1.2 Functional Neuroanatomy
13
that rely heavily on the parietal lobe, are impaired early in the course
of Alzheimer’s disease. In contrast, personality changes, which reflect
frontal lobe function, are relatively late consequences of Alzheimer’s
disease. A rarer, complementary cortical degeneration syndrome, Pick’s
disease, first affects the frontal lobes while sparing the temporal and
parietal lobes. In Pick’s disease, disinhibition and impaired language
expression, which are signs of frontal dysfunction, appear early, with
relatively preserved language comprehension and memory.
Memory loss can also result from disorders of the subcortical gray
matter structures, specifically the basal ganglia and the brainstem
nuclei, from disease of the white matter, or from disorders that affect
both gray and white matter.
Emotion
Individual emotional experiences occupy the attention of all men-
tal health professionals. Emotion derives from basic drives, such
as feeding, sex, reproduction, pleasure, pain, fear, and aggression,
which all animals share. The neuroanatomical basis for these
drives appears to be centered in the limbic system. Distinctly
human emotions, such as affection, pride, guilt, pity, envy, and
resentment, are largely learned and most likely are represented
in the cortex (see Color Plate 1.2-3). The regulation of drives
appears to require an intact frontal cortex. The complex inter-
play of the emotions, however, is far beyond the understanding
of functional neuroanatomists. Where, for example, are the rep-
resentations of the id, the ego, and the superego? Through what
pathway are ethical and moral judgments shepherded? What pro-
cesses allow beauty to be in the eye of the beholder? These philo-
sophical questions represent a true frontier of human discovery.
Several studies have suggested that within the cortex exists
a hemispheric dichotomy of emotional representation. The left
hemisphere houses the analytical mind but may have a limited
emotional repertoire. For example, lesions to the right hemi-
sphere, which cause profound functional deficits, may be noted
with indifference by the intact left hemisphere. The denial of
illness and of the inability to move the left hand in cases of
right hemisphere injury is called
anosognosia.
In contrast, left
hemisphere lesions, which cause profound aphasia, can trigger
a catastrophic depression, as the intact right hemisphere strug-
gles with the realization of the loss. The right hemisphere also
appears dominant for affect, socialization, and body image.
Damage to the left hemisphere produces intellectual dis-
order and loss of the narrative aspect of dreams. Damage to
the right hemisphere produces affective disorders, loss of the
visual aspects of dreams, and a failure to respond to humor,
shadings of metaphor, and connotations. In dichotic vision
experiments, two scenes of varied emotional content were dis-
played simultaneously to each half of the visual field and were
perceived separately by each hemisphere. A more intense emo-
tional response attended the scenes displayed to the left visual
field that were processed by the right hemisphere. Moreover,
hemisensory changes representing conversion disorders have
been repeatedly noted to involve the left half of the body more
often than the right, an observation that suggests an origin in
the right hemisphere.
Within the hemispheres, the temporal and frontal lobes play a prom-
inent role in emotion. The temporal lobe exhibits a high frequency of
epileptic foci, and temporal lobe epilepsy (TLE) presents an interesting
model for the role of the temporal lobe in behavior. In studies of epi-
lepsy, abnormal brain activation is analyzed, rather than the deficits in
activity analyzed in classic lesional studies. TLE is of particular interest
data yet support the notion, it is conceivable that the hippocam-
pal cognitive map is inappropriately reactivated during a
déjà vu
experience.
The most famous human subject in the study of memory is
H. M., a man with intractable epilepsy, who had both his hippoc-
ampi and amygdalae surgically removed to alleviate his condition.
The epilepsy was controlled, but he was left with a complete ina-
bility to form and recall memories of facts. H. M.’s learning and
memory skills were relatively preserved, which led to the suggestion
that declarative or factual memory may be separate within the brain
from procedural or skill-related memory. A complementary deficit
in procedural memory with preservation of declarative memory may
be seen in persons with Parkinson’s disease, in whom dopaminer-
gic neurons of the nigrostriatal tract degenerate. Because this defi-
cit in procedural memory can be ameliorated with treatment with
levodopa (Larodopa), which is thought to potentiate dopaminergic
neurotransmission in the nigrostriatal pathway, a role has been pos-
tulated for dopamine in procedural memory. Additional case reports
have further implicated the amygdala and the afferent and efferent
fiber tracts of the hippocampus as essential to the formation of
memories. In addition, lesional studies have suggested a mild lat-
eralization of hippocampal function in which the left hippocampus
is more efficient at forming verbal memories and the right hippoc-
ampus tends to form nonverbal memories. After unilateral lesions
in humans, however, the remaining hippocampus may compensate
to a large extent. Medical causes of amnesia include alcoholism,
seizures, migraine, drugs, vitamin deficiencies, trauma, strokes,
tumors, infections, and degenerative diseases.
The motor system within the cortex receives directives from
the association areas. The performance of a novel act requires
constant feedback from the sensory and association areas for
completion, and functional neuroimaging studies have dem-
onstrated widespread activation of the cortex during unskilled
acts. Memorized motor acts initially require activation of the
medial temporal lobe. With practice, however, the performance
of ever-larger segments of an act necessary to achieve a goal
become encoded within discrete areas of the premotor and pari-
etal cortices, particularly the left parietal cortex, with the result
that a much more limited activation of the cortex is seen during
highly skilled acts, and the medial temporal lobe is bypassed.
This process is called the
corticalization of motor commands.
In
lay terms, the process suggests a neuroanatomical basis for the
adage “practice makes perfect.”
Within the diencephalon, the dorsal medial nucleus of the
thalamus and the mammillary bodies appear necessary for
memory formation. These two structures are damaged in thia-
mine deficiency states usually seen in chronic alcoholics, and
their inactivation is associated with Korsakoff’s syndrome. This
syndrome is characterized by severe inability to form new mem-
ories and a variable inability to recall remote memories.
The most common clinical disorder of memory is Alzheimer’s
disease. Alzheimer’s disease is characterized pathologically by the
degeneration of neurons and their replacement by senile plaques and
neurofibrillary tangles. Clinicopathological studies have suggested that
the cognitive decline is best correlated with the loss of synapses. Ini-
tially, the parietal and temporal lobes are affected, with relative sparing
of the frontal lobes. This pattern of degeneration correlates with the early
loss of memory, which is largely a temporal lobe function. Also, syntac-
tical language comprehension and visuospatial organization, functions
