The Brain Rhythms and Sleep
The electroencephalogram (EEG) is a measurement that enables us to glimpse the generalized activity of
the cerebral cortex.
Recording Brain Waves
o Recording an EEG is relatively simple
o The electrodes are wires taped to the scalp; some two dozen electrodes are fixed to standard
positions on the head and connected to banks of amplifiers and recording devices.
o Different regions of the brain can be examined
o An EEG measures voltages generated by the currents that flow during synaptic excitation of the
dendrites of many pyramidal neurons in the cerebral cortex, which lies under the skull and makes
up 80% of the brain’s mass
The electrical contribution of any single cortical neuron is exceeding small, however, and
must penetrate several layers of non-neural tissue to reach the electrode
So it takes many thousands of underlying neurons, activated together, to generate an
EEG signal big enough to see all.
o The amplitude of the EEG signal strongly depends, in part, on how synchronous the activity of the
underlying neurons is.
o When a group of cells is excited simultaneously, the tiny signals sum to generate on large surface
signal. However, when each cell receives the same amount of excitation, but spread out in time,
the summed signals are meagre and irregular.
The total amount of excitation may not have changes, only the timing of the activity.
o An alternative way to record the rhythms is with magnetoencephalography (MEG).
The capabilities of MEG complement those of other methods that measure brain
It is better than EEG at localizing the sources of neural activity in the brain.
MEG can record rapid fluctuations of neural activity but cannot provide the spatially
detailed images of fMRI.
EEG and MEG directly measure the activity of neurons, whereas fMRI and PET detect
changes in blood flow or metabolism controlled in part by neuronal activity.
o EEG rhythms vary dramatically and often correlate with particular states of behaviour and
o The rhythms are categorized by their frequency range
Beta rhythms are the fastest—anything greater than about 14Hz, and signal an activated
Alpha rhythms are about 8-13 Hz and are associated with quiet, waking states
Theta rhythms are about 4-7 Hz and occurring during some sleep states
Delta rhythms are quite slow, less than 4 Hz, large in amplitude and the hallmark of
o EEG will never tell us what a person is thinking, but it can help us know if a person is thinking
o Low-frequency, high amplitude rhythms are associated with non-dreaming sleep states, or
pathological state of coma
This is logical because when the cortex is actively engaged in processing information,
the activity level of cortical neurons is relatively high, but also relatively unsynchronized
Low synchrony means that the EEG amplitude is low and beta rhythms dominate
By contrast, during deep sleep, cortical neurons are not engaged in information
processing and so synchrony is high and the EEG amplitude is high
The Functional States of the Brain
o Several times during a night, you enter a state called rapid eye movement sleep (REM sleep) EEG looks more awake than asleep, you body (except for the eyes and respiratory
muscles) is immobilized, you conjure up vivid, detailed illusions (dreams)
o The rest of the time you spend in non-REM sleep in which the brain does not usually generate
complex dreams (sometimes called slow-wave sleep; also dominated by large, slow EEG rhythms)
o These fundamental behavioural states--awake, non-REM sleep, and REM sleep—are produced by
three distinct states of brain function. Each behavioural state is also accompanied by large shifts
in body function
o Non-REM seems to be a period for rest.
Muscle tension is reduced, and movement is minimal
The body is capable of movement but only rarely does it do so (briefly to adjust the
The temperature and energy consumption of the body are lowered
There is an increase in activity of the parasympathetic division of the ANS; heart rate,
respiration, and kidney function all slow down and digestive processes speed up
The brain also seems to rest
Its rate of energy use and the general firing rates of its neurons are at their
Slow, large amplitude EEG rhythms indicate that the neurons of the cortex are
oscillating in relatively high synchrony
Studies indicate that mental processes also hit their daily low during non-REM
o REM sleeps is an active, hallucinating brain in a paralyzed body
The physiology of REM sleep is also bizarre. The EEG looks almost indistinguishable from
that of an active, waking brain, with fast, low-voltage fluctuations.
The oxygen consumption of the brain is higher in REM sleep than when the brain is
awake and concentrating on difficult math problems
The paralysis that occurs during REM is almost a total loss of skeletal muscle tone—
atonia. Most of the body is actually incapable of moving.
Respiratory muscles do continue to function but barely
The muscles controlling eye movement and the inner ear are strikingly active.
Physiological control systems are dominated by sympathetic activity during REM sleep
The body’s temperature control system simply quits, and core temperature
begins to drift downward
Heart and respiration rates increase but become irregular
Frequency of Dominant Rhythm
Alpha 8-13, Beta 15-30, Delta less than 4, Gamma 30-90, Theta 4-7
The Sleep Cycle
o Roughly 75% of total sleep time is spent in non-REM and 25% in REM, with periodic cycles
between these states throughout the night
o Non-REM is divided into four distinct stages
During a normal night, we slide through the stages of non-REM, then into REM, and so
on. These cycles repeat about every 90 minutes and are examples of ultradian rhythms,
which have faster periods than circadian rhythms.
o EEG rhythms during the stages of sleep:
Stage 1 non-REM sleep is transitional sleep. It is when EEG alpha rhythms of relaxed
waking become less regular and wane and the eyes make slow, rolling movements. It is
fleeting and usually lasts only a few minutes. It is the lightest stage of sleep.
Stage 2 is slightly deeper and may last 5-15 minutes. It has occasional 8-14 Hz
oscillations of the EEG called the sleep spindle which is known to be generated by a
thalamic pacemaker. Also a high-amplitude sharp wave called the K complex is
observed. Eye movements almost cease. Stage 3 consists of large-amplitude, slow delta rhythms. Eye and body movements are
Stage 4 is the deepest stage of sleep and my persist for 20-40 minutes.
Then sleep begins to lighten again, ascends to stage 2 for 10-15 minutes, and suddenly
enters a brief period of REM sleep, with its fast EEG beta rhythms and sharp, frequent
o As the night progresses, there is a general reduction in the duration of non-REM sleep (esp.
stages 3 and 4) and an increase in REM periods.
o Half of the night’s REM sleep occurs during its last third, and the longest REM cycles may last 30-
o There is an obligatory refractory period of about 30 minutes between periods of REM—each
REM cycle is followed by at least 30 minutes of non-REM sleep before the next REM period can
Why Do We Sleep?
o Only mammals and some birds have a REM phase.
o No single theory of the function of sleep is widely accepted, but the most reasonable ideas fall
into two categories: theories of restoration and theories of adaptation.
First theory: we sleep to rest and recover and to prepare to be awake again.
Second theory: we sleep to keep us out of trouble, to hide from predators when we are
most vulnerable or from other harmful features of the environment, or to conserve
o Prolonged sleep deprivation can lead to serious physical and behavioural problems
o Evidence indicates that sleep is not a time of increased tissue repair for the body.
o Adaptation theories of sleep take many forms.
Some large animals eat small animals; a stroll in the moonlight is far too risky for a
squirrel living in owl and fox territory. The squirrel’s best strategy may be to stay safely
tucked away in an underground burrow during the night, and sleep is a good way to
enforce such isolation. At the same time, sleep may be an adaptation for conserving
Functions of Dreaming and REM Sleep
o It is possible to deprive sleeper of REM sleep specifically, by waking them every time they enter
the REM state; when they fall asleep a minute or two later, it is inevitably into a non-REM state,
and they can accumulate an entire night of relatively pure non-REM sleep
o One researcher observed, after several days of this annoying treatment, sleepers attempt to
enter the REM state much more frequently than normal. They experience REM rebound and
spend more time in REM proportional to the duration of their deprivation.
o Allan Hobson and Robert McCarley of Harvard University propose an “activation-synthesis
Dreams are seen as the associations and memories of the cerebral cortex that are
elicited by the random discharges of the pons during REM sleep.
Thus the pontine neurons, via the thalamus, activate various areas of the cerebral
cortex, elicit well-known images or emotions, and the cortex then tries to synthesize the
disparate images into a sensible whole.
o Depriving humans or rats of REM sleep can impair their ability to learn a variety of tasks.
Israeli neuroscientist Avi Karni found that if people were deprived of REM sleep their
learning of the task did not improve overnight.
Depriving them of non-REM sleep actually enhanced their performance.
Neural Mechanisms of Sleep
o Sleep is an active process that requires the participation of a variety of brain regions
The neurons most critical to the control of sleeping and waking are part of the diffuse
modulatory neurotransmitter systems The brain stem modulatory neuron neurons using norepinephrine (NE) and serotonin (5-
HT) fire during waking and enhance the awake state; some neurons using acetylcholine
(ACh) enhance critical REM events, and other cholinergic neurons are active during
The diffuse modulatory systems control the rhythmic behaviours of the thalamus, which
in turn controls many EEG rhythms of the cerebral cortex; slow, sleep-related rhythms
of the thalamus apparently block the flow of sensory information up into the cortex.
Sleep also involves activity in descending branches of the diffuse modulatory systems,
such as the inhibition of motor neurons during dreaming.
Wakefulness and the Ascending Reticular Activating System
Lesions in the brain stem of humans can cause sleep and coma, suggesting that the
brain stem has neurons whose activity is essential to keeping us awake
Giuseppe Moruzzi found that lesions in the midline structures of the brain stem caused
a state similar to non-REM sleep, but lesions in the lateral tegmentum, which
interrupted ascending sensory inputs, did not.
o Electrical stimulation of the midline tegmentum of the midbrain transformed
the cortex from the slow, rhythmic EEGS of non-REM sleep to a more alert and
aroused state with an EEG similar to that of waking.
o This region is called the ascending reticular activating system
Several sets of neurons increase their firing rates in anticipation of awakening and
during various forms of arousal.
o Include cells of the locus coeruleus (contains NE), serotonin-containing cells of
the raphe nuclei, ACh-containing cells of the brain stem and basal forebrain and
neurons of the midbrain that use histamine as a neurotransmitter.