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Sleeping origin?

Simon Schultz schultz at sedal.su.OZ.AU
Mon Jul 31 17:10:33 EST 1995

In article <3vfotb$hih at nntp3.u.washington.edu>, dfitts at u.washington.edu (Douglas Fitts) writes:
|> bill at nsma.arizona.edu (Bill Skaggs) writes:
|> >Yes, this is known as the "radiator hypothesis".  It was presented in
|> >a Behavioral and Brain Sciences article a little while back, together
|> >with assorted critical review.
|> >As for my own opinion, it seems to me that Mother Nature could have
|> >worked out a better way of cooling off the brain, something that
|> >wouldn't turn animals into inert blobs and leave them at the mercy of
|> >any predator that happens to wander along.
|> If you fell asleep at the water hole you probably wouldn't pass on your
|> genes. If you hid and built a nest, maybe you would.  I haven't heard many
|> really appealing theories of sleep, but it seems to be metabolically
|> efficient -- shutting down arousal/activity for hours a day can reduce
|> energy demands, hence the number of hours necessarily spent
|> foraging/hunting.  Probably also reduce water demands.  If this time was 
|> spent in a nest, hole, quietly perched on a twig, or standing up in the 
|> middle of a herd, it might also *reduce* predation risk, no?

How about the following idea? That sleep is in fact an "active refresh"
system for memory, analogous to the active refresh we need to apply to 
dynamic RAM memory chips to extend the storage time associated with the physics
of the storage medium. (Even though in the DRAM the memory is not associative, 
the analogy still holds).

Assume for a moment that the fundamental cortical long-term storage mechanism
is associative long-term potentiation (LTP). The physics of this mechanism
appears to dictate a time for complete decay of a stored synaptic
enhancement of a few weeks. This means that we should refresh all important
memories at least every few days, preferably once per night. Assume that
memories, both in hippocampus and neocortex are both stored in an autoassociator
(error-correcting coder) of some kind [1]. Sharp-wave rhythm corresponding to
communication back from hippocampus to neocortex is found in slow-wave sleep.
This could correspond to the actual active-refresh memory consolidation
mechanism. REM sleep would involve reloading the hippocampus with the
important information learned during the preceding day. [2] Memory is thus
cycled through a hippocampal-neocortical active refresh loop.

Note: this does not preclude different "daytime" function of the hippocampus
in episodic memory etc. Possibly the hippocampal episodic memory system
would be involved itself in working out what are the important memories
from the day that need to be refreshed. Projections from amygdala etc. would
of course help in this.

Actually, I've no idea how memories are stored neocortically, but whatever
the circuit is, I think its guaranteed to need refresh of some kind. I
suspect that it may be computationally more desirable to use a refreshed
moderate-term storage mechanism (discard the rubbish) than to evolve an
indefinte-duration storage device.

[1] Treves & Rolls, Hippocampus 4(3) 374-391
[2] Buzsaki, Neuroscience 31(3), 551-570


   Simon R. Schultz,			Ph:  ( +61-2 351 4768 )
   Dept. of Electrical Engineering,	Fax: ( +61-2 660 1228 )
   Building J03,			http://www.sedal.su.oz.au/~schultz/
   University of Sydney, NSW 2006.	email: schultz at sedal.su.oz.au


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