14
Chapter 1: Neural Sciences
in psychiatry because patients with temporal lobe seizures often mani-
fest bizarre behavior without the classic grand mal shaking movements
caused by seizures in the motor cortex. A proposed TLE personality is
characterized by hyposexuality, emotional intensity, and a perseverative
approach to interactions, termed
viscosity.
Patients with left TLE may
generate references to personal destiny and philosophical themes and
display a humorless approach to life. In contrast, patients with right TLE
may display excessive emotionality, ranging from elation to sadness.
Although patients with TLE may display excessive aggression between
seizures, the seizure itself may evoke fear.
The inverse of a TLE personality appears in persons with bilat-
eral injury to the temporal lobes after head trauma, cardiac arrest,
herpes simplex encephalitis, or Pick’s disease. This lesion resem-
bles the one described in the Klüver-Bucy syndrome, an experimen-
tal model of temporal lobe ablation in monkeys. Behavior in this
syndrome is characterized by hypersexuality, placidity, a tendency
to explore the environment with the mouth, inability to recognize
the emotional significance of visual stimuli, and constantly shifting
attention, called
hypermetamorphosis.
In contrast to the aggres-
sion–fear spectrum sometimes seen in patients with TLE, complete
experimental ablation of the temporal lobes appears to produce a
uniform, bland reaction to the environment, possibly because of an
inability to access memories.
The prefrontal cortices influence mood in a complementary way.
Whereas activation of the left prefrontal cortex appears to lift the mood,
activation of the right prefrontal cortex causes depression. A lesion to
the left prefrontal area, at either the cortical or the subcortical level,
abolishes the normal mood-elevating influences and produces depres-
sion and uncontrollable crying. In contrast, a comparable lesion to the
right prefrontal area may produce laughter, euphoria, and
witzelsucht,
a tendency to joke and make puns. Effects opposite to those caused by
lesions appear during seizures, in which occurs abnormal, excessive
activation of either prefrontal cortex. A seizure focus within the left
prefrontal cortex can cause gelastic seizures, for example, in which the
ictal event is laughter. Functional neuroimaging has documented left
prefrontal hypoperfusion during depressive states, which normalized
after the depression was treated successfully.
Limbic System Function
The limbic system was delineated by James Papez in 1937. The
Papez circuit consists of the hippocampus, the fornix, the mam-
millary bodies, the anterior nucleus of the thalamus, and the cin-
gulate gyrus (Fig. 1.2-4). The boundaries of the limbic system
were subsequently expanded to include the amygdala, septum,
basal forebrain, nucleus accumbens, and orbitofrontal cortex.
Although this schema creates an anatomical loop for emo-
tional processing, the specific contributions of the individual
components other than the hippocampus or even whether a
given train of neural impulses actually travels along the entire
pathway is unknown.
The amygdala appears to be a critically important gate
through which internal and external stimuli are integrated.
Information from the primary senses is interwoven with inter-
nal drives, such as hunger and thirst, to assign emotional sig-
nificance to sensory experiences. The amygdala may mediate
learned fear responses, such as anxiety and panic, and may
direct the expression of certain emotions by producing a par-
ticular affect. Neuroanatomical data suggest that the amygdala
exerts a more powerful influence on the cortex, to stimulate or
suppress cortical activity, than the cortex exerts on the amyg-
dala. Pathways from the sensory thalamic relay stations sepa-
rately send sensory data to the amygdala and the cortex, but
the subsequent effect of the amygdala on the cortex is the more
potent of the two reciprocal connections. In contrast, damage to
the amygdala has been reported to ablate the ability to distin-
guish fear and anger in other persons’ voices and facial expres-
sions. Persons with such injuries may have a preserved ability
to recognize happiness, sadness, or disgust. The limbic system
appears to house the emotional association areas, which direct
the hypothalamus to express the motor and endocrine compo-
nents of the emotional state.
Figure 1.2-4
Schematic drawing of the major anatomic structures of the limbic system. The cingulate and parahippocampal gyri form the “limbic lobe,”
a rim of tissue located along the junction of the diencephalon and the cerebral hemispheres. (Adapted from Hendelman WJ.
Student’s
Atlas of Neuroanatomy
. Philadelphia: WB Saunders; 1994:179.)
