Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!caen!uflorida!shark!tomh From: tomh.bbs@shark.cs.fau.edu (Tom Holroyd) Newsgroups: sci.bio Subject: Echidnas & REM sleep Message-ID: Date: 26 Apr 91 15:51:23 GMT Sender: bbs@cs.fau.edu (Waffle BBS) Organization: Florida Atlantic University Lines: 55 1. Echidnas have no desynchronized EEG during sleep (I don't know about eye movements, but that's not the point; I'm interested in the cortical activity). 2. They have a very large (proportionaly) neocortex, particularly frontal. In humans: Deep sleep EEG is synchronized. Periodically, during sleep, the cortex becomes active (desynchronized) accompanied by rapid eye movements (REM), PGO spikes, etc. (Hippocampal theta, too, I think). Synchronized EEG is a lower dimensional state than desynchronized EEG. The system doesn't wander around much, it mostly stays where it is, or drifts around slowly. The active cortex, on the other hand, is high-dimensional - there are many more degrees of freedom available to the system. REM sleep may be the brain's way of ensuring that we don't get "stuck" in the lower-diemsional synchronized state during deep sleep. Periodic bursts of activity kick the system out of the relatively small limit- cycle attractor. Patients in coma, for example, do not have REM sleep, and measurements of their EEG dimension show an even more synchronized state than for normal deep sleep. They are "stuck." Echidnas may not have desynchronized sleep because their larger neocortex gives them the added dimensionality they need to make it through the night - but this would be a very inefficient way to do it. Periodic desynchronizing bursts would be better, since the dimensionality can be raised higher and faster. Some added thoughts: The neurons involved in the cyclic behavior would adapt themselves to the activity level, becoming less active in the process. This would make it even more difficult to get out of the low D state. The stimuli that normally cause waking, including the internal ones, are merely perturbations of a vastly larger system. These signals would be enough to cause cortical activation if the brain isn't in an abnormally low D state. REM sleep activity in the cortex keeps the pathways sensitive to stimulation, so when morning comes around, the various stimuli wake you up. This doesn't explain REM rebound or why REM deprivation is bad. Presumably, waking the person should activate the cortex, taking the place of REM. Waking activation may be ineffective however, due to the lack of modulatory influences present during sleep (such as, perhaps, theta rhythm). Comments? Tom Holroyd Center for Complex Systems Florida Atlantic University tomh@bambi.ccs.fau.edu