EEG Patterns in Sleep and Wakefulness

Tutorial 20: EEG Patterns in Sleep and Wakefulness

William James wrote about the stream of consciousness; the contents of which he described as continuous, personal, changing, and selective. Sigmund Freud focused on what he believed to be occurring below this stream of conscious activity, the unconscious. Each scholar concentrated on different levels of awareness to the external and internal stimuli that characterize our existence, our sense of self. Consciousness is clearly not an all-or-none phenomenon. Levels of awareness vary across a continuum ranging from alert and focused conscious activity (engaged as we attend to a challenging topic of interest) to the negligible awareness associated with sleep, anesthesia, or even coma. Somewhere in between are the moments, we turn on the "automatic pilot" inside, such as when driving a familiar route.

These varying states of consciousness are associated with changes in brain electrical activity. In 1929, Hans Berger invented the technique called electroencephalography (EEG), which allows us to measure this changing electrical activity using electrodes secured to the scalp. EEG measures and amplifies the sum of electrical activity generated by action potentials and graded synaptic potentials (see Figures 2, 3 and 4) of neurons located in the cerebral cortex. The summary of this activity is recorded finally as brain waves (or traces) of varying amplitude (height) and frequency (cycles per second, cps). Human brain waves are typically divided into four principle bands based on frequency. These bands are identified by Greek letters, and are beta (13-24 cps), alpha (8-12 cps), theta (4-7 cps), and delta (under 4 cps). In general, beta wave activity is associated with alert, conscious thinking and behavior, alpha with deep relaxation and meditative states, theta with light sleep, and delta with deep sleep.

EEG recordings made during sleep are painstaking performed by sleep researchers who monitor this activity of volunteer subjects throughout the night. In addition to EEG, the electromyograph (EMG - muscular activity and tension), electrooculograph (EOG - eye movements, and electrocardiograph (EKG - heart contractions) are also recorded to consider possible contamination of the EEG signal, and because these measures are of interest in their own right. Breathing and pulse rates and body temperature are also routinely measured. The activity of brain neurons both while awake and asleep are controlled by nuclei found in the medulla and pons (see Figure 8).

Figure 20 illustrates the characteristics of the human EEG while awake and during the four stages of sleep. The discussion focuses on the nature of consciousness and physiology associated with each of these stages.

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We spend approximately one third of our life in the state of sleep. Volition (the will and choice to respond to the environment) and consciousness are partially or completely deferred, sensitivity to external stimuli is reduced, and physiological functions are partially suspended. What is the purpose of this behavior that constitutes such a major portion of our existence yet provides us with such little conscious gratification?

Two primary theories are proposed to answer this question: recuperation theories and circadian theories. The foundation of the recuperation theories argues that the homeostasis or internal stability of the body is disturbed by the waking state; sleep is needed to restore normal function. The circadian theories argue that animals sleep during the time of day when it is unnecessary for them to be engaged in survival behaviors. Wakefulness is necessary only to allow the animal to engage in behaviors that will satisfy basic needs. In this context, sleep conserves energy for the behaviors that are necessary for survival.

There is significant variation across species in the time each spends sleeping. Humans typically need 8 hours of sleep; the giant sloth needs 20 hours of sleep, and the horse only 2 hours. These differences provide a backdrop for contrasting the two major theories. For example, recuperation theories would predict that animals that expend more energy would need more sleep. There is, however, no correlation between the activity level of a species and the amount of time spent in sleep. Conversely, there is a correspondence between the level of vulnerability of a species during sleep and the amount necessary. Vulnerable animals such as zebra spend little time in sleep compared with the amount of time their predators (lions, tigers) spend in sleep. Hence, there is greater support for the circadian theories. This stronger support, however, does not eliminate the importance of the recuperative aspects of sleep. Indeed, an integration of the two purposes likely provides the most complete explanation for all the data.

Each of the theories of sleep predicts a different outcome in the aftermath of sleep deprivation. The recuperative theory predicts that the longer an animal is deprived of sleep, the greater the physiological and behavioral disturbance. The circadian theories predict no debilitating effects following sleep deprivation, a greater tendency to want to sleep during the usual time of day for this behavior, and little or no compensation of sleep loss when sleep again is possible. The research supports both sets of prediction. Although long bouts of deprivation do not substantially increase compensatory sleep time, the brain does compensate for sleep loss by increasing the proportion of time spent in the presumably more restorative stages of slow-wave sleep. REM sleep is also an important behavior. When a subject is repeatedly interrupted while in REM sleep leading to deprivation, subjects show a rebound phenomenon. That is, when the subject is allowed to return to a natural sleep cycle, they spend a greater percentage of time in REM sleep than is normally the case.

Many different hypotheses have been offered to explain the purpose of sleep (particularly REM sleep). Among them are the vigilance hypothesis, learning or memory consolidation hypothesis, species- typical reprogramming hypothesis, and the brain development hypothesis. The vigilance hypothesis suggests that REM sleep provides a period of time when the animal is more aware to external stimuli and, therefore, provides a periodic opportunity to monitor for the presence of predators. The learning or memory consolidation hypothesis suggests that during REM sleep the brain is allowed to process the events of the day, integrating these with pre-existing memories. The species-typical reprogramming hypothesis speculates that REM sleep allows the brain to integrate what was learned during the day with the circuits that control instinctual behaviors. Finally, human infants spend the majority of their sleep time in REM sleep, and is has been speculated that this is associated with the extensive process of brain development underway.

Sleep deprivation studies with humans suggest that the brain, but not the rest of the body, needs slow-wave sleep in order to recovery from the day's events. In support of this conclusion, quadriplegics (who are unable to be physically active) show only a small decrement of time spent in slow-wave sleep compared to physically active people. In addition, experiments that have increased the metabolic activity of specific brain regions have shown an increase in slow-wave delta activity in the same brain region the night following the manipulation.

Sleep is but one of the circadian rhythms that affect life. Indeed, all behavioral, psychological, and physiological process is affected in some way by circadian rhythms. Each rhythm is affected by the light - dark cycle of the environment in which we live. When exposed to alternating periods of 10 hours of light and 10 hours of dark, the circadian rhythm will shift to accommodate the new 20-hour day. When no light dark cycles are present, the so-called "free-running rhythms" that develop will vary for the individual; although the typical cycle will be greater than 24 hours and more like 25. These free-running rhythms reflect the natural activity of an internal biological clock, free of external cues. When traveling between time zones, which differ in light-dark cues, body function is adversely affected. This phenomenon, called jet lag, illustrates the uneasy adjustment made by the internal biological clock in response to changing external cues of light and dark.

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Tutorial 20: EEG Patterns in Sleep and Wakefulness

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