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PERIODICALS OF IMPLICIT COGNITION (2006) 7, 1-7 Implicit Laboratory Association, Budapest, Hungary Sleep, Dreaming and Memory Péter Simor ImpLab Association, Budapest & Institute of Psychology, Eötvös Loránd University, 46, Izabella utca, Budapest, 1076, Hungary [email protected] http://www.implab.org Abstract: In this target article I briefly summarize some of the experimental results and theoretical assumptions about the information processing mechanisms during the different stages of sleep. The main differences between the phenomenological aspects of NREM and REM sleep are discussed along with their different contributions for informational processes. Neuropsychological evidences of the dream state will also be considered. Keywords: dreaming, episodic memory, emotion, explicit memory, implicit memory, memory consolidation, NREM sleep, REM sleep, semantic memory. Introduction Mr. Z. has no formal training in psychology. For him it might be a really strange idea that during the ostensibly passive state of sleeping, a lot of important things might happen in his mind. However, if Mr. Z. could recall his last night dream where he was riding a red horse and saving the world from the evil conspiracy of his own boss he may realize something. Although he thought he was „switched off” his mind was obstinately working. Imagine that Mr. Z. is extremely curious and he wants to understand the above mentioned paradox, so he starts to look for all the scientific publications concerning the role of sleep and dreaming. It can be taken granted that after reading a few articles Mr. Z. will become even more confused then ever as a result of the numerous contradictory results, opinions and theories. In the last few decades sleep has been associated with mood-regulation, coping and with stress (e.g., Berger, Riemann, & Lauer, 1986 in Bódizs, 2000), memory consolidation (e.g., Payne & Nadel, 2004), semantic rewriting of episodic memories (Stickgold, 2002), emotional information processing (Hartmann, 1996), facilitation of social bonding (McNamara, 1996), or even that sleep has nothing to do with information processing, but only serves pure biological functions (e.g., Vertes & Eastman, 2000). 1. The memory consolidation hypothesis 1.1 The first steps The first (mainly animal) experiments focusing on the effect of sleep in learning and memory used the different methods of sleep deprivation paradigm. According to the main hyphothesis (Fishbein 1970 in Ellman, Spielman, Luck, Steiner, & Halperin, 1991), the newly aquired informations are stored in a fragile state, and REM sleep is the necessary process to consolidate the new information. In one popular experimental design for example the animal’s sleep was selectively blocked after a learning task, and then the investigators analized if the REM deprived, NREM deprived and the control group showed different performance in the previously 1 Simor: Sleep, dreaming and memory learnt task. In some studies the REMdeprived group showed some performance deficits compared with the other groups (Bódizs, 2000), but in other studies the investigators failed to demonstrate such relationship (Siegel, 2001). Among the great number of the righteous methodological critics, maybe the most important is, that the sleep deprivation technique apart from depriving the subject from a certain stage of sleep, introduces a lot of uncontrolled variables such as stress, hipersexuality, irritability, hiperphagia, etc., which can interfere with the required learning process. 1.2 Implicit and explicit memory After the first REM deprivation techniques, with the appearance of more sophisticated experimental paradigms (including human experiments), interesting results enriched our knowledge about the relationship between sleep and memory consolidation. One important finding is that different types of memory (implicit vs. explicit) are best consolidated during specific stages of sleep (Payne & Nadel, 2004). The experimental data suggests the following conclusions: 1. REM sleep may facilitate the consolidation of implicit, procedural memories (Plihal & Born, 1997). 2. REM sleep is an opened state for information processing: the classical conditioning can be generated in REM, and the conditioned response appears in waking too. Equally in REM sleep the subject responds (with the activity in the specific brain areas) to the conditioned stimulus learnt while awake (Bódizs, 2000). 3. NREM sleep, especially slow wave sleep is critical for the consolidation of explicit, episodic memories. 4. The effect of NREM sleep applies both for verbal tasks and spatial tasks (Plihal & Born, 1997). 1.3 Emotional and neutral information Another interesting result is that REM sleep can also facilitate the consolidation of explicit memories, if the information is emotionally relevant and even more if it is emotionally threatening for the subject (Karni, Tanne, Rubenstein, Askenasy, & Sagi, 1994). While the emotionally neutral information may benefit preferentially from NREM sleep. The emotional information processing in REM sleep is probably related to the mood-regulating effect of REM sleep and the emotional aspect of dreams, but the mechanism underlying this emotional functioning is still unclear, and open for debate. 1.4 The sceptics However some researchers (e.g., Siegel, 2001; Vertes & Eastman, 2000) are still sceptical about the information processing in sleep; they say that sleep serves “cruder”, purely biological functions. For example, according to Vertes the general purpose of sleep is the recuperation of the CNS, and the function of REM sleep is to “mobilize”, prepare the brain for the recovery of sleep. Perhaps the most persuasive argument of these “sceptics” is that a lot of SSRI antidepressant reduces REM sleep, but people taking these antidepressants doesn’t show any cognitive or memory deficits. As an alternative hypothesis we can suggest, that in the cases of reduced REM phase maybe other compensatory mechanisms (in NREM sleep or waking) assume the “job” of the “dismissed” REM sleep. 2. Dreams: The movie of the sleeping consciousness 2.1 The explicit reflection of unconscious processes If we speak about dreams we have to be aware of that we jumped to the phenomenological, psychological level of analysis. Are the aforementioned implicit processes reflected in dreams? The experimental results regarding this question PERIODICALS OF IMPLICIT COGNITION (2006) 7 2 Simor: Sleep, dreaming and memory are really scarce, but there is some evidence that the consolidating memory traces can be incorporated to our dreams (Koulack, 1975 in Bódizs, 2000; Schwartz, 2003), but in a transformed way, influenced by the “logic” of the dream narrative. For example if I dream that I fight against a terrible orc, using a newly aquired aikido techique, the dream may reflect a procedural memory consolidating process (the movement that I learnt the day before). And the face of the orc could be the phenomenological correlate of the activation of the face processing modules of my brain, working with an emotionally relevant dvd experience. Of course these assumptions are speculative, but maybe with the merger of the Brain Imaging methods and sleep mentation techniques this kind of young research trend will also gain strength. 2.2 NREM dreams vs. REM dreams – phenomenological aspects The nature of the dream mentations of the awakened subjects, is different if awakened of NREM or REM sleep. Generally we can say that REM dreams are more bizarre, story-like, visually dominated and emotionally more intense, than NREM dreams, which are more thought-like, more logical, less visual, and emotionally less absorbing, less vivid mental experiences (Bódizs, 2000) 2.3 The memory sources of dreaming Although our dreams are influenced by our memories, they are not a replay of complete episodic memories (Fosse, Fosse, Hobson, & Stickgold, 2003), only isolated fragments, or metamorphosed elements appear in the healthy1 subjects dream. But once again REM and NREM dreams seem to be different according to the memory sources 1 In PTSD the traumatic event realistically (with coherent perceptual and spatio-temporal details) “returns” in the nightmares of the traumatized person (Stickgold, 2002). of the dream formation (Baylor & Cavallero, 2001). While REM dreams are mainly associated with the semantic, and abstract self-referential knowledge system, NREM dreams may be built mainly by elements of episodic memory (Cavallero & Cicogna, 1993). 3. The neuropsychological perspective 3.1 The neocortical-hippocampal dialog Apart from the slight differences among the theorists, there is a consensus, that altered hippocampal functioning during sleep accounts for the fragmented episodic memories during dreaming (Nielsen & Stenstrom, 2005), for the different memory consolidating effects of the different stages, and for the different memory sources in the two main stages of the sleeping brain. In waking the hippocampal-neocortical dialog is bi-directional, information flowing in both directions. In NREM sleep only the hippocampus sends information to the cortex, while in REM sleep only the neocortex send information to the hippocampus, and the hippocampalneocortical outflow is blocked (Buzsáki, 1996, in Stickgold, 2002). 3.2 The Emotional Brain and the Dreaming Brain After borrowing the Emotional Brain and Dreaming Brain denomination from LeDoux (1996) and Hobson (1988) respectively, now let’s see some of the neuroanatomical correlates of dreaming, and the aforementioned (1.2; 1.3; 2.3) memory processes. According to the neuropsychological perspective (Muzur, Pace-Schott, & Hobson, 2002; Hobson, 2000) the lack of self-awareness, and the impairment of logic planning, and other executive functions is due to the selective deactivation of the prefrontal cortex (namely the deactivation of the dorsolateral prefrontal cortex) during REM sleep. At the same time the elevated activation of the limbic structures, such as the amigdala can PERIODICALS OF IMPLICIT COGNITION (2006) 7 3 Simor: Sleep, dreaming and memory contribute to the emotional aspect of dreams, and the enhancement of emotional information processing during REM sleep, because the amigdala can modulate the consolidation of emotional memories (McGaugh, 2000). 3.3 Emotions as the deep structure of dream narratives If the higher-order cognitive functions can’t organize the dream scenario, what are the main principles that structure these often surrealistic stories? One proposal is that emotional patterns, or the individuals main emotional concerns interlaces the dream content (Nielsen & Stenstrom, 2005). So maybe at the background of the bizarre associations in dreams there are ego-relevant emotional patterns. Conclusion Sleep and dreaming may serve several functions simultaneously, according to the different needs of the organism, so taking Hunt’s (1989) opinion it’s worth to be really “polytheist” if we want help MR.Z to resolve his confusion. Invited Peer Commentary Slow wave sleep, brain oscillations and the reptilian wake state Andras Czurko Institute of Behavioral Sciences, University of Pecs, 12, Szigeti Street, Pecs, H-7624, Hungary [email protected] 3.1 In connection to the hippocamponeocortical dialogue (Buzsaki, 1996), I think it is important to note that the (declarative) memory trace formation is supposed to happen in two stages (Buzsaki, 1989). The acquisition of information may happen during the theta/gamma oscillations (awake state and REM sleep). The consolidation of the memory, the hippocampal-neocortical transfer, on the other hand, is carried by discrete quanta of cooperative neuronal bursts during slow wave sleep (SWS or NREM) and associated with sharp wave/ripple (SWR) network patterns (140200Hz frequency oscillations) in hippocampus and enthorhinal cortex (Buzsaki, 1998) (Chrobak & Buzsaki, 1996; Csicsvari, Hirase, Mamiya, & Buzsaki, 2000; Ylinen et al., 1995). There is also direct experimental evidence that certain firing sequences are replayed in a time-compressed form during SWS and their occurrence is correlated with SWRs (Lee & Wilson, 2002; Nadasdy, Hirase, Czurko, Csicsvari, & Buzsaki, 1999). Recently on humans explicit memory formation during SWS can be boosted by transcranial application of slow oscillating potentials (Marshall, Helgadottir, Molle, & Born, 2006). 3.2 In case of implicit memory formation, it is well known that not the hippocampus and the mediotemporal memory system is the key player. From the neuropsychological perspective, not only the prefrontal cortex and the amygdala but equally the caudate nucleus, the cerebellum and sensory cortical areas are involved in the different domains of implicit memory (Squire & Zola, 1996). From this perspective I think it is easy for Mr. Z. to understand that here the REM sleep is likely to play the primary role (1.2). Finally, I think an evolutionary perspective is missing from the target article. I’m sure that first it can be surprising for Mr. Z that our mammalian SWS can be a homologous states to the reptilian waking state, but I’m sure that from this perspective several of the confusing issues of our sleep and dreams can be resolved (Nicolau, Akaarir, Gamundi, Gonzalez, & Rial, 2000; Rial, Nicolau, Lopez-Garcia, & Almirall, 1993). PERIODICALS OF IMPLICIT COGNITION (2006) 7 4 Simor: Sleep, dreaming and memory Dream and memory - in waking Szinovszki Bence Faculty of Pedagogy, Eötvös Loránd University, 23/27, Kazinczy utca, Budapest, 1075, Hungary [email protected] 1. Adding thoughts to the target article from the perspective of waking consciousness I sketch some connections between the previous article about unintended implicit recall (Marno, 2006) and dreams. In the cultural tradition of dream interpretation (included some scientific researches nowadays as well) the main question of the waking consciousness is if the dream movies contain any useful information for the waking mind or not (Halász, 2000 in Bódizs, 2000). In scientific approaches could transform this point of view to the question of adaptation: an important part of that could be the analysis of dream's recall mechanism. How can we access to the movie seen in sleep? What are the conditions of recall? And what could be the role of the access? This approach is not interested in the differences of explicit and implicit structures but take notes of their connections, relationships and communications. 2. In waking it is easy to forget and difficult to recall the dream. Memory or oblivion depends on awakening. Hobson's (1988, in Atkinson, 1996) fixed that the distraction of awakening fails the dream consolidation and dream contents disappear from memory. On the other hand we may presume these contents only get unaccessible for the intentional search in waking consciousness. In many cases after in a natural situation (not in laboratory) the dream comes back "by accident". For example when Mr. Z going home from his work suddenly set to remember his dream of last night. Possibly he can't tell us why he recalls or what kind of stimulus led him to call it back. While he insist that he dreamt today and he didn't remember it in the morning. About process of non-strategic recall Marno cites Moskovitch's theory (2000, in Marno, 2006) which appointed that during spontaneous recall the limbic structures are activated without prefrontal cortex and this allows to associate directly the abstract ambient stimulus with particular memory experiences. In addition, Kónya (2006) makes a distinction between the two principle part of spontaneous memory contents: actual pre-semantic core deriving from amygdala (unintended or automatic memory) and the associated semantic part of that (memory without intention). Comparing this mechanism with the neuropsychological aspects of dream mentioned in the target article (3.2) we could explore analogue processes between the dream arising in REM phase and spontaneous recall in waking. Using the previous distinction in the field of dream it seems that during REM pre-semantic memories, namely amygdala-memories are associated with semantic reflections in consciousness. Otherwise it would offer a good explanation that the deep structures of returning dreams and their emotional connotations and reflections given by the individual in waking are consistent in spite of the contrasts in their content. 3. I would like to bring back once again the example about Mr Z, who suddenly recalls the dream of his last night. We could see it as a special case within the spontaneous memory when effected by the actual perception, an unintended memory interlaces with the semantic element coming from representation of dream and not from the representation of autobiographical memory. Marno agrees with Mace (2004, in Marno, 2006) that for supporting to realize the intentions during unconscious recall process the individual's aim is to recall a PERIODICALS OF IMPLICIT COGNITION (2006) 7 5 Simor: Sleep, dreaming and memory particular memory from the past that is accordant with his actual situation. The memory of a dream could be more informative to interpret a particular situation than memories coming from waking stage? Probably, it could. If we presume the primary role of pre-semantical memories in this interpretation process. Anyway this example can illuminates the working mechanism of implicit memory showing the connections between the sleeping and waking consciousness and emphasizing the important role of emotions in the adaptation. In addition, referring a previous issue about the evolution of consciousness I would like to cite those theories within the authors presumed that the function of implicit structures is the adaptation to the environment, contrary to explicit structures which support the adaptation of the environment to ourselves (Holyoak and Spellman, 1993 in Aczél, 2006). REFERENCES (together for all the articles in this issue) Aczél, B. (2006). A tudat evolúciója. 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