Behavioral Neuroanatomy
The brain, and the brain alone, is the source of our pleasures, joys, laughter, and
amusement, as well as our sorrow, pain, grief, and tears. It is especially the organ we
use to think and learn, see and hear, to distinguish the ugly from the beautiful, the bad
from the good, and the pleasant from the unpleasant. The brain is also the seat of
madness and delirium, of the fears and terrors which assail by night or by day, of
sleeplessness, awkward mistakes and thoughts that will not come, of pointless
anxieties, forgetfulness and eccentricities. —Hippocrates
The human nervous system consists of the central nervous
system (CNS) and the peripheral nervous system (PNS). The
CNS contains the brain (cerebral hemispheres, basal ganglia,
and thalamus); brainstem (pons, medulla, and midbrain); and the
spinal cord. The PNS consists of somatosensory (afferent)
neurons, motor (efferent) neurons, and autonomic neurons.
The cerebral cortex
The area of the brain most closely associated with behavior is
the cerebral cortex, although subcortical areas are also involved.
The activity of the cortex can be segregated functionally into
sensory, motor, and association areas that act together to
ultimately affect behavior.
The cortex also can be divided anatomically into frontal,
temporal, parietal, and occipital lobes, as well as limbic lobes
that contain the medial parts of the frontal, temporal, and
parietal lobes.
Frontal lobes
The frontal lobes have four major subdivisions. The first two,
the motor strip and the supplemental motor area, are involved in
motor behavior; the third (Broca's area) in language. The fourth
division is the prefrontal cortex.
Clinically occurring events,
surgically imposed changes, and neuroimaging studies provide
evidence for the behavioral and personality functions of the
prefrontal cortex. The famous case of Phineas Gage, a man who
received a large prefrontal lobe lesion in an accident in the mid-
nineteenth century, first demonstrated the personality functions
of the frontal lobes. Although he had remarkably few obvious
neurological problems, Gage demonstrated a significant
personality change after his brain lesion healed. A formerly
nonaggressive person, Gage showed outbursts of anger after the
accident. A respectful, energetic, persistent, and organized
person before his accident, he began to show an inability to
carry out plans and a lack of self-control and concern for others.
In a similar way, some patients who have had bilateral
prefrontal lobotomy, a surgical procedure used in the past to
treat serious psychiatric illness, retain intellectual functioning
but show uncharacteristic apathy and lack of goal-directed
behavior after the surgery.
Perseveration, engaging in repeated unnecessary behavior and
thought, disinhibition, and sudden outbursts of temper, as well
as reinstatement of the infantile sucking ,palmomental and
rooting reflexes (frontal release signs) are seen in patients with
prefrontal lobe damage; this is now known as prefrontal lobe
syndrome. Interestingly, schizophrenia and obsessive-
compulsive disorder (OCD), both of which are characterized by
personality and affective changes, are associated with decreased
bilateral prefrontal cortical activity as measured by functional
magnetic resonance imaging (fMRI) and positron emission
tomography (PET) .
Although personality changes are associated with damage to the
entire prefrontal cortex, clinical and other evidence indicates
that the three major prefrontal subdivisions”the orbitofrontal
region, the dorsolateral convexity, and the medial region”have
specialized behavioral functions. The dorsolateral convexity
influences behavior and personality and has executive
responsibilities involving activities like formulating plans,
maintaining attention and concentration, and changing problem-
solving strategies when needed. The orbitofrontal cortex is a
center for the biological control of inhibition, emotions, and
drive states. It is also part of the dopamine-driven reward circuit
of the brain and is activated in addicts exposed to drug-related
cues . The medial region has connections to the basal ganglia
and accessory cortical motor areas and is involved primarily in
motor activity. Damage to each of these subdivisions results in
characteristic behavioral abnormalities The emotional-
behavioral functions of the frontal lobes are lateralized. Lesions
of the left prefrontal area, both cortical and subcortical, can
result in depression, whereas lesions of the right are more likely
to produce manifestations of elevated mood. Similarly, fMRI
studies reveal that positive mood is associated with activation of
the left prefrontal cortex and stress with activation of the right
prefrontal cortex.
Limbic lobe
Because the neurons within the limbic lobe form circuits that
play a major role in emotions, the limbic lobe has been called
the limbic system. Its primary functions are to mediate between
the hypothalamus and cerebral cortex and to modulate the
activity of the autonomic nervous system . First described by
Papez in 1937 and expanded later to include other areas, the
limbic system or Papez circuit contains the hippocampus, fornix,
amygdala, septum, part of the thalamus, the cingulate gyrus, and
related structures . The limbic system also acts on the
hypothalamus, which in turn influences endocrine control of
emotions via secretion of hormones. Damage to the limbic lobe,
particularly the amygdala and hypothalamus, results in
behavioral abnormalities. Recently, neuroimaging studies
indicate that, like the prefrontal cortex, the volume of limbic
structures like the amygdala and hippocampus are reduced in
patients with schizophrenia.
The basal ganglia
The basal ganglia are a group of nuclei that receive information
from the entire cerebral cortex and project it to the frontal lobes
via the thalamus. There are four structural components of the
basal ganglia:
•
Striatum (containing the caudate nucleus and the putamen)
•
Pallidum (also called the globus pallidus)
•
Substantia nigra
•
Subthalamic nucleus
The basal ganglia function to translate the desire to execute
movement into actual movement. Conditions causing damage to
its nuclei can result in neuropsychiatric illnesses with motor
symptoms. For example, over activity of the striatum or damage
to the substantia nigra results in Parkinson's-like symptoms like
the inability to initiate movement (bradykinesia). Underactivity
of the striatum and shrinking of the caudate nucleus are
associated with the symptoms of Huntington's disease. Damage
to the caudate is associated also with Tourette's syndrome,
whereas lesions of the pallidum and subthalamic nucleus result
in conditions characterized by sudden, uncontrolled limb
movements (hemi….?)
Hemispheric specialization
The left side of the brain controls the right side of the body.
Because approximately 90% of the population preferentially
uses the right hand, the left hemisphere of the brain is referred to
as the dominant hemisphere. Communication between the
cerebral hemispheres occurs via structures including the corpus
callosum, anterior commissure, hippocampal commissure, and
habenular commissure.
Because the left hemisphere is associated with language
function, damage to this hemisphere results in impairment of
skills such as speech, writing, and reading in almost all right-
handed people and in most left-handed people. The right, or
nondominant hemisphere, is associated primarily with
perception and also with spatial relations, body image,
recognition of faces and music, puzzle-solving, map-reading,
and musical and artistic ability. Damage to the right hemisphere
has motor sequelae and indirect effects on behavior but does not
usually affect intelligence or personality directly.
There are sex differences in functional organization of the brain.
For example, women generally have a larger corpus callosum
and anterior commissure and appear to have better
interhemispheric communication than men. When doing a
verbal task, women typically use both hemispheres, whereas
men show activation of only one hemisphere. The better-
developed right hemispheres of men may in part explain the
consistently documented male advantage in executing spatial
tasks
Consciousness, coma, and brain death
The thalamus and reticular formation, a network of neurons in
the brainstem, are the brain regions most closely involved in
arousal and consciousness. In contrast to cortical lesions
(particularly left-sided lesions), which must be extensive to
cause loss of consciousness, relatively small, localized lesions of
either of these structures, particularly the reticular formation,
can cause profound loss of consciousness or coma, nonsleep loss
of consciousness that extends for a prolonged period. Similarly,
lesions that disrupt connections between the brainstem and
thalamus can result in coma.
A patient in a profound coma has no conscious cognitive
function. If this condition is not reversible, the person is said to
be in a persistent vegetative state. Whether to maintain a person
who is in this state on life support is an important ethical issue
in medicine
TABLE Neuropsychiatric Anatomy: Function and Dysfunction
REGION/DIVISION MAJOR FUNCTIONS
EFFECTS OF LESION ON
BEHAVIOR
Frontal lobes
Dorsolateral
convexity
Planning for future action
(executive functions)
•
Decreased motivation,
concentration and attention
•
Disorientation
•
Mood disturbances
Orbitofrontal cortex Control over biological
drives
•
Disinhibition and
inappropriate behavior
•
Poor judgment
•
Lack of inhibition or remorse
(pseudopsychopathic
behavior)
Medial cortex
Control of movement
•
Apathy
•
Decreased spontaneous
movement (akinesia)
•
Gait disturbances
•
Incontinence
Temporal lobes
•
Memory
•
Learning
•
Emotion
•
Impaired memory
•
Psychomotor seizures
•
Changes in aggressive
behavior
•
Auditory
processing
•
Inability to understand
language (i.e., Wernicke's
aphasia [left-side lesions])
LIMBIC LOBES
Hippocampus
•
Memory storage
•
Poor new learning
Amygdala
•
Coordination of
emotional states,
particularly anger
and aggression,
with somatic
responses
•
Klأuer-Bucy syndrome
(decreased aggression,
increased sexuality,
hyperorality)
•
Decreased conditioned fear
response
•
Inability to recognize facial
and vocal expressions of
anger in others
Parietal lobes
•
Somatic sensation
and body image
•
Impaired IQ
•
Impaired processing of visual-
spatial information, (i.e.,
cannot copy a simple line
drawing or a clock face
correctly [right-sided lesions])
•
Gerstmann's syndrome (i.e.,
cannot name fingers, write,
tell left from right, or do
simple math, and impaired
processing of verbal
information [left-sided
lesions])
Occipital lobes
•
Vision
•
Visual hallucinations and
illusions
•
Inability to identify
camouflaged objects
•
Blindness