Why do we sleep?

hcorbett at garnet.berkeley.edu hcorbett at garnet.berkeley.edu
Sat Mar 5 20:46:17 EST 1994


There is another theory as to "why we need to sleep" outside of the 
regeneration hypothesis that was mentioned earlier in the thread. This
is a possibly inaccurate portrayal of the justification put forward
by Bruce McNaughton of the University of Arizona during a lecture of
his I recently heard here in Berkeley. Consult his works for more
accuracy.

In brief, he works on neural ensemble modelling and backs up his models
with controlled experiments in rats. A lot of work has been done to sort
out the connectivity, inputs, and outputs of neurons in the hippocampus
(an area of the brain like the neocortex but with fewer layers). Basically,
it seems that patterns of neuronal activity can influence the recipient's
capacity to relay a signal, and so on... and that influence is usually
temporary. This influence can either make the later neurons more or less
receptive and capable of firing when a new stimulus comes along. Since
the effect can be long-lasting, much research has focussed on the 
hippocampus as an area of memory integration.

McNaughton's work convincingly shows that based on patterns of activity
that he is able to measure in a rat hippocampus, that the hippocampus
is probably dedicated to integrating positional information. His neural
network model will predict a path that a rat will have taken as it 
wanders around a featureless box, and it will very closely map the actual
path the rat took! THat is, the model is just going on correlated
activity patterns that are recorded in the hippocampus
.
What has this to do with sleep? In humans, lesions to the hippocampus
tends to eliminate recent memories but leave older memories intact. It
is possible that during sleep, the hippocampus is "offline:" from any more
stimulation, and that it reactivates the neural patterns stimulated by
the animal's day's experience. The purpose would be to integrate these
patterns in a more robust representation in the cortex (long-term memory).
McNaughton mentioned at the very end of his talk that the correlation
patterns in the cells being recorded in the hippocampus actually persisted
during the sleep of the animal after it had had a chance to run around,
over and over.

So, the "purpose" of sleep (and the reason for sleep paralysis, closed
eyes and the works) could be to give the long-term memory a chance to
catch up with the day's experiences - a way of knowing what happened to
you. I think this is a fascinating hypothesis and would welcome more
discussion.

P.S. Thanks to everyone for the help with the student questions I posted
 a little while ago.

Heather Corbett
UC Berkeley




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