for publication, we learnt from Dr should be turned off, so that the system is very small amount of it, occupying perhaps John Hopfield that he and his colleagues largely isolated. It should then be given SUC- 5% of their sleept3. There are no very had independently arrived at the idea of cessive‘ random’ activations, from internal convincing reports of REM sleep (as reverse learning, though not in connection sources, so that any incipient parasitic judged by the EEG) in reptiles, amphibia with dreams. In a parallel communica- modes would be excited, especially if the or fish. tion’* they have shown that the behaviout general balance of excitation to inhibition If an animal is deprived of REM sleep for of their very idealized neural net is indeed had been temporarily tilted towards excita- one or more nights (but allowed non-REM improved by reverse learning. That is, it tion. Some mechanism is then needed to sleep) then it will usually have more REM equalizes the accessibility of stored make changes so that these potentially sleep in subsequent nights14*t5. memories and suppresses most of the parasitic modes are damped down. Such a All this evidence suggests that REM spurious ones. We have since repeated their rough outline description immediately sleephas an important function, at least for simulations and confirmed their general reminds one of REM sleep and the mammals. Since the majority of dreams are conclusions. It remains to be seen how well hallucinoid dreams associated with it. not remembered, that function is more reverselearning acts on other more realistic likely to be associated with the unconscious neural nets. We have revised our paper iI’ REM sleep dreaming process - that is, with REM the light of their results. It was discovered in the 1950sthat in mam- sleep without awakening - rather than Note that the amount of reverse learninE mals there are two main types of sleep. with the few dreams which are recalled. per step in these simulations was very small Periods of REM sleep (also called D sleep It has been shown that during REM sleep (only about 107’of0 the amount needed f@’ or paradoxical sleep) alternate with periods the forebrain is periodically and widely complete learning), although several bun- of non-REM sleep (also called S sleep, stimulated by the brain stem. This activity dred such steps were used. This alerts us t0 slow-wave sleep, or orthodox sleep) of in the brain stem can happen even in the the possibility that the changes produced iI1 which four stages of increasing depth of absence of the cortex. Hobson and Mc- REM sleep may individually be very small sleep are usually distinguished. During Carleyr6, following the pioneer work of but cumulative over many PGO spikes and &uRE VolM 14 JULYIs3 II3 C~MMLNTAR\/ $T gmlly Ilig~ts’+p~ ‘-, 1’ :’ L i :‘. learning process.. The latter is a positive nature of REM dreams. The ObJection mrght be raised that some mechanism which. does not merely fail to The effects of REM sleep deprivation are .&reriments have shown that REM may alter synaptic strengths (or other long- harder to explain. It is well established that appear to help the retention of memory, lasting brain parameters) but changes them REM deprivation often produces a re- whereas the process of reverse learning so that the dream is not just forgotten but bound - more REM sleep than usual oc- would tend to make the memory fade. The actively ‘unlearned’. The result is that the curs when the subject is eventually allowed results of Hopfield et aI.‘* show that this dream (or some of the elements of it) is less to sleep without interruption. We would need not be the case. After reverse learning likely to recur in the future. have expected that REM deprivation, if the recall of their net was less confused and The terms ‘reverse learning’ or ‘unlearn- severe enough, might cause hallucinations more uniform. : ing’ are not ideal because they rather imply - that is, structured visual and auditory If there is indeed a mechanism for that one has to learn something first in responses to ‘noise’ - and perhaps delu- reverse learning, many questions arise order to unlearn it. What does a fetus sions and obsessions. There is a little about its character. Does it act via the same ‘learn’ that has to be unlearned? Our evidence for thisz5, but usually the effects ntechanism as normal learning (whatever answer is that, during development, the are either small or absentz6. This is partly that is) or Is a special, quite separate, semirandom process of making synaptic because it is extremely difficult to produce mechanism involved? Is the mechanism cdnnections is likely to produce parasitic long periods of complete REM deprivation associatedwith one particular system in the modes. it is these which must be ‘unlearn- in humans by selective arousal. After a brain stem? Another possibility is that a ed’ in order to obtain a well-behaved week or two it becomes almost impossible small amount of reverse learning is always system. to awaken them promptly at every onset of present but is normally overwhelmed by We need some explanation for recurrent REM sleep, so that prolonged experiments the positive plasticity produced by one or dreams. We propose the ud hoc hypothesis have not been done. One cannot help but more of the diffuse systems from the brain that a recurrent dream is one which, for one wonder whether similar experiments on stem. When their activity is greatly reduc- reason or another, tends to wake up the food deprivation might lead to the conclu- ed, as it is in REM sleep9, the residual sleeper, perhaps because of the anxiety sion (if unsupported by other evidence) reverse learning can then exert its effect often associated with them. This will have that food also had no essential function. unopposed, at least on recently modified the effect that the learning process changes However, REM deprivation in animals synapses. sign, passing from reverse learning to does appear to lower the threshold for cor- In its simplest form our theory postulates positive learning, so that the underlying tical instability produced by electro- that there is no intelligent supervisor inside spurious associations remain, and so a convulsive shock27-30, which is what we the brain which decides in detail which similar dream is likely to occur on some might expect. REM deprivation in humans potential neural activities should be left un- later occasion. This mechanism does sometimes produces irritability and an in-, touched -and which should be damped postulate a supervisor of a kind but its sole ability to concentrate. One might suggest down. This choice is made solely by the function is to decide whether the sleeper that these are the effects of the attention response of the forebrain to the relatively should wake up or not. Thus for a dream to mechanism being forced to subdue sub- nonspecific signals from the brain stem. In become recurrent it must have two proper- threshold parasitic modes which would very general terms, the brain stem gives the ties. It must be related to a potentially otherwise break into consciousness. REM forebrain a varied pattern of bangs (the parasitic mode and it must wake up the deprivation can also allow feelings and PGO waves). Any resulting activity is then dreamer in such a way that he remembers it wishes to appear which had previously modified so that it is less likely to occur in rather vividly. been kept out of consciousness3r, or, in cer- the future. Our theory, in its present state, says tain subjects, can show changes towards in- It would of course be possible to nothing about the function of non-REM creased internal fantasy during waking32. postulate a more complex mechanism. For sleep. These stages of sleep usually have A further difficulty is that some drugs, example, in REM sleep, especially in early less of the hallucinoid type of dream which such as certain monoamine oxidase in- development, there could be innate testing we associate with our reverse learning hibitors, appear to prevent REM sleep en- programmes, together with a ‘supervisor’ mechanism. Non-REM sleep is likely to tirely33 without producing very obvious to decide what to store and what to erase, have the restorative function often psychological deficits. This is a difficulty depending on the result of the tests. postulated for it but it may also have some for any theory which assumesREM sleep is Various workers have made proposals informational function. For example, it important and runs in the face of all the along these linesrPV2”As. far as we know, might be used for the process of ‘con- other evidence about it. We can only plead nobody has previously suggested that the solidating’ memory in some way. It is that such drugs may have complicated side testing procedure involves the removal of worth noting that the first REM period of ‘effects which make the observations Potentially parasitic modes. the night is normally preceded by a misleading. It has been customary to believe that substantial period of non-REM sleep. A direct test of our postulated reverse during an unconscious dream the content learning mechanism seems extremely dif- of the dream is stored in some form of very Testing the theory ficult. It would be necessary to show that short-term ‘memory’ but that the As far as we can tell, our theory is broadly our unconscious dreams (dreams we do not mechanism for transferring it into longer compatible with a large amount of ex- remember - a new word for this is really term memory is inoperative. We normally perimental data. Starting from a plausible needed, we suggest ‘remination’) reduce become conscious of our dreams only if we hypothesis about cortical function, it ex- the probability of such thoughts occurring wake up while dreaming is in progress. If plains in an effortless way both the need for in the future. This is far beyond the we then pay attention to our dream, some REM sleep in adult life and the large methods we have available today. It would of its content can be maintained in very amount of it during the development of the be interesting to know if the threshold for short-term memory and may eventually be brain. We believe no previous theory ex- hallucination, induced by drugs or other transferred to longer-term memory as the plains this distribution of REM sleep in means, is lowered as a result of REM transfer mechanism becomes activated. such a simple manner. Any purely deprivation. Another approach would be Otherwise our dream fades. Thus we can psychological theory (such as Freud’s) is to look for the structural and chemical cor- speak of forgetting our dreams, meaning hard-pressed to explain the large amount relates of the postulated reverse learning that we know that we had a dream, but are of REM sleep in the womb, and any purely mechanism, but exactly how to do this is at somewhat uncertain of its content. developmental theory must account for the the moment unclear. Without further This forgetting of a dream, which has quite appreciable amount of REM sleep in evidence of this kind our theory must be often been remarked on, does not adult life. Our theory accounts for both. It regarded as speculative. necessarily involve our postulated reverse is also compatible with the hallucinoid It is clear that useful insights can come NATURE VOL.305 14 JULY 19~2 from neural modelling. This appr6ach has- necessary: :a fairly constant internal caused by a defect in the reverse learning its limitations, sin&it is difficult .to pro- temperature, so that its function is not process should not be overlooked. duce realistic models and even more disturbed by temperature fluctuations, and In this model, attempting to remember difficult to simulate them effectively, in addition a cleaning-up mechanism, to one’s dreams should perhaps not be en- especially if the. hypothetical neural nets remove potentially parasitic modes. In couraged, because such remembering may approach a realistic size, when the com- short, without REM dreams evolution help to retain patterns of thought which are putational time becomes prohibitively could not have produced the highly refined better forgotten. These are the very pat- long. However, such theoretical studies neocortex we have today. terns the organism was attempting to damp should at least reveal some of the types of If the reverse learning mechanism we down. networks which would benefit from our have postulated exists, one might wonder Finally we should remark that even if it proposed mechanism. They might also what effects its failure might have. A com- turns out that our ideas are wrong and that help to give more life to our otherwise plete failure might lead to such grave nature does not employ the reverselearning rather vague characterization of the cor- disturbances - a state of almost perpetual mechanism we have postulated, the process tical system. obsession or spurious, hallucinatory may well be useful for artificial intelligence Another approach would be to under- associations - that it would probably be machines of the future, especially those take comparative studies. There is one severely selected against. A partial failure having extensive parallel processing, a mammal which, although possessinga well should produce unwanted responses to learning mechanism and a certain amount developed neocortex”, appears not to random noise, perhaps as hallucinations, of randomness in their construction. show any signs of REM sleep (at least in delusions, and obsessions, and produce a We thank our resident and visiting co]- young adults), even though it exhibits nor- state not unlike some schizophrenias. leagues at The Salk Institute for many mal non-REM sleetis. This is the Echidna ‘It has been postulated before that there useful discussions. We are especially Tachyglossusaculeatus (the spiny anteater) might be a relation between REM sleepAndy grateful to Drs Allan Hobson and Jim found in Australia. The Echidnas and the schizophrenia, but studies have shown that Horne who made detailed comments on duck-billed platypus are primitive egg- there is little oi no connection between the our draft manuscript, and to Dr John Hop- laying mammals (monotremes). outward signs of REM sleep and field for communicating his work to us Griffiths” has written that “. . . the schizophrenia3’. However, a partial failure before publication and for helpful discus- gyrencephalic cerebrums of the of the reverse learning mechanism would sions. This work has been supported by the Tachyglossidae have been and are a source not necessarily alter the amount of REM J. W. Kiechkhefer Foundation, Samuel of wonder to neurobiologists”. He quotes sleep, since the control mechanisms for the Roberts Noble Foundation, US Air Force Elliott SmithM who in ;1902 wrote “The occurrence of REM sleep might be Grant number AFOSR-82-0042 and the most,obtrusive feature of this brain is the somewhat distinct from the reverse learn- System Development Foundation. 0 relatively enormous development of the ing process itself. Thus the possibility that cerebral hemispheres . . . The meaning of some forms of schizophrenia might be Received 8 October 1982; accepted I2 May 1983. this large neopallium is quite incomprehen- sible . . .“. Griffiths adds: “Determinants of modern neurophysiology also fail to ex- I. Hartmann, E.L. The Functions of S/up (Yale University 20. Gaarder. K: Arch. @n. Psychiut. 14.253-260 11%6). plain how echidnas come by this cortex”. Press. London. 1973). 21. &an. E.M. in Slew and Dreaminn led. Hartmann. E.I We suggestthat Tachyglosncs needs such a 2. Cartmight. R.D. A Primer on sleep and Dreaming 295.jO7 (Little-B&n, Boston, 1970). (Addison-Wesley, Reading. Massuchusetts. 1978). 22. Greenberg. R. & Pearlman. C. Penp. Rio/. Med. 1. large cortex because it cannot tune it up by 3. Schmitt. F.O.. Worden. F.G.. Addman, G. & Dennis. 513-521 (1974). the process of reverse learning. Experience S.G. (cds) The Organizalion of the Cerebral Cortex 23. Jouvet. M. Prw. Bruin Rex. 53, 33l-346(1980). (MIT Press. Cambridge. Massachusetts. 1981). 24. Hobson, J.A. in Brain Mechanisms and Percepreol with idealized neural nets shows that one 4. Siegel, R.K. Scienl. Am. 237, 132-141 (1977). Awarwwss (eds Pomp&no. 0. & Marsan, C.A.1 can usually avoid overloading a net, and 5. Emuntrout. G.B. & Cowan. J.D. Biol. Cybernetics 379-404 (Raven. New York, 1981). thus the confusion such an overloading 34.137-Iti(1979). 25. Dement. W. in Academy of Psychoanalysis: Science and 6. Palm. G. Nrural Assemblies: An Allernalive Approach lo Aychwnolysis Vol. 7 (ed. Masxrman. J.) 129.184 creates, by making the net bigger. Artif&llnlclii~en~ (Sarinner. New York. 1982). (Grune and Stratton, New York, 1964). Tachyglouw can be studied in captivity. 7. K&&en. T. A&ciadw Me&y (Springer. New York. 26. Vogel. G.W. Arch. II!. Psychiul. 18.312-329 (1968). 1977). 27. Cohen, H.B. and Dement, W.C. Science 150, 1318.1319 It might be rewarding to examine in more 8. Hinton, G.E. & Anderson, J. feds) Porullel ModeIs ol (l%5). detail its behaviour, neurophysiology and Associrrliw Memory (Erlbaum. Hillsdale. New Jersey, 28. Hartmann. E.. Marcus. I. & Leinoff. A. Psychono!ll. 1981). Sri. 13. 141.142 (1968). neuroanatomy compared to a primitive 9. Hobson.J.A., McCarlcy. R.W. & Nelson, J.P. J. Nt-uro- 29. Cohen. H.. Thomas. J. & Dement. W.C. Bruin &‘I. placental mammal, such as a hedgehog, physiol. (in the press). 19,317-321 (1970). which does show REM sleep. If REM sleep IO. Roffwarg, H.P., Muzio, J.N. & Demml. W.C. Sciencee’ 30. Cohen, H.B. & Dement. W.C. Brain Res. 22. 421.4:: 152.604-619 (1966). 119701. serves an important function; this should II. Allison, T. & Van Twyver, H. Ngr. His/. 79. 56-65 31. G&b&g. R.. Pearlman, C.. Fingar, R.. Kantrowitz. J. be reflected in some way in its absence in (1970). & Kawliche. S. Br. J. med. Psycho/. 43. I-It 12. Allison. T. & Cicchetti, D.V. Science 194. 732-734 (1970). the spiny anteater. (1976). 32. Cartwright, R.D. & Ratzel. R.W. Arch. ,een. P$ychioi 13. Klein. M., Michel. F. & Jouvet, M. C.r. hrbd. 27.277-280 (1972). Possible implications She. Acad. Sci.. Porti 158.99-103 (1964). 33. Wyatt, R.J., Kupfer. D.J., Scott, J.. Robinson. D.S. k 14. Demcnt, W. S-km-e 131,1705-1707 (1960). Snyder. F. Psychopharmacolopia IS, 236244 (19691. If it turns out that our ideas are broadly IS. Kaks. A., Hoedemachcr, F.. Jacobson. A. & Lichrenstein. 34. Criffithr. M. ?ht &?lo~y of Monorremes (Academic. correct, they could help us to understand E. Nalure 204. 1337-1338 (19641. New York. 1978). 16. Hobson. J.A. & McCarley. R.W. Am. J. Psychial. 35. Allison, T.. Van Twyner. H. & Golf. W.R. Archs. lwl the evolution of the neocortex which is so 134.1335-1348(1977). biol. 110. 145.184 (1972). typical of mammals. It seemslikely that in 17. Jouvct. M. Arch% ital. Biol. 100, 125-206 (1962). 36. Smith, G.E. R. Co//. Sarg. Mus. Cur Physiol. Ser. 2. IS. Hopfield. J.J.. F&stein, D.1. & Palmer. R.C. A’UfUre 133.157(19021. order for a highly tuned system to perform 304. l58-159(1983). 37. Vogel, G.W. & Traub. A.C. Arch. wn. Ps.vrhio’. efficiently at least two requirements are 19. Newman. E.A. &Evans, C.R. Nalure 206,534 (l%S). 18.287-300 (1968).