Limbic System
The Neurophysiology of Emotions
The brain is the organ of the mind. Anatomists have
described the brain in terms of the evolutionary path we are on. We have
old-age, middle-age and new-age parts, each with different properties. The old
modules have remained much the same over the millennia. New structures grow out
of and over the older brain modules, linked together with the brain equivalents
of wiring harnesses and interface modules. Some of the interfaces between old
and new modules seem to be incomplete and unstable, leaving a lot to be desired
about human behavior.
The idea of the limbic system originated with attempts to
explain how emotions are generated. You can begin with four interconnected
modules: the hypothalamus, the anterior thalamic nucleus, the cingulate gyrus
and the hippocampus (the Papez circuit).
McLean added the orbitofrontal and medialfrontal cortices (prefrontal area), the parahippocampal gyrus and
subcortical modules such as the amygdala, the medial thalamic nucleus, the
septal area, basal nuclei and a few brainstem formations. The limbic system
generates feeding, fighting, fleeing and sexual behaviors. At the same time, the
limbic system generates feelings, emotions and participates in the regulation of
body functions.
Paul McLean suggested that the human brain could be viewed as three systems of different
ages - an old reptilian brain, a middle (early mammalian) brain, topped off with
a new, advanced brain, the neocortex. The neocortex allows us to learn, adapt
and create new modes of behavior. The neocortex has the computer equivalent of random access memory (RAM),
allowing the input of new information
This new information is used to interpret and adjust the
operation of read-only memory (ROM) which is built into old and middle brain
modules and cannot be modified. New babies are not born with the new brain
programs. Old programs are built into us and need not be learned. Old programs
include some of the most negative qualities – predatory and territorial
aggression, anger, and jealousy, for example. Some of our most positive
qualities are also innate such as the tendencies to mate, bond and form social
units with altruistic features.
The limbic system involves old brain modules that keep us
alive and involved in worldly interactions. These innate programs link us to the
world with appetites and drives. Appetites are states of need, giving rise to
drives which energize our behavior and send us searching for gratification out
there in the competitive, not-always-forgiving world. Hunger for food, food
searches, and eating gratification are
priorities for living creatures. After you are fed you can attend to the other
needs.
The organs of in the chest and abdomen (viscera)
interact with the brain through the autonomic nervous system that has three
major divisions:
- An old vagal system that fosters digestion and that responds to threats by cardiac output reduction and immobilization.
- A spinal sympathetic nervous system that inhibits vagal influence on digestion and
increases the metabolic rate to mobilize for "fight or flight."
- A myelinated vagal system that regulates cardiac output and breathing in response to inner
and outer signals.
Visceral regulation is a component of emotional processing
and is affected by social experience. Dysfunction in visceral regulation systems can be both causes and effects of
emotions. Anger for example, involves increased breathing, heart rate and blood
pressure.
The vagus nerve is the tenth cranial nerve, integrated with
the activities of other cranial nerves involved in social engagement: hearing,
facial expression via facial musculature and eye movements; the coordination of
breathing with sound production via laryngeal,
pharyngeal and head-moving muscles.
Old programs nested in the limbic system include
the most essential routines of animals:
establishment and fighting in defense of territory, foraging, hunting,
homing, hoarding, formation of social groups, greeting, grooming, courtship, mating,
flocking, and establishment of social
hierarchy by ritualistic display. Nested
in these old modules are programs inherited from reptiles that continue to
dominate the human experience.
Three Important Reptilian Behaviors
- Perseveration is the tendency for a fixed behavior to repeat or persist,
even when the path is blocked or the real benefit of a behavior disappears.
Eating behaviors, for example, are highly automated and resist change;
"...reptiles are slaves to routine, precedent, and ritual..."
- Frustration and anger are responses whenever seeking behaviors are
blocked, or threats are perceived. The least degree of drive-blocked discomfort
is frustration; "...there is hardly anything more surely to upset than the
alteration of long-established
routines."
- Displacement occurs when there is no further supply of the gratification
for the appetite or drive. Seeking behaviors shift laterally to find an alternative.
The limbic system contributes the for-me-ness of
experiences and generates cathexes. McLean stated: "The primitive limbic system has the capacity to generate the strong
feelings of conviction that we attached to our beliefs, regardless of whether
they are true or false..."
The anterior cingulate cortex (ACC) is the cortical part of
the limbic system. The cingulate gyrus lies above the corpus callosum and is
reciprocally connected with the anterior nuclear group of the thalamus through
the thalamic radiations and with the hypothalamus by way of the mammillothalamic
tract. The cingulate cortex is also connected with the parahippocampal gyrus and
septal area by way of the cingulum.
This central position gives the ACC control over cognitive
and emotional processing. The ACC is part of a brain system that monitors
performance to regulate behavior. For example
the left anterior cingulate cortex (ACC) is coupled with left inferior frontal
gyrus during letter-recognition decisions. The right ACC is coupled with right parietal areas during
visuospatial decisions. The ACC can detect errors in action but also predicts
the probability of error based on past experience. The ACC provides a
competitive advantage by anticipating and avoiding errors.
Conflict monitoring by the ACC can determine the need for
greater cognitive control and cause greater prefrontal cortex activity and
appropriate adjustments in behavior.
Decision making involves prediction of the outcome of a
given action. Obviously, the expectation of a pleasant outcome makes a behavior
more likely. When outcomes are not
pleasurable, regret is attached to the decision to act.
Cricelli et al found that regret involved the orbitofrontal, and
anterior cingulate cortex. They stated: ‘We measured brain activity using
functional magnetic resonance imaging (fMRI) while subjects selected between two
gambles; regret followed information about the favorable outcome of choices not
made. Subjects became increasingly regret-aversive, a cumulative effect
reflected in enhanced activity within the medial orbitofrontal cortex and
amygdala. This pattern of activity reoccurred just before making a choice,
suggesting that the same neural circuitry mediates direct experience of regret
and its anticipation. “
The posterior cingulate cortex assesses the risk involved
in obtaining rewards. Lee suggested: “Choosing to accept enough risk, but not
too much, is an important survival skill, and depending on the circumstances,
animals may either seek or avoid risk. Given the choice between a sure bet and a larger but
uncertain reward, Macaques consistently take the riskier option, and posterior
cingulate cortex neurons represent the risk of those choices.”
Drives to maintain brain concentrations of food (and drug)
substances are organized in the old brain or limbic structures which do not ask
consciousness for permission to operate. The old brain is characterized by a
low-level consciousness and a high level of automaticity. Most insightful people
will describe the split in their personalities as though automatic behaviors
take over and consciousness simply monitors the events which follow.
The idea of "will-power" is difficult to substantiate since
there does not seem to be a brain procedure with which we reach from high-level
thinking down into machine level programming where automatic behaviors are
produced. The characteristic of addictive behavior is the recurrent search for a
food, beverage, or drug which supplies specific molecules. Drugs can be
ingested, inhaled or injected. This substance search mode may be concealed by
layers of social behavior that are more or less acceptable. The drinker,
smoker and drug user develop repertoires
of social behaviors and mannerisms which conceal and support their habit.
