get up and turn around to the right, until the
remaining food appeared on her right side, as
she was entirely unable to conceive that the
plate or space itself had a left side. Similarly, a
patient with aphasia due to damage to Wer-
nicke’s area in the dominant temporal lobe not
only cannot appreciate the language symbol
content of speech, but also can no longer com-
prehend that language symbols are an operative
component of speech. Such a patient continues
to speak meaningless babble and is surprised
that others no longer understand his speech
because the very concept that language sym-
bols are embedded in speech eludes him.
This concept of fractional loss of conscious-
ness is critical because it explains confusional
states caused by focal cortical lesions. It is also a
common observation by clinicians that, if the
cerebral cortex is damaged in multiple locations
by a multifocal disorder, it can eventually cease
to function as a whole, producing a state of
such severe cognitive impairment as to give the
appearance of a global loss of consciousness.
During a Wada test, a patient receives an in-
jection of a short-acting barbiturate into the
carotid artery to anesthetize one hemisphere so
that its role in language can be assessed prior
to cortical surgery. When the left hemisphere is
acutely anesthetized, the patient gives the ap-
pearance of confusion and is typically placid but
difficult to test due to the absence of language
skills. When the patient recovers, he or she
typically is amnestic for the event, as much of
memory is encoded verbally. Following a right
hemisphere injection, the patient also typically
appears to be confused and is unable to orient
to his or her surroundings, but can answer sim-
ple questions and perform simple commands.
The experience also may not be remembered
clearly, perhaps because of the sudden inability
to encode visuospatial memory.
However, the patient does not appear to be
unconscious when either hemisphere is acutely
anesthetized. An important principle of exam-
ining patients with impaired consciousness is
that the condition is not caused by a lesion whose
acute effects are confined to a single hemi-
sphere. A very large space-occupying lesion may
simultaneously damage both hemispheres or
may compress the diencephalon, causing im-
pairment of consciousness, but an acute infarct
of one hemisphere does not. Hence, loss of
consciousness is not a typical feature of unilat-
eral carotid disease unless both hemispheres
are supplied by a single carotid artery or the
patient has had a subsequent seizure.
The concept of the cerebral cortex as a mas-
sively parallel processor introduces the question
of how all of these parallel streams of informa-
tion are eventually integrated into a single con-
sciousness, a conundrum that has been called
the binding problem.
97,98
Embedded in this
question, however, is a supposition: that it is
necessary to reassemble all aspects of our ex-
perience into a single whole so that they can
be monitored by an internal being, like a small
person or homunculus watching a television
screen. Although most people believe that they
experience consciousness in this way, there is
no a priori reason why such a self-experience
cannot be the neurophysiologic outcome of the
massively parallel processing (i.e., the illusion of
reassembly, without the brain actually requiring
that to occur in physical space). For example,
people experience the visual world as an un-
broken scene. However, each of us has a pair of
holes in the visual fields where the optic nerves
penetrate the retina. This blind spot can be dem-
onstrated by passing a small object along the
visual horizon until it disappears. However, the
visual field is ‘‘seen’’ by the conscious self as a
single unbroken expanse, and this hole is pa-
pered over with whatever visual material bor-
ders it. If the brain can produce this type of
conscious impression in the absence of reality,
there is no reason to think that it requires a
physiologic reassembly of other stimuli for pre-
sentation to a central homunculus. Rather, con-
sciousness may be conceived as a property of
the integrated activity of the two cerebral hemi-
spheres and not in need of a separate physical
manifestation.
Despite this view of consciousness as an
‘‘emergent’’ property of hemispheric informa-
tion processing, the hemispheres do require
a mechanism for arriving at a singularity of
thought and action. If each of the independent
information streams in the cortical parallel pro-
cessor could separately command motor re-
sponses, human movement would be a hopeless
confusion of mixed activities. A good example
is seen in patients in whom the corpus callosum
has been transected to prevent spread of epi-
leptic seizures.
99
In such ‘‘split-brain’’ patients,
the left hand may button a shirt and the right
hand follow along right behind it unbuttoning.
If independent action of the two hemispheres
can be so disconcerting, one could only imagine
Pathophysiology of Signs and Symptoms of Coma
27
the effect of each stream of cortical processing
commanding its own plan of action.
The brain requires a funnel to narrow down
the choices from all of the possible modes of
action to the single plan of motor behavior that
will be pursued. The physical substrate of this
process is the basal ganglia. All cortical regions
provide input to the striatum (caudate, putamen,
nucleus accumbens, and olfactory tubercle). The
output from the striatum is predominantly to
the globus pallidus, which it inhibits by using
the neurotransmitter GABA.
100,101
The pallidal
output pathways, in turn, also are GABAergic
and constitutively inhibit the motor thalamus,
so that when the striatal inhibitory input to the
pallidum is activated, movement is disinhibited.
By constricting all motor responses that are not
specifically activated by this system, the basal
ganglia ensure a smooth and steady, unitary
stream of action. Basal ganglia disorders that
permit too much striatal disinhibition of move-
ment (hyperkinetic movement disorders) result
in the emergence of disconnected movements
that are outside this unitary stream (e.g., tics,
chorea, athetosis).
Similarly, the brain is capable of following
only one line of thought at a time. The con-
scious self is prohibited even from seeing two
equally likely versions of an optical illusion si-
multaneously (e.g., the classic case of the ugly
woman vs. the beautiful woman illusion) (Figure
1–7). Rather, the self is aware of the two alter-
native visual interpretations alternately. Simi-
larly, if it is necessary to pursue two different
tasks at the same time, they are pursued alter-
nately rather than simultaneously, until they
become so automatic that they can be per-
formed with little conscious thought. The stri-
atal control of thought processes is implemen-
ted by the outflow from the ventral striatum to
the ventral pallidum, which in turn inhibits the
mediodorsal thalamic nucleus, the relay nu-
cleus for the prefrontal cortex.
100,101
By dis-
inhibiting prefrontal thought processes, the
striatum ensures that a single line of thought
and a unitary view of self will be expressed
from the multipath network of the cerebral
cortex.
An interesting philosophic question is raised
by the hyperkinetic movement disorders, in
which the tics, chorea, and athetosis are thought
to represent ‘‘involuntary movements.’’ But the
use of the term ‘‘involuntary’’ again presupposes
a homunculus that is in control and making de-
cisions. Instead, the interrelationship of invol-
untary movements, which the self feels ‘‘com-
pelled’’ to make, with self-willed movements is
complex. Patients with movement disorders of-
ten can inhibit the unwanted movements for
a while, but feel uncomfortable doing so, and
often report pleasurable release when they can
carry out the action. Again, the conscious state
is best considered as an emergent property of
brain function, rather than directing it.
Similarly, hyperkinetic movement disorders
may be associated with disinhibition of larger
scale behaviors and even thought processes. In
this view, thought disorders can be conceived
as chorea (derailing) and dystonia (fixed delu-
sions) of thought. Release of prefrontal cortex
inhibition may even permit it to drive mental
imagery, producing hallucinations. Under such
conditions, we have a tendency to believe that
somehow the conscious self is a homunculus
that is being tricked by hallucinatory sensory
experiences or is unable to command thought
processes. In fact, it may be more accurate to
view the sensory experience and the behavior as
manifestations of an altered consciousness due
to malfunction of the brain’s machinery for main-
taining a unitary flow of thought and action.
Neurologists tend to take the mechanistic
perspective that all that we observe is due to ac-
Figure 1–7. A classic optical illusion, illustrating the in-
ability of the brain to view the same scene simultaneously
in two different ways. The image of the ugly, older woman
or the pretty younger woman may be seen alternately, but
not at the same time, as the same visual elements are used
in two different percepts. (From W.E. Hill, ‘‘My Wife and
My Mother-in-Law,’’ 1915, for Puck magazine. Used by
permission. All rights reserved.)
28
Plum and Posner’s Diagnosis of Stupor and Coma