Consciousness, the Brain, and Behavior
239
One of the areas that plays an important role in ori-
enting and selective attention is in the brainstem, where the
interaction of various sensory modalities in single cells can be
detected experimentally. The receptive fi elds of the different
modalities overlap. For example, a visual and auditory input
from the same location in space will signifi cantly enhance the
fi ring rates of certain of these so-called multisensory cells,
whereas the same type of stimuli originating at different places
will have little effect on or may even inhibit their response.
Thus, weak clues add together to enhance each other’s sig-
nifi cance so we pay attention to the event, whereas we might
ignore an isolated small clue.
The locus ceruleus, a nucleus in the brainstem pons,
which projects to the parietal cortex and to many other parts
of the central nervous system, is also implicated in selective
attention. The system of fi bers leading from the locus ceru-
leus helps determine which brain area is to gain temporary
predominance in the ongoing stream of the conscious experi-
ence. Norepinephrine, the transmitter these neurons release,
acts as a neuromodulator to enhance the signals transmitted
by certain sensory inputs so the difference increases between
them and weaker signals. Thus, neurons of the locus ceruleus
improve information processing during selective attention.
There are also multisensory neurons in association areas
of the cerebral cortex (Chapter 6). Whereas the brainstem
neurons are concerned with the orienting movements associ-
ated with paying attention to a specifi c stimulus, the cortical
multisensory neurons are more involved in the perception of
the stimulus. Neuroscientists are only beginning to under-
stand how the various areas of the attentional system interact.
Some insights into neural mechanisms of selective atten-
tion are being gained from the study of individuals diagnosed
with
attention defi
cit hyperactivity disorder (ADHD).
This
condition typically begins early in childhood and is the most
common neurobehavioral problem in school-aged children (3
to 5 percent). ADHD is characterized by abnormal diffi culty
in maintaining selective attention, and/or impulsiveness and
hyperactivity. Investigation has yet to reveal clear environ-
mental causes, but there is good evidence for a genetic basis
because ADHD tends to run in families. Functional imaging
studies of the brains of children with ADHD have indicated
dysfunction of brain regions in which catecholamine signaling
is prominent, including the basal nuclei and prefrontal cortex.
In support of this, the most effective medication used to treat
ADHD is
methylphenidate
(Ritalin
®
), a drug that increases
synaptic concentrations of dopamine and norepinephrine.
Neural Mechanisms of Conscious Experiences
All conscious experiences are popularly attributed to the work-
ings of the “mind,” a word that conjures up the image of a
nonneural “me,” a phantom interposed between afferent and
efferent impulses. The implication is that the mind is some-
thing more than neural activity. Most neuroscientists agree,
however, that the mind represents a summation of neural
activity in the brain at any given moment and does not require
anything more than the brain. Physiologists, however, have
only a beginning understanding of the brain mechanisms that
give rise to mind or to conscious experiences.
We will describe in this section how some neuroscien-
tists have speculated about this problem. The thinking begins
with the assumption that conscious experience requires neu-
ral processes—either graded potentials or action potentials—
somewhere in the brain. At any moment, certain of these
processes correlate with conscious awareness, and others do
not. A key question here is: What is different about the pro-
cesses we are aware of?
A further assumption is that the neural activity that
corresponds to a conscious experience resides not in a single
anatomical cluster of “consciousness neurons,” but rather in
a set of neurons that are temporarily functioning together in
a specifi c way. Because we can become aware of many differ-
ent things, we further assume that this grouping of neurons
can vary—shifting, for example, among parts of the brain that
deal with visual or auditory stimuli, memories or new ideas,
emotions or language.
Consider the perception of a visual object. As we dis-
cussed in Chapter 7, different aspects of something we see are
processed by different areas of the visual cortex—the object’s
color by one part, its motion by another, its location in the
visual fi eld by another, and its shape by still another—but we
see
one
object. Not only do we perceive it; we may also know
its name and function. Moreover, as we see an object, we can
sometimes also hear or smell it, which requires participation
of brain areas other than the visual cortex.
The simultaneous participation of different groups of
neurons in a conscious experience can also be inferred for the
olfactory system. Repugnant or alluring odors evoke differ-
ent reactions, although they are both processed in the olfac-
tory pathway. Neurons involved in emotion are also clearly
involved in this type of perception.
Neurons from the various parts of the brain that simul-
taneously process different aspects of the information related
to the object we see are said to form a “temporary set” of neu-
rons. It is suggested that the synchronous activity of the neu-
rons in the temporary set leads to conscious awareness of the
object we are seeing.
As we become aware of still other events—perhaps a mem-
ory related to this object—the set of neurons involved in the
synchronous activity shifts, and a different temporary set forms.
In other words, it is suggested that specifi c relevant neurons in
many areas of the brain function together to form the unifi ed
activity that corresponds to awareness.
What parts of the brain might be involved in such a tem-
porary neuronal set? Clearly the cerebral cortex is involved.
Removal of specifi c areas of the cortex abolishes awareness
of only specifi c types of consciousness. For example, in a syn-
drome called
sensory neglect,
damage to association areas of
the parietal cortex causes the injured person to neglect parts
of the body or parts of the visual fi
eld as though they do not
exist. Stroke patients with parietal lobe damage often do not
acknowledge the presence of a paralyzed part of their body
or will only be able to describe some but not all elements in a
visual fi eld.
Figure 8–8
shows an example of sensory neglect
as evidenced in drawings made by a patient with parietal lobe
damage on the right side of the brain. Patients such as these
are completely unaware of the left-hand parts of the visual
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