Reddit reviews Learning & Memory

>So I think researchers don't believe our brains do backpropagation like is used in machine learning neural networks

Absolutely correct.

Also, there's multiple memory systems in the brain, and they each operate using different mechanisms (at least on the level of analysis you're discussing). You're discussing motor memories, which is fairly distinct from other types of memories like declarative, episodic, or semantic.

The good news is that people have been working on the questions you have for years, and there's some useful textbooks that you might read to get a start at better understand the psychology and neurobiology of learning and memory (which is a very rich area of research). One thing you'll learn is that there are analogies between the way neural networks learn and operate and how brains do it, but the actual mechanisms are very different.

Here's one from the late great Howard Eichenbaum: https://www.amazon.com/Learning-Memory-Howard-Eichenbaum/dp/0393924475

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>This could correspond to the neocortex having access to the initiation of each action, and sending the signal to initiate each action in the sequence required. Eventually each action grows direct connections from one action initiation to the next, so that the actions happen smoothly, right after one another. I was wondering if this could be the kind of learning that happens while we sleep.

So, this is an interesting thought. We know in rodents that memories for episodes (temporally ordered sequences of events) are replayed during sleep. The belief is that this reflects (in part) consolidation of that memory (a process that moves the dependence of episodic memories from the hippocampus to the cortex, and makes them highly resistant to degradation). We know this happens for episodic-like memory representations (and that this sequential replay likely happens because the circuitry of the hippocampus is well-suited for representing sequences), and while I'm sure consolidation happens for motor memories, I'm not sure if the mechanism is similar as that for episodic memory.

What you're asking about is called declarative memory. There are two kinds of declarative memory: episodic memory, which is for things that happened to you, like your store example; and semantic memory, which is for facts and procedures, like your coding example. (Wikipedia pages linked.)

There's a lot we still don't understand about declarative memory - for example, I don't have anything to tell you about the sensation of memory finally "clicking" - but we do know that the hippocampus is a crucial brain region involved. Studies of people with amnesia tell us that injuries to the hippocampus tend to result in impaired episodic and/or semantic memory. We also know that sleep is important for memory consolidation, and a lot of research has been done on what the hippocampus does during sleep.

Connections between the cortex and hippocampus are also important. One of the things the cortex does is process sensory information, so a lot of the information contained in memories comes to the hippocampus from cortex. So a very simplified theory might say that sensory information is processed in the cortex, then stored in the hippocampus, consolidated during sleep, and then sent back to the cortex when you remember it.

The hippocampus is also very important for spatial memory and awake spatial representation, and it's been shown that when rats are sleeping after learning how to navigate a new environment, hippocampal cells "replay" the activity they had when the rat was awake and exploring the environment. It's not completely clear how the hippocampus' role in spatial representation interacts with its role in episodic memory (yet).

Another interesting idea, which has been debated a lot over the years, is that there might be neurons with very specific jobs that contribute to memory. For example, is there a single neuron in your brain that recognizes your grandmother? People call these "grandmother cells" or "concept cells", and disagree on whether or not this idea is even plausible.

Here are articles about the things I've mentioned:

• Science article about hippocampal injuries and amnesia
  
  
• PLOS ONE article on sleep and declarative memory
  
  
• Nature article that's a good starting point for learning about hippocampal place cells and spatial memory
  
  
• Nature Reviews Neuroscience article giving a theory of how cortical-hippocampal circuits underlie declarative memory
  
  
• Neuron article about integrating the hippocampus' role in spatial memory with its role in declarative memory
  
  
• Nature Reviews Neuroscience opinion article about concept cells
  
  
  If you're not affiliated with a university, it may be difficult to get ahold of these articles. Here's a pretty good textbook on memory that you should be able to get on interlibrary loan at your local library if you're persistent.
  
  EDIT: fixed some formatting problems and typos