Continual-Activation Theory of Dreaming
Jie Zhang
4418 Villa ParadisoCres., Windsor, Ontario, Canada N9G 2L7
Abstract:
Key Words:continual-activation theory, dreaming, memory, NREM, REM, working memory.
Introduction
Every person on earth dreamsevery night. But "Why do we dream?" or "What is the function ofdreaming?" Although these questions have been the subject of a debate forcenturies, the answers to these questions are still unknown.
In 1900, Sigmund Freud publishedhis famous theory of dreams. He theorized that dreams involved complicatedpsychological processes that, when decoded, revealed the hidden desires of thedreamer. Freud's theory of dreams dominated sleep science in the first half ofthe 20th century until the 1950s, when Aserinsky and Kleitman (1953)discovered REM (rapid eye movement) sleep. Since then, many theories ofdreaming have been proposed.
Allan Hobson and Robert McCarley(1977) presented a neurophysilogical model of dreaming, the Activation-Synthesishypothesis. They hypothesized that dreaming resulted from the interpretation bythe cortex of information concerning eye movements and activated brain stemmotor pattern generators. They proposed that in REM sleep, the ascendingcholinergic PGO (ponto-geniculo-occipital) waves stimulate higher midbrain andforebrain cortical structures, producing rapid eye movements. The activatedforebrain then synthesizes the dream out of internally generated information.According to Hobson and McCarley "the forebrain may be making the best ofa bad job in producing even a partially coherent dream imagery from therelatively noisy signals sent up to it from the brain stem." The model isbased primarily on animal research of REM sleep, and assumed that the same structuresthat induce REM sleep also generate sensory information. The theory givesneither answer to the question why the brain should generate any images duringREM sleep, nor explanation why this kind of vivid dream does not occur duringwaking, since the PGO waves occur at a fairly constant daily rate in the cat(14,000 + 3000 waves per day) (Jouvet, 1969), although it is markedly reducedin waking (Hobson, 2002). However, the activation-synthesis model, which hasdominated the field of study of dreaming ever since, has been recentlychallenged by Mark Solms's neuropsychological – psychoanalytic model (Solms,2000). Supported by the brain lesions study, Solms pointed out that dreamingand REM sleep are controlled by different brain mechanisms. That is, REM canoccur without dreaming and dreaming can occur without REM.
Jie Zhang (2004) recentlypresented a new human memory model. By introducing a temporary memory stageinto the memory process to bridge the gap between short-term memory and thelong-term memory, Zhang proposed that during our waking time, the memory formedfrom the working memory is not saved directly into the long-term memory;instead it is saved into a temporary memory. The function of sleep is toprocess, encode and transfer the data from the temporary memory to thelong-term memory. Zhang suggested that, besides the central executive (acontrol system that mediates attention and regulation of processes) (Baddeley& Hitch, 1974), the working memory could be divided into two groups of subsidiarysystems: one for processing declarative (conscious) memory and one forprocessing procedural (non-conscious) memory. Corresponding to the type ofmemory been transferred, Zhang proposed that, sleep has two different stages:NREM (non rapid eye movement) sleep for processing the declarative memory, andREM sleep for processing the procedural memory. He further suggested that thereare two types of dreams. The type I dream, a thought-like dream, is the resultof the memory replay when the declarative memory is transferred from thetemporary memory to the long-term memory during NREM sleep. The type II dream,a more dream-like dream, mainly occurs when the procedural memory istransferred from the temporary memory to the long-term memory during REM sleep.Based on this model, Zhang theorized that, although occurring often in REMsleep, the type II dream has no direct causality with the REM state.
Continual-Activation Theory
Continual-activation theoryhypothesizes that both conscious and non-conscious subsidiary systems ofworking memory have to be continually activated to maintain proper brainfunctioning. When the level of activation of either subsidiary system descendsto a certain threshold, the continual-activation mechanism in the brain will betriggered to generate a data stream from the memory stores to flow through thesubsidiary system in order to maintain brain continual activation. Toillustrate the continual-activation theory, a detailed Zhang memory model(Zhang, 2004) is given in figures 1 to 3.
During waking time, the sensorymemory is continuously receiving information from all five senses. Theinformation is first held in the sensory memory store. By attention, some ofthe information will then be passed to the conscious subsidiary system ofworking memory for processing. To process the incoming information, theconscious subsidiary system of working memory will retrieve related informationfrom the declarative (conscious) temporary memory and or the declarative(conscious) long-term memory. The processed information will then be sent tothe declarative temporary memory for rapid memory saving. In the meanwhile, theconscious subsidiary system of working memory will send a request to thenon-conscious subsidiary system of working memory for a response output. Toprocess this request, the non-conscious subsidiary system of working memorywill also retrieve corresponding procedural memory from the procedural (non-conscious)temporary memory and or the procedural (non-conscious) long-term memory. Theprocessed request will then be sent to the procedural temporary memory forrapid memory saving. At the same time, the processed request will also bepassed to the motor output. The results of the motor output will then bemonitored by five senses and fed back to the working memory for modifying.While awake, both the declarative and the procedural long-term memories are inretrieval-only mode. This process has been illustrated in figure 1. Here, thecentral executive will alternately pay attention to both subsidiary systems ofworking memory (illustrated by shaded symbols in the flow chart). During wakingtime, both the conscious and the non-conscious subsidiary systems of workingmemory are continually activated.
During NREM sleep period, thesensing rates of all the sensors are slowed and the arousal thresholds areincreased. The central executive is now mainly engaged with the conscioussubsidiary system of working memory for declarative memoryprocessing/transferring. The conscious subsidiary system of working memory willretrieve data from the declarative temporary memory, compare it with relatedfiles previously saved in the declarative long-term memory, and delete anyunwanted, duplicate or overlapping data. The information deemed relevant as newor updated, is encoded and transferred to the declarative long-term memory.While asleep, both declarative and procedural temporary memories are inretrieval-only mode. If awakened during this period time, the sleeper will havea thought-like mentation. This type of mentation has been defined as type Idream (Zhang, 2004). Sleep terrors may occur during this period of time whenthe brain is retrieving/processing/filling the most scary image memory data.Nevertheless, during this period of time, there is no attended data streamflowing through the non-conscious subsidiary system of working memory since thecentral executive (attention center) is busy with the conscious subsidiarysystem of working memory for memory processing. According to thecontinual-activation hypothesis, a continual-activation mechanism inside thebrain will be triggered to automatically and randomly retrieve data from theprocedural memory stores. This randomly retrieved data stream will flow throughthe non-conscious subsidiary system of working memory to maintain the braincontinual activation (figure 2). Sleepers do not act out of this randomlyretrieved procedural memory, because, by nature, their muscle tones arelowered. However, sometimes, for some reason, either the lowered muscle tonedoes not lower enough or the data stream is too strong, and the following sleepdisorders might occur: sleep walking, sleep talking, tooth grinding andperiodic limb movement disorder.
During the REM sleep period, thecentral executive is mainly engaged with the non-conscious subsidiary system ofworking memory for procedural memory processing/transferring. This time, thenon-conscious subsidiary system of working memory will perform the data fillingprocess similar to what happened in the conscious subsidiary system during NREMsleep. The so-called rapid eye movement (REM) is, in fact, a externalmanifestation of this filling process. Again, during this stage of sleep, thesleeper is prevented from acting out of this strong procedural memory datastream because the main muscle system is paralyzed. However, for some peoplethis paralysis fails to occur during REM sleep, leading to REM sleep behaviordisorder (RBD). As well, sometimes, the paralyzed muscle does not recover fastenough after waking, leading to sleep paralysis. During this period of time,there is no attended data stream flowing through the conscious subsidiarysystem of working memory. Similar to the case in NREM sleep, thecontinual-activation mechanism for the conscious subsidiary system of workingmemory will be initiated (figure 3). This mechanism will randomly retrievedeclarative memory data from the declarative memory stores to feed to theconscious subsidiary system of working memory in order to meet the minimumlevel required for brain activation. The easiest, most accessible information,such as day residue, will be more salient for retrieval. Since the data streamcomes from random activation, the sleeper should have a mentation similar tothe type I dream during this time. One thing that needs to be pointed out isthat not every type I dream can be recalled: the dream without visual contentis surely more difficult to recall. However, as the randomly retrieved memoriesflow through the conscious subsidiary system of working memory, the brain will attemptto make sense and interpret them. With the involvement of the brain association(associative thinking) system (believed to be mainly looped through the area ofthe frontal lobes) and the emotion system, the vivid dreaming event (type IIdream) will start shortly after. The level of brain activation in the conscioussubsidiary system of working memory will then be increased to a higher level.Based on the dreamer's thinking, the association system will associativelyretrieve memories from the memory stores and quickly turn them into pictures orrealistic-like events. This may, sometimes, bring dreamers a feeling of theirthoughts being broadcast, as some of their thoughts (judgment, decision, etc.)end within the next dream scene in someway. Although the data randomlyretrieved by the continual-activation mechanism will be interpreted by thedreaming brain and synthesized into a dream plot, the type II dream plot isstrongly influenced by the dreamer's own thinking. From this point of view, itis easy to understand that a person with brain damage to the frontal lobes(association system), such as the lobotomy patients, or to the"visual-spatial sketch-pad" (spatial imagery loop) (Baddeley &Hitch, 1974) may not have a vivid dream-like dream (type II dream), althoughthe continual-activation mechanism is still functioning.
As we shall now see, thecontinual-activation theory of dreaming hypothesizes that the type II dream isa result of involvement of the conscious subsidiary systems of working memorywith this continual-activation mechanism. The theory is in agreement withHobson and McCarley's Activation-Synthesis hypothesis in the aspect of thatdreaming (type II dream) is based on physiological mechanisms not psychicforces, and is the results of brain activation and synthesis. However, thecontinual-activation theory differs with the activation-synthesis hypothesis inone main ground. The continual-activation theory proposes that REM sleep anddreaming have different physiological mechanisms and serve different functionalpurposes. Dreaming can occur without REM and REM can occur without dreamingwhich support Mark Solms's finding (Solms, 2000).
Conclusion
Besides the central executive,the working memory could be divided into two subsidiary systems: the consciousand the non-conscious subsidiary systems. Both subsidiary systems have to becontinually activated through their life times. It is very much like the brainhas "hearts" of its own. When the flow of data stream in eithersubsidiary system drops to a given threshold, a continual-activation mechanismwill be triggered to randomly retrieve memory data from the memory stores tomaintain brain activation. Dreaming, as a conscious experience, is only aresult of the involvement of conscious subsidiary system of the brain with thiscontinual-activation data stream.
Acknowledgements: Theauthor would like to thank Jeremy Tebbens and Bill Boyle for valuablediscussions on this manuscript.
References
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