Waking brain activation

One of the most instructive examples of state boundary crossing is the tendency to experience dreamlike visuomotor sensations at sleep onset. These are called hypnagogic hallucinations if the subject is still awake enough to notice or be aroused by them. Apparently, one need only carry waking brain activation over the sleep boundary and dreaming will im-... 

Non-REM sleep is usually considered as a compensatory resting state for the brain, following the intense waking brain activity. Indeed, previous brain imaging studies showed that the brain was less active during periods of non-REM sleep as compared to periods of wakefulness. 

To have such vivid consciousness while asleep is paradoxical. Because the memory for dreams is so fleeting, it would be natural to assume that the dreams occurred in the instant before awakening as a by-product of the brain activation process causing me to wake up. But this hypothesis is incorrect, or at least incomplete. We know this because had I been sleeping in a sleep lab, instead of my bed at the Hotel Miramare in Strom-boli, a distinctive constellation of physiological events would have preceded my awakening—perhaps by as long as 30, 40, or even 50 minutes. 

Activation-synthesis ascribes dreaming to brain activation in sleep. The principle engine of this activation is the reticular formation of the brain stem, just as it is in waking, but the chemical mode of activation is distinctly different. It is for that reason and that reason alone that dreaming and waking consciousness are so different. In waking, the noradrenergic... 

It is already clear from the forgoing discussion that the brain must be fully activated to process information efficiently. Brain activation may have quite different cellular and molecular mechanisms and a quite different regional distribution according to whether it is activated in waking... 

The net result is that in one brain-activated state, waking, the brain is in touch with the outside world and can act upon it, whereas in another equally activated state, REM sleep, it cannot do either. In both cases, the activation is real and important and must constitute a dimension of any model. But so diametrically opposed are the input-output conditions of waking and REM that they cannot possibly be dealt with by an activation-only model. We need the input-output (TO) dimension. 

The expression of the immediate early gene (IEG) c-fos has been extensively used as a functional marker of brain activity in neuroscience including sleep research (53-55). In most cell types, the basal level of c-fos expression is relatively low however, c-fos messenger ribonucleic acid (mRNA) and Fos protein can be rapidly and transiently induced by a diverse range of extracellular stimuli. Several laboratories have found that both c-Fos protein and mRNA levels decrease in the cerebral cortex during sleep relative to wakefulness (56-59). This decline has been linked to a reduction in the firing rate of the locus ceruleus during sleep and the consequent reduction of norepinephrine release in the cortex (60). 

Sleep is basically divided into two major states - rapid eye movement (REM) sleep and non-REM sleep. These two states of sleep differ as much from each other as either of them differ from wakefulness and therefore early in the modern era of sleep research REM sleep was referred to as a third state of existence [3], REM sleep is a state of dreaming, increased brain activity and variability in breathing and heart function. At the same time, it is a sleep stage where most of the muscles reach their maximum relaxation. The amount of REM sleep does not change much with aging, and remains stable at around 20 % of total sleep. 

Although counterintuitive, the discovery of brain activation in sleep was rapidly accepted by those dream scientists who had sudden and transformative ah-ha experiences when they read about it. In the ensuing excitement about the similarities between waking and dreaming consciousness, few stopped to wonder what bit of this sleep-dependent brain activation could account for the difference, which, after all, is every bit as important as the similarities ... 

Could physiology help us answer these questions as well Certainly not if all we paid attention to was brain activation and the common aspects of its occurrence in waking and sleep. 

As it turned out, acetylcholine was a prime mover not only of muscles, but also of the central brain state. Acetylcholine-containing neurons fire in both waking and REM sleep, so they may help to mediate EEG activation in both states. Their excitability is apparently enhanced in REM sleep because of the decrease in inhibition from the serotonin-containing neurons, which you will remember are turned off in REM. In their variety and complexity, these neurophysiological details can become confusing. The main point to keep in mind is that in REM sleep the brain, although electrically as activated as in waking, is activated in a chemically very different way. 

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