Astral Codex Ten

hippocampus

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hippocampus

Article

hippocampus is a recurring concept in the Astral Codex Ten archive, appearing 2 times across 2 issues between March 16, 2021 and October 20, 2022. The archive places it in contexts such as “REM sleep renormalizes the hippocampus”; “The hippocampus, a structure involved in memory, has its own rhythm called hippocampal theta”; “The hippocampus is traditionally associated with memory”. It most often appears alongside Alaska, Alpha, Andres.

Metadata

  • Category: Concepts
  • Mention count: 2
  • Issue count: 2
  • First seen: March 16, 2021
  • Last seen: October 20, 2022

Appears In

Source Context

Recovered passages from the original issue text. When the raw archive preserved outbound links inside the source passage, they are listed directly under the quote.

Sleep Is The Mate Of Death
March 16, 2021 · Original source
What is the role of REM vs. non-REM sleep? Depressed people have much more REM sleep than non-depressed people. Serotonin seems to decrease REM sleep, so unsurprisingly SSRI antidepressants decrease REM sleep a lot (not just in depressed people, in everybody). This would lend itself very nicely to a theory where REM sleep is involved in decreasing synapse strength, depressed people have too much of it, they end up with overly weak synapses, and that's what depression is. In this model, antidepressants would treat depression by increasing serotonin levels in a way that represses REM. The problem with this is that in Tononi's original paper, he says that the best evidence supports synaptic renormalization in non-REM sleep; he doesn't have a great idea what REM is doing. He does mention one possibility is that non-REM sleep renormalizes most of the brain, but for some reason it doesn't work on the hippocampus, and REM sleep renormalizes the hippocampus. And some of the studies on depression and synaptic density point to the hippocampus in particular. But others don't, and this connection seems kind of forced. I think R&K mostly focus on slow-wave sleep and think it's renormalizing incorrectly rather than just too much or too little.
Inline links: decrease REM sleep a lot, original paper
Book Review: Rhythms Of The Brain
October 20, 2022 · Original source
Similarly, brain waves determine (or reflect?) which structures are “talking” to which other structures. The hippocampus, a structure involved in memory, has its own rhythm called hippocampal theta. When some part of your auditory cortex is trying to remember something, it goes into hippocampal theta, “links up with” the hippocampus, and for a while that part of the cortex and the hippocampus effectively form a single fluid computational unit. Then when it’s done, it goes back into its normal rhythm and separates from the hippocampus again. The progress of waves might even reflect causal priority; when a cat is using sight to navigate and smell to confirm, waves progress from a visual region to an olfactory region; when the cat switches to using the smell to navigate and vision to confirm, the waves reverse direction.
Theta is most important in the hippocampus. The hippocampus is traditionally associated with memory, but Buzsaki cites recent research showing that it also handles navigation (eg a rat navigating a maze, or a person walking through a familiar city). Why this combination of tasks? Rhythms sort of suggests that brain areas are less about specific tasks than about specific graph-theoretic arrangements, which are convenient for specific algorithms, which are convenient for specific tasks. The neural connections in the hippocampus are mostly local in a way that makes it a good “auto-associater”, ie you give it part of a pattern and it can recognize which pattern is being suggested. For complicated reasons this works well for both spatial tasks and memory. In fact, Buzsaki sketches out a way that episodic memory might have evolved from our navigation system, where eg remembering a list is equivalent to remembering a path through space, and remembering a specific episode is equivalent to remembering a landmark (he doesn’t specifically say so, but this is a fascinating match for the method of loci mnemonic device). Theta rhythm is an oscillation that helps divide navigation into time steps so you can figure out where you are now, where you were a moment ago, and where you’re going.
Inline links: method of loci
In one particularly fascinating section, Buzsaki notes that the frequency of gamma waves is about 7x the frequency of theta waves. He shows how this means that the hippocampus (a sort of master regulator of short-term memory) can “call” conscious attention “as a function” seven times within one of its time steps, which he links to the old adage that you can fit seven plus or minus two items within short-term memory.