This is my understanding:
1: both HC and the cortex learn during day.
2: cortical learning is STM, which must be consolidated via HC. Probably an exaggerated version of Hebbian coincidence (HC AND CC) learning, through burst mode.
3: unconsolidated cortical memory can’t last until sleep, and most likely needs to be used during day anyway.
4. thus, HC has to be consolidating CC quite frequently. probably during alpha waves.
Check out the engram paper.
For sleep to take so long, it must be doing some heavy lifting at the molecular/cellular level. My money is on manufacturing a resource that gets consumed during daily mental activity.
If it’s new neurons, then they also have to be wired in and the old ones disposed of. It’s easy to imagine all the other phenomena (sleep cycles, dreaming etc) being a side-effect of rewiring, not the primary purpose.
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That’s my point, routine memory consolidation (STM->LTM) should not depend on sleep. Sleep involves a lot of synaptic pruning, so the purpose of sleep spindles may be to protect / rebalance important new memories during this pruning. But it’s all about connections, I don’t believe there is much neurogenenisis going on.
That paper covers a lot of hyposesses, could you be more specific?
For example, there is transfer theory of HC function: " As proposed by David Marr in his model of hippocampus-dependent memory [3] and supported by many experimental and clinical studies, episodic memories are transferred after acquisition from the hippocampus to the neocortex for long-term storage.".
That I think is contrary to a pointer theory: primary memories are formed and remain in CC. HC forms a more abstract copy that latter reconsolidates CC memories, this is different from forming brand new CC memories via transfer.
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Really appreciate the info, lots more learning ahead…
Is it really known or is it know in a Donald Rumsfeld sense and more of a known unknown ? We know what goes on but have no idea as to how or where as per “entirely possible”.
Still to read half a dozen other papers…
I like the first paper, with the really interesting section “We also tested the possibility that memory traces are embedded to the cortical network during the state preceding sleep (e.g., wakefulness), and we asked the question how these traces would be modified by slow oscillations. We found that spike sequences of cortical neurons reflecting those embedded synaptic patterns replayed during ongoing slow oscillations and that led to further enhancement of characteristic synaptic patterns (data not shown).”
The slow wave replay I get and can agree with as to a process as this very much fits “within” my perception as to what goes on. Weather it is occuring in HC or Cortex the paper does not address at all as it theorises it could happen in either. The slow wave need would seem to be partially negated if Max’s theta cycles occur (reinforcement within the 200mS windows - which coincidentally is near the magic number 7 if looking at 25mS LTP…)
My theory on this is the parallel replay of memories creates a hallucinatory effect (what we then think of as dreaming) as the brain makes a reality from the out of temporal order memory replay. Wakeful state prevents parallel memory replay due to focus of attention limitations (winner takes nearly all). Parallel replay is then a critical requirement for sleep spindles to have an effect as it is the combination which allows for a different type of pooling to occur.
I would agree with this proposition. How can the cortex block any surprise from being learnt within a column after it has fired within the LTP window ? Sensory input has to arrive at the cortex to identify surprise.
I believe the HC learns a completely different representation to the cortex as it is a more abstract process of temporal phasing which is a function of hierarchical navigation, i.e. creating short cut paths to the right “next” palce in the cortex.
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If I implied that the cortical portion formed during sleep is new memories I must have worded it poorly - that is not what I think.
My take is that the EC/HC and cortex are specialists in different kinds of learning, with the EC/HC being good at one-shot learning and the cortex more oriented at classic Hebbian learning. The cortical traces formed during the day fade at a exponential rate to some very low level latent memory. The sleep process transfers from one limited capacity learning system (EC/HC) to the higher capacity cortex by reinforcement of the weak latent engrams. I think of it like developing an image on film.
The EC/HC index part is the access to the latent cortical information that will be consolidated during the sleep cycle until the information is boosted in the cortex. Being located at the top of the hierarchy as the EC/HC is, pushing these keys back on the top of hierarchy ripples back down through the hierarchy to re-access and reinforce the engrams that make up the declarative (episodic) memories, normalizing the engram between between the two systems.
It is very likely that the combination of cycle time primes the cortex to facilitate the process over and above what happens during the initial learning process. If the “awake learning” used the same process it is likely that it would saturate as the EC/HC does if not “flushed” by the sleep process when it fills up.
It is well known that sleep deprivation causes memory errors like hallucinations. I assume that this is what happens when the EC/HC memories reach the limits of coding capacity and bleed into each other.
I should add that the cortex layer 2/3 pattern memory is different than the layer 5/6 sequence memory in this context. I see the 2/3 layers as the primary parts of the cortex that is affected by this transfer of memory contents. The map-2-map direct connections are primarily connected to layers 2/3.
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In that case they won’t be accessible to the cortex during day, which is seems necessary to me. Even during sleep, signal-to-noise in such faded memory must be horrible, I don’t think the reinforcement by HC would actually work.
I violently disagree. HC is nowhere close to the top, and kind of lateral to the cortical hierarchy anyway. The top of task-positive network is dlPFC, and that’s below DMN. In DMN, the top is mPFC-PCC loop: https://neurosciencenews.com/brain-evolution-abstract-thought-20364/
I don’t see how this is related, HC can be emptied during sleep regardless. And there is plenty of such emptying / pruning during sleep in CC too.
Not just HC, CC can be overloaded during the day too.
Opinions vary.
I see the temporal lobe as the top of the WHAT/WHERE sensory hierarchy before being presented to the EC/HC. At that general point in the hierarchy (temporal lobe) the cortical projections are sent to various sub-cortical structures.
There is a top-down motor hierarchy that originate with the sub-cortical projections to the prefrontal cortex and after several hops through frontal cortical maps, terminating in the motor cortex along the central sulcus.
The two hierarchies are broadly interconnected, with a prominent example being the two speech centers Broca’s area in the prefrontal cortex and Wernicke’s area in the posterior temporal lobe.
The motor output and sensory input areas along the central sulcus are very heavily interconnected.
So we have an ascending and descending hierarchies, connected at the top through the subcortical structures and at the bottom along the central sulcus - somewhat like the ouroboros - a circular structure.
Deciding what is the start and end presents some definition difficulties but I chose to put the boss, the sub-cortex, at the top of the sensory hierarchy. I chose to put the output driving the body as the terminus of the sense-act chain. This is, after all, why nature puts a brain in control of the body.
Well, you know what I think about your boss 
Some new confirmation, memory-consolidating ripples are not specific to sleep: Brain Ripples May Help Bind Information Across the Human Cortex - Neuroscience News
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The UC San Diego team, led by Halgren, found that ripples also occur in all areas of the human cortex, in waking as well as sleep.
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Personally, I hate science journalism. The original paper (PNAS, open access) is here.
…and thanks for the tip, excellent reading.
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So, that was on micro-sleep. And now we have more on micro arousals: Stress Transmitter Wakes Your Brain More Than 100 Times a Night, and It Is Perfectly Normal - Neuroscience News
The basic mechanisms of memory consolidation don’t seem to be very different, in sleep or awake.
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