Consciousness, the Brain, and Behavior
233
States of Consciousness
The term
consciousness
includes two distinct concepts:
states
of consciousness
and
conscious experiences.
The fi rst con-
cept refers to whether a person is awake, asleep, drowsy, and
so on. The second refers to experiences a person is aware of—
thoughts, feelings, perceptions, ideas, dreams, reasoning—
during any of the states of consciousness.
A person’s state of consciousness is defi ned in two ways:
(1) by behavior, covering the spectrum from maximum atten-
tiveness to coma, and (2) by the pattern of brain activity that
can be recorded electrically. This record, known as the
elec-
troencephalogram (EEG),
portrays the electrical potential
difference between different points on the surface of the scalp.
The EEG is such an important tool in identifying the different
states of consciousness that we begin with it.
Electroencephalogram
Neural activity is manifested by the electrical signals known as
graded potentials and action potentials (Chapter 6). It is possi-
ble to record the electrical activity in the brain’s neurons, par-
ticularly those in the cortex near the surface of the brain, from
the outside of the head. Electrodes, which are wires attached
to the head by a salty paste that conducts electricity, pick up
electrical signals generated in the brain and transmit them to
a machine that records them as the EEG.
While we often think of electrical activity in neurons
in terms of action potentials, action potentials do not usu-
ally contribute directly to the EEG. Rather, EEG patterns are
largely due to graded potentials, in this case summed postsyn-
aptic potentials (Chapter 6) in the many hundreds of thou-
sands of brain neurons that underlie the recording electrodes.
The majority of the electrical signal recorded in the EEG orig-
inates in the pyramidal cells of the cortex. The processes of
these large cells lie perpendicular to the brain’s surface, and
the EEG records postsynaptic potentials in their dendrites.
EEG patterns are complex waveforms with large varia-
tions in both amplitude and frequency (
Figure 8–1
). (The
properties of a wave are summarized in Figure 7–21.) The
wave’s amplitude, measured in microvolts (μV), indicates how
much electrical activity of a similar type is going on beneath
the recording electrodes at any given time. A high amplitude
indicates that many neurons are being activated simultaneously.
In other words, it indicates the degree of synchronous fi ring of
whichever neurons are generating the synaptic activity. On the
other hand, a low amplitude indicates that these neurons are
less activated or are fi ring asynchronously. The amplitude may
range from 0.5 to 100 μV. Note that EEG amplitudes are about
1000 times smaller than the amplitude of an action potential
The wave’s frequency indicates how often the wave cycles
from its maximal amplitude to its minimal amplitude and
back. The frequency is measured in hertz (Hz, or cycles per
second) and may vary from 1 to 40 Hz or higher. Four dis-
tinct frequency ranges are characteristic of EEG patterns. In
general, lower EEG frequencies indicate less responsive states,
such as sleep, whereas higher frequencies indicate increased
alertness. As we will see, one stage of sleep is an exception to
this general relationship.
The cause of the wavelike nature, or rhythmicity, of the
EEG is not certain, nor is it known exactly where in the brain
it originates. Current thinking is that clusters of neurons in the
thalamus play a critical role; they provide a fl uctuating output
through nerve fi bers leading from the thalamus to the cortex.
This output, in turn, causes a rhythmic pattern of synaptic
activity in the pyramidal neurons of the cortex. As noted previ-
ously, the cortical synaptic activity—not the activity of the deep
thalamic structures—comprises most of a recorded EEG signal.
The synchronicity of the cortical synaptic activity (in
other words, the amplitude of the EEG) does refl ect the
degree of synchronous fi
ring of the thalamic neuronal clus-
ters that are generating the EEG. These clusters receive input
from other brain areas involved in controlling the conscious
state. The purpose these oscillations serve in brain electrical
activity is unknown. Theories range from the “idling hypoth-
esis,” which says that it is easier to get brain activity up and
running from an “idle” as opposed to a “cold start,” to the
“epiphenomenon hypothesis,” which says that the oscillations
are simply the by-product of neuronal activity and have no
functional signifi cance at all.
The EEG is a useful clinical tool because wave pat-
terns are abnormal over diseased or damaged brain areas (e.g.,
because of tumors, blood clots, hemorrhage, regions of dead
tissue, and high or low blood sugar). Moreover, a shift from a
less synchronized pattern of electrical activity (low-amplitude
EEG) to a highly synchronized pattern can be a prelude to the
electrical storm that signifi es an epileptic seizure.
Epilepsy
is a
common (occurs in about 1 percent of the population) neuro-
logical disease that appears in mild, intermediate, and severe
forms. It is associated with abnormal synchronized discharges
of cerebral neurons. These discharges are refl ected in the EEG
as recurrent waves having distinctive high amplitudes (up to
1000 μV) and individual spikes or combinations of spikes and
waves (
Figure 8–2
). Epilepsy is also associated with stereotyped
Figure 8–1
EEG patterns are wavelike. This represents a typical EEG recorded
from the parietal or occipital lobe of an awake, relaxed person over
approximately four seconds. EEG wave amplitudes are generally
20–100 μV, with a duration of about 50 msec.
Time
Onset of seizure
Wave
Spike
Time
Figure 8–2
Spike-and-wave pattern in the EEG of a patient during an epileptic
seizure.
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