238
Chapter 8
The problem now becomes practical. How do we know
when a person (e.g., someone in a coma) is brain dead? There
is general agreement that the criteria listed in
Table 8–2
, if
met, denote brain death. Notice that the cause of a coma must
be known because comas due to drug poisoning are usually
reversible. Also, the criteria specify that there be no evidence
of functioning neural tissues above the spinal cord because
fragments of spinal refl exes may remain for several hours or
longer after the brain is dead. The criterion for lack of spon-
taneous respiration (apnea) can be diffi
cult to check; if the
patient is on a respirator, it is of course inadvisable to remove
him or her for the 10-min test because of the danger of fur-
ther brain damage due to lack of oxygen. Therefore, apnea is
diagnosed if there is no spontaneous attempt to “fi
ght” the
respirator; that is, if the patient’s refl exes do not drive respira-
tion at a rate or depth different from those of the respirator.
Conscious Experiences
Having run the gamut of the states of consciousness from the
awake, alert state to coma and brain death, we deal now with
the conscious experiences during the awake state.
Conscious experiences are those things we are aware
of—either internal, such as an idea, or external, such as an
object or event. The most obvious aspect of this phenomenon
is sensory awareness, but we are also aware of inner states such
as fatigue, thirst, and happiness. We are aware of the passing
of time, of what we are presently thinking about, and of con-
sciously recalling a fact learned in the past. We are aware of
reasoning and exerting self-control, and we are aware of direct-
ing our attention to specifi c events. Not least, we are aware of
“self.”
Basic to the concept of conscious experience is the ques-
tion of attention.
Selective Attention
The term
selective attention
means avoiding the distraction
of irrelevant stimuli while seeking out and focusing on stimuli
that are momentarily important. Both voluntary and refl ex
mechanisms affect selective attention. An example of volun-
tary control of directed attention familiar to students is ignor-
ing distracting events in a busy library while studying there.
An example of selective attention occurs when a novel
stimulus is presented to a relaxed subject showing an alpha
EEG pattern. This causes the EEG to shift to the beta rhythm.
If the stimulus has meaning for the individual, behavioral
changes also occur. The person stops what he or she is doing
and looks around, listening intently and turning to face
toward the stimulus source, a behavior called the
orienting
response.
If the person is concentrating hard and is not dis-
tracted by the novel stimulus, the orienting response does
not occur. It is also possible to focus attention on a particular
stimulus without making any behavioral response.
For attention to be directed only toward stimuli that are
meaningful, the nervous system must have the means to eval-
uate the importance of incoming sensory information. Thus,
even before we focus attention on an object in our sensory
world and become aware of it, a certain amount of processing
has already occurred. This so-called
preattentive processing
directs our attention toward the part of the sensory world that
is of particular interest and prepares the brain’s perceptual
processes for it.
If a stimulus is repeated but is found irrelevant, the behav-
ioral response to the stimulus progressively decreases, a pro-
cess known as
habituation.
For example, when a loud bell is
sounded for the fi rst time, it may evoke an orienting response
because the person might be frightened by or curious about
the novel stimulus. After several ringings, however, the indi-
vidual makes progressively less response and eventually may
ignore the bell altogether. An extraneous stimulus of another
type or the same stimulus at a different intensity can restore
the orienting response.
Habituation involves a depression of synaptic transmission
in the involved pathway, possibly related to a prolonged inacti-
vation of calcium channels in presynaptic axon terminals. Such
inactivation results in a decreased calcium infl
ux during depolar-
ization and, therefore, a decrease in the amount of neurotrans-
mitter released by a terminal in response to action potentials.
Neural Mechanisms for Selective Attention
Directing our attention to an object involves several distinct
neurological processes. First, our attention must be disen-
gaged from its present focus. Then, attention must be moved
to the new focus. Attention must then be engaged at the new
focus. Finally, there must be an increased level of arousal that
produces prolonged attention to the new focus.
Table 8–2
Criteria for Brain Death
I. The nature and duration of the coma must be known.
a. Known structural damage to brain or irreversible
systemic metabolic disease.
b. No chance of drug intoxication, especially from
paralyzing or sedative drugs.
c. No sign of brain function for 6 h in cases of known
structural cause and when no drug or alcohol is
involved; otherwise, 12–24 h without signs of brain
function plus a negative drug screen.
II. Cerebral and brainstem function are absent.
a. No response to painful stimuli other than spinal cord
refl exes.
b. Pupils unresponsive to light.
c. No eye movement in response to ice-water stimulation
of the vestibular refl ex.
d. Apnea (no spontaneous breathing) for 10 min.
e. Systemic circulation may be intact.
f. Purely spinal refl exes may be retained.
III. Supplementary (optional) criteria:
a. Flat EEG (wave amplitudes less than 2 (μV).
b. Responses absent in vital brainstem structures.
c. Greatly reduced cerebral circulation.
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