250
Chapter 8
SUMMARY
States of Consciousness
I. The electroencephalogram provides one means of defi ning the
states of consciousness.
a. Electrical currents in the cerebral cortex due predominantly
to summed postsynaptic potentials are recorded as the
EEG.
b. Slower EEG waves correlate with less responsive behaviors.
c. Rhythm generators in the thalamus are probably responsible
for the wavelike nature of the EEG.
d. EEGs are used to diagnose brain disease and damage.
II. Alpha rhythms and, during EEG arousal, beta rhythms
characterize the EEG of an awake person.
III. NREM sleep progresses from stage 1 (faster, lower-amplitude
waves) through stage 4 (slower, higher-amplitude waves) and
then back again, followed by an episode of REM sleep. There
are generally four or fi ve of these cycles per night.
IV. Aminergic and cholinergic brainstem centers, via their
projections forward to the cerebrum as components of the
reticular activating system, interact with the thalamus to
regulate the sleep-wake cycles. Hypothalamic nuclei also play
a role.
Conscious Experiences
I. Brain structures involved in selective attention determine
which brain areas gain temporary predominance in the
ongoing stream of conscious experience.
II. Conscious experiences may occur because a set of neurons
temporarily function together, with the neurons that comprise
the set changing as the focus of attention changes.
Motivation and Emotion
I. Behaviors that satisfy homeostatic needs are primary motivated
behaviors. Behavior not related to homeostasis is a result of
secondary motivation.
Repetitive Transcranial Magnetic Stimulation
Although shocking the brain with electroconvulsive therapy
is certainly effective in treating depression and other mental
disorders, the use of this therapy has been limited due to
the requirement for general anesthesia and the potential for
side effects.
Repetitive transcranial magnetic stimulation
(rTMS)
shows great promise not only as a gentler alternative
to shock therapy, but also as a powerful research tool for
exploring the human brain. (Do not confuse this technology
with the practice of placing ordinary magnets on the
skin to treat various ailments, which has no scientifi cally
documented basis.)
In rTMS, circular or fi
gure-eight shaped metallic coils
are placed against the skull overlying specifi c brain regions,
and then brief, powerful electrical currents are applied
at frequencies between 1 and 25 pulses per second. The
resulting magnetic fi eld induces current to fl ow through
cortical neuronal networks directly beneath the coil. The
immediate effect is similar to shock therapy: Neural activity
is transiently disordered or sometimes silenced in that brain
region. However, no anesthesia is required and no pain,
convulsion, or memory loss occurs. Depending on the
frequency and treatment regimen applied, the lasting effects
of rTMS can cause either an increase or a decrease in the
overall activity of the targeted area.
This technology provides a powerful, noninvasive
research tool for studying brain function in normal, healthy
people. Functional mapping of the human cortex is an
example of how rTMS is being used in basic research.
For example, placing the coil just above a person’s left ear
will cause the muscles of the right thumb to twitch. Such
functional associations have historically been demonstrable
only in subjects with their brains exposed during surgical
procedures. Researchers are currently using rTMS to
investigate cortical pathways involved in vision, language,
learning, and drug reactions.
Electrical stimulation through rTMS also shows great
promise as a therapeutic tool for the treatment of some
mental disorders. In recent clinical trials, two to four weeks of
daily rTMS stimulation of the left prefrontal cortex resulted
in marked improvement of patients with major depression
who hadn’t responded to medication. Medical scientists
are hopeful that refi nements in technique will also lead to
breakthroughs in the treatment of obsessive-compulsive
disorder, mania, schizophrenia, and other psychiatric illnesses.
Head Trauma and Conscious State
Knowing what we do about consciousness and cognitive
function, it is not surprising that damage to the cerebral
cortex might result in confused and disoriented behaviors
and even epilepsy-like seizures and coma.
Concussion
is the
usually brief loss of consciousness that occurs after injury
due to a blow to the head, a fall, or some other form of
blunt trauma. Pupillary constriction in response to light
is normal. When the patient awakens, he or she is usually
confused for several hours, exhibiting amnesia for events that
occurred immediately before and after the injury. In contrast,
intracranial
hemorrhage
results from damage to vasculature
in and around the brain and can be associated with skull
fracture, violent shaking, and sudden accelerative forces such
as those that would occur during an automobile accident.
Blood may collect between the skull and the dura mater (an
epidural hematoma
), or between the arachnoid mater and
the surrounding meninges or within the brain
(
subdural
hematoma
).
Intracranial hemorrhage often occurs without
loss of consciousness, and symptoms such as headache, motor
dysfunction, and loss of pupillary refl exes may not occur until
several hours or days afterward. It is treated when necessary
by drainage of the blood from the affected area. One reason
why it is important to watch the behavior of a person with
concussion is to recognize whether the initial trauma has
resulted in an intracranial hemorrhage.
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