editione1.0.2Updated November 2, 2022
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And it is only after seeing man as his unconscious, revealed by his dreams, presents him to us that we shall understand him fully. For as Freud said to Putnam: ‘We are what we are because we have been what we have been.’André Tridon*
It is a well-known fact that memory formation and learning are related to sleep. A rested mind is more capable of learning concepts, and the human brain does not have as detailed a memory of yesterday as it has of the present day. In this chapter, I detail how the brain learns during sleep, describing hippocampal replay, visual cortex replay, and amygdala replay. They are all mechanisms the brain uses to convert short-term memory into long-term memory, encoding the knowledge stored throughout the day. The same circuitry responsible for decoding information from the neocortex to support memory recall is also used for imagination, which indicates that the brain does not record every moment and spends time learning during the night.
In 1995, the complementary learning systems (CLS) theory was introduced,* an idea that had its roots in earlier work by David Marr.* According to this theory, learning requires two complementary systems. The first one, found in the hippocampus, allows for rapid learning of the specifics of individual items and experience. The second, located in the neocortex, serves as the basis of the gradual acquisition of structured knowledge about the environments.
The neocortex gradually acquires structured knowledge,* and the hippocampus quickly learns the particulars. The fact that bilateral damage to the hippocampus profoundly affects memory for new information but leaves language, general knowledge, and acquired cognitive skills intact supports this theory. Episodic memory, that is the memory related to collections of past personal experiences occurring at a particular time and place, is widely accepted to depend on the hippocampus.
Figure: Hippocampus location inside the human brain.
The hippocampus is responsible for spatial memory (where am I?), declarative memory (knowing what), explicit memory (recalling last night’s dinner), and recollection (retrieval of additional information about a particular item like the color of your mother’s phone).
Hippocampal replay is the process by which, during sleep or awake rest, the same cells in the hippocampus activated during an initial activity are activated during sleep in the same order, or the completely reverse order, but at a much faster speed. Hippocampal replay has been shown to have a causal role in memory consolidation.
Howard Eichenbaum and Neal J. Cohen captured this view in 1988 with their suggestion that these hippocampal neurons should be called relational cells rather than the narrower term “place cells.”*
The hippocampus is an essential part of how memories form.* When a human experiences a new situation, the information about it is encoded and registered in both the hippocampus and cortical regions. Memory is retained in the hippocampus for up to a week after the initial learning. During this stage, the hippocampus teaches the neocortex more and more about the information. This process is called the hippocampal replay. For example, during the day, a mouse is trapped in a labyrinth and learns the path to get out. That night, the hippocampus replays the same neurons that were fired in the hippocampus and encodes the spatial information into the neocortex. The next time that the mouse is in the same labyrinth, it will know where to go based on the encoded information.
In this theory, the hippocampus, where synapses change quickly, is in charge of storing memories temporarily, whereas neocortical synapses change over time. Lesions made in the hippocampus and associated structures in animals are associated with deficits in spatial working memory and a failure to recognize familiar environments. Hence, consolidation may be an active process by which new memory traces are selected and incorporated into the existing corpus of knowledge at variable rates and with differential success according to their content.
The visual cortex presents the same kind of replay and acts in synchrony with the hippocampus.* Experiments show that the temporarily structured replay occurs in the visual cortex and hippocampus in an organized way called frames. The multicell firing sequences evoked by awake experiences replay during these frames in both regions. Not only that, but replay events in the sensory cortex and hippocampus are coordinated to reflect the same experience.
Frightening awake rats reactivates their brain’s fear center, the amygdala, when they next go to sleep.* In 2017, scientists at New York University (NYU), György Buzsáki and Gabrielle Girardeau, demonstrated this by adding rats to a maze and then giving them an unpleasant but harmless experience such as a puff of air.* From then on, the rats feared that place. “They slowed down before the location of the air puff, then [ran] super fast away from it.” The team also recorded the activity at the amygdala cells, which showed the same pattern of firing as the hippocampus. Their amygdalae became more active when they mentally revisited the fearsome spot.* These events may happen in order to store retained information in a different, lower-level part of the brain as well as in the neocortex, which is a more evolutionarily advanced part of the brain.
Buzsáki noted that it is unclear if the rats experienced this as a dream or if the experience led to nightmares. “We can’t ask them.” He went on to say, “It has been fairly well documented that trauma leads to bad dreams. People are scared to go to sleep.”
When people have new experiences, the memory formed by them is stored in the brain in different parts of the hippocampus and other brain structures. Different areas of the brain store different parts of the memory, like the location of where the event happened and the emotions associated with it.*
For a long time, neuroscientists who studied the brain believed that when we recall memories, our brains activate the same hippocampal circuit as when the memories initially formed. But a study in 2017,* conducted by neuroscientists at MIT, showed that recalling a memory requires a detour circuit, called a subiculum, that branches off from the original memory circuit.*
“This study addresses one of the most fundamental questions in brain research—namely how episodic memories are formed and retrieved—and provides evidence for an unexpected answer: differential circuits for retrieval and formation,” says Susumu Tonegawa, the Picower Professor of Biology and Neuroscience.*
The study also has potential insights regarding Alzheimer’s and the subiculum circuit. While researchers did not specifically study the disease, they found that mice with early-stage Alzheimer’s had difficulty recalling memories although they continued to create new ones.
In 2007, a study published by Demis Hassabis showed that patients with damage to their hippocampus could not imagine themselves in new experiences.* The finding shows that there is a clear link between the constructive process of imagination and episodic memory recall. We’ll discuss that further in the next chapter.
All low-level parts of the brain—including the hippocampus, visual cortex, and amygdala—replay during sleep to encode information. That is why it is easy to remember what you had for lunch on the same day but hard to remember what you ate yesterday. Short-term memories in the lower levels stay until your brain stores them and encodes all the knowledge during sleep. The neocortex stores relevant information encoded and compacted.
Deep neural networks also serve as a way of encoding information. For example, when a deep neural network classifies an image, it encodes it into the classified objects because the image contains more bits of data than merely a tag. An apple can look a thousand different ways, but they are all called apples. Turning short-term memory into long-term memory involves compressing all the information, including visual, tactile, and any other sensory material into compact data. So, someone can say that they ate a juicy apple yesterday but not remember all of the details of how the apple looked or tasted.
Memory recall and imagination serve as a way of decoding information from the higher parts of the brain, including the neocortex, into the lower parts of the brain, including the amygdala, visual cortex, and hippocampus. Memory recall and imagination may be only decoding the information that is stored in the neocortex.
John Anderton: Why’d you catch that?
Danny Witwer: Because it was going to fall.
John Anderton: You’re certain?
Danny Witwer: Yeah.
John Anderton: But it didn’t fall. You caught it. The fact that you prevented it from happening doesn’t change the fact that it was going to happen.
—Minority Report (2002)