>Top 2-3. Dreams are for memory consolidation:
- Dreams are for memory consolidation
- Perhaps the leading contemporary theory is that dreams somehow involve memory consolidation and storage, often via a proposed form of memory replay. The dominant metaphor for this theory of consolidation is that of the computer: memories need to be “stored” somewhere in the brain, like storing a computer file on a hard drive, and therefore there must be a storage process. This viewpoint is held by much of traditional cognitive neuroscience, wherein the goal of the brain is to “store” memories as veridically as possible, although there is growing recognition that veridical “computer-like” storage is not desirable for complex learning and that forgetting is just as important. According to the memory consolidation hypothesis, memory storage occurs during dreams, or alternatively dreams, by accessing previously stored memories, strengthen them, or that somehow dreams are a byproduct of integrating new memories with older ones.
- >Top There is a significant line of research that draws from this theory, including many neuroimaging studies, and a full review of the literature would be beyond the scope of this paper. However, there is also debate. Specifically the consolidation hypothesis is both very broad and rarely meant to specify just dreams rather than sleep in general. For example, there is evidence that learning a new task leads to a greater activation during both REM and slow wave sleep in the task-relevant cortical areas, which indicates there is no preferential consolidation during dreaming. This is true even when comparing a wake/sleep condition versus a control condition without sleep but over the same time, which has found that blood-oxygen-level-dependent (BOLD) activity increased in associated brain regions with the task. But these sorts of neuroimaging studies are not very specific, since increased activation in relevant areas does not actually mean storage, nor replay, nor integration with existing memories. Indeed, they could be interpreted just as easily for evidence of the OBH (Overfitted Brain Hypothesis ) (see “Evidence from neuroscience”).
- A significant line of direct evidence for the consolidation theory comes in the form of “replay” of memories during sleep, a specific hypothesis with a clear thesis and standards of evidence. Replay was originally discovered in the hippocampus of rats, although the original analysis was again for slow wave sleep, not correlating this process to dreaming specifically. Indeed, the same statistically increased firing in correlated neurons that counts as “replay” occurs during quiet wakefulness, indicating it has nothing specific to do with dreams. In general, if two neurons potentiate at the same time and from the same cause, they are more likely to be correlated in the sense of increased firing in the future, regardless of whether they are replaying anything specific, a view supported by the finding that “replay” events are much faster in terms of their firing.
- There are a number of significant issues with the specific hypotheses that dreams are replaying memories. First, offline replay of episodic memories may not actually assist memorization; since ground truth is absent offline, such replays might actually introduce errors. Indeed, neuroscience has shown that re-accessing memories generally changes them, rather than enforces them. Due to the issues in assuming that specific memories are actually replayed, a number of authors have proposed more complex theories of consolidation, such as that the idea there exists two complementary learning systems and replay connects these two systems together. Furthermore, the hypothesis must grapple with the fact that well-controlled neuroimaging experiments show little evidence for exact sequence replication and strong evidence for mostly never-before-seen firing patterns. In light of this sort of evidence, some have argued that sleep promotes gist extraction from specific memories, although these sorts of hypotheses have not specifically been about dreaming.
- >Top Overall, replay is unlikely to be the purpose of dreams, since, as previously discussed, based on the most detailed studies on dream reports after awakenings, dreams are not connected or, at most, only vaguely connected with the day's events. Overall, it appears that less than 1% to 2% of dream reports have anything to do with episodic memories. Except in cases of posttraumatic stress disorder (PTSD), dreams do not repeat specific memories, and those that do are considered pathological; for instance, closely after Hurricane Andrew, the only hurricane-related dreams, even from a sample of the population from the hardest hit area, were from those already diagnosed with PTSD. Close studies of dream journals have in general found that replays of specific memories or a day's events are generally either rare or nonexistent, although they do in general involve actions and people the dreamer is familiar with. One study examining dreams after the terrorist attacks of 9/11 found that “not a single one of the 880 dreams (440 of them after 9/11) involved planes hitting tall buildings or similar scenarios, even though all the participants had seen these events many times on television (and it was clearly an emotionally important experience). No scenes were pictured that were even close to the actual attacks.”
- Indeed, there is significant evidence that episodic memory and dreams are dissociated. >Top While there is behavioral evidence that repetitive daily tasks, like having subjects play Tetris for significant periods of time, can lead to Tetris-inspired dreams, such inculcated images or sequences do not represent replay in that they are not veridical repeats of previous games, being more hallucinatory and sparse in content and only being loosely related to the played game, such as dreaming of playing some altered version of a maze game after being exposed to it during the day. Moreover, dreams triggered by such repetitive games appear even in patients completely lacking all memory, those with clinically diagnosed amnesia. It can take several days for repetitive tasks to show up in dreams, a form of “dream lag,” and almost always these tasks appear in partial forms that are, again, only loosely similar. Overall, the behavioral evidence suggests that dreams are not replays of memories or waking events.
- There is a strong line of evidence from Tetris-studies to sudden wake-ups to dream-lag effects showing that partial or loosely similar dreams can be triggered most reliably by recently learned tasks, and yet such inculcated dreams generally take the form of never-before-seen experiences or sequences with the traditional dream-like properties of sparseness and hallucination, matching no specific memory but rather a seeming exploration of the state-space of the task itself.
- >Top It is worth noting that in most cases the sparse, hallucinatory, and narrative properties of dreams are unaccounted for by the consolidation hypothesis. Most dreams do not involve specific memories at all, making the integration of new memories a questionable purpose for dreaming. Indeed, it is openly admitted that the consolidation hypothesis still views dreams themselves as epiphenomena. As we will see, much of the supporting results for the integration, replay, or storage of memories actually fits better with the OBH (Overfitted Brain Hypothesis) (“Evidence from neuroscience”).
- Dreams are for selective forgetting
- >Top Notably, Francis Crick and Graeme Mitchinson proposed an alternative purpose for dreams in 1983, which they called “reverse learning.” In this hypothesis, the point of dreaming is somehow to remove “undesirable” connections and help the brain “unlearn.” Yet this hypothesis has been largely ignored in contemporary dreaming research. Instead, the alternative hypothesis, that dreams involve replay or consolidation of memory, became favored by the community due to the excitement around early replay results. Contemporary neuroscientific research often views there as being both a consolidation phase as well as a forgetting phase for memories, although this is again predominately associated with slow wave sleep, rather than dreaming specifically. Within deep learning it is known that averaging together models and renormalizing can assist in learning which could broadly be thought to resemble some sort of selective forgetting in real neural networks. More specifically, it has been proposed that Boltzmann machines may actually implement something very similar to the Crick and Mitchinson notion of reverse learning wherein synaptic down-scaling could eliminate the discrepancy between environmental inputs and a system's internal model of the environment, which is an earlier version of the sorts of predictive processing theories discussed in the section “Dreams benefit predictive processing by refining generative models.” Indeed, it has been argued that down-scaling of synapses might themselves prevent overfitting, thus providing a possible link between SHY (Synaptic Homeostasis Hypothesis), the OBH, and the original reverse learning hypothesis.
- Recently there has been explicit computational modeling in spiking neural networks showing that “reverse learning” in the form of reverse learning rules can indeed prove helpful. Specifically, the authors showed that an anti-learning rule during a “sleep phase” of the network, in the form of anti-Hebbian learning, could break up attractor states that were detrimental to learning. It is worth noting that breaking up detrimental attractor states could lead to similar outcomes as the OBH. However, in general any sort of “reverse learning” is not necessary from the perspective of the OBH. This is because “reverse learning” approaches differ significantly by focusing on how specific memories are destroyed (via anti-learning mechanisms like a hypothetical “reverse Spike-timing-dependent plasticity”) rather than how corrupted inputs or top-down noise can improve generalization like in the OBH.
- veridical: truthful
- memory consolidation: 記憶の固定化
- gist: 要点、趣旨
- 現代の代表的な理論は、夢は何らかの形で記憶の 強化と保存に関わっているというもので、多くの場合、記憶の再生という形で提案されている。
- 9.11のテロ後に見た夢を調査したある研究では、880回の夢 (440回は9.11以降) の中で、飛行機が高層ビルに衝突するなどのシナリオを見たものは一つもなかったが、参加者全員がこれらの出来事をテレビで何度も見ていた。
- 繰り返し行う作業が夢に現れるまでには数日かかることがあり (これは"Dream lag"と呼ばれる) また、ほとんどの場合、大まかにしか似ていない部分的な形で現れる。夢は記憶や覚醒時の出来事の再現ではないことが示唆される。
- Francis CrickとGraeme Mitchinsonは1983年に、夢の別の目的を"逆学習"と名付けて提案している。この仮説は、夢を見ることで、"望ましくない"結合を取り除き、脳が"学習しなくなる"のを助けるというもの。
- 逆学習: シナプスを縮小することで、環境の入力とシステムの内部モデルとの間の矛盾を解消する。
- 実際、シナプスのダウン・スケーリング自体が過剰適合 (overfitting)を防ぐのではないかと言われており、SHY、OBH、そして当初の逆学習仮説の間に関連性があると考えられている。
- "逆学習"のアプローチは、OBHのように、破損した入力やトップダウンのノイズがどのようにして汎化を向上させるかではなく、特定の記憶がどのようにして破壊されるか (仮説的な"逆スパイクタイミング依存可塑性" のような反学習メカニズムを介して) に焦点を当てている点で大きく異なるからである。
- Cf: Sigmoid Function: