The brain may be viewed as a hierarchy of three “separate brains”: vegetative/ reflective brain, a higher adaptive/skilled brain, and an intermediate brain concerned with emotions and instincts. The vegetative brain corresponds roughly to the brain stem and is concerned with controlling vital bodily functions (respiration, digestion, circulation) as well as with integrating brain reflexes. The adaptive and skilled brain corresponds to the cerebral cortex (the neocortex). It has sensory, motor, and association areas that serve in complex perception and execution of skilled motor functions (e.g., hand movements, speech) as well as higher mental functions (e.g. learning, thoughts, introspection, planning).
Limbic system (LS) structures are concerned with central (neural) control over the expression of emotions, instinctive behaviors, drives, motivation, and feelings. In lower vertebrates, the LS is called the rhinencephalon (small brain) due to its intimate connection to the central olfactory structures. In these animals, many instinctive behaviors are guided by the sense of smell. Both the cerebral cortex and the LS have ample access to the brain stem motor areas, permitting them to exercise their respective adaptive and instinctive (stereotyped) controls over behavior.
LS STRUCTURES and CONNECTIVITY.
The main LS structures are the amygdala (almond), septum (wall), hippocampus (sea horse), cingulate gyrus, anterior thalamus, and hypothalamus. The relative mass of the LS remains fairly uniform among the different vertebrates. The LS structures are connected by a number of complicated but no necessarily reciprocal pathways, some of which constitute a loop. The loop of Papez (hypothalamus—anterior thalamus—cingulate gyrus—hypocampus—hypothalamus) has been considered, for a variety of reasons including the fact that patients with lesions in this loop exhibit abnormalities of emotional expression, as one of the neural circuits serving in emotional and instinctive behavior.
The LS was once thought to be involved only in emotional/instinctive behavior and that the cerebral cortex and LS had few connections and little communication. This view is changing. Thus, the hippocampus, a prominent LS structure, thanks to its numerous connections to both the older (e.g., olfactory and limbic) and the newer (e.g., the cortex) parts of the brain, plays an essential role in learning and memory. Similarly, the cingulate gyrus (located on the medial hemispheric surface and possibly regulating such social behaviors as parental care) is part of both the LS and the cerebral cortex; the cingulate gyrus, like the hippocampus provides an important link between the adaptive and the instinctive brains. LS structures like the amygdala and septum can also communicate, via their reciprocal connections to the cingulate gyrus, with the higher cortical association areas. The LS structures also receive abundant sensory input and send motor input to both voluntary and involuntary motor centers via the cingulate gyrus to the motor cortex and via the hypothalamus to the brain stem.
EFFECTS OF LS STIMULATION/ LESIONS
In the 1940’s Walter Rudolph Hess (Swiss physiologist and Nobel laureate) discovered that electrical stimulation of certain areas in the hypothalamus and neighboring structures in cats can evoke the behavior patterns of fear or aggression similar to those observed during a cat fight (hissing, spitting, hair standing, back hunching, and slapping). Lesion studies indicated that disconnecting the forebrain from these lower limbic areas leaves the expressiono of these emotions intact while removing purpose and directedness from them.
Additionally, stimulation of certain areas in the amygdala often activates aggressive responses. Bulls can be forced to charge under these conditions. Stimulation of the amygdala in a normally submissive monkey causes the animal to exhibit aggressive gestures more often. As a result, the monkey temporarily moves up in the group’s dominance order. Violent attacks of rage are occasionally seen in humans with epileptic seizure discharges (causing excesslive local electrical activity) in the amygdala; surgical removal of the amygdala eliminates the rage attacks. Removal of the amygdala in monkeys also results in timidity and passivity. The hippocampus is one place where stimulation does not elicit any emotional/instinctive behaviors.
In the 1960’s, James Olds (American psychobiologist) found that certain areas in the LS may function in pleasure or reward. When rats with electrodes in the septum or associated pathways (e.g., medial forebrain bundle) are taught to self-stimulate at will, they do so for a long time, preferring this electrical stimulation of the brain to food rewards (hence the label “pleasure centers”). Humans also report pleasure when stimulated in similar locations.
Little is known about how exactly LS circuits function in experience of feelings and expression of emotions/instincts. A person’s responses to the smell and sight of a rose may help explain. These stimuli bring pleasure, fond visual associations, a smile, and autonomic responses (e.g.,changes in heartbeat). The rose scents find access to the LS via the olfactory system, which feeds into the amygdala and the hypothalamus. The rose’s visual sensations activate the LS either via the reticular formation pathways to the hypothalamus (LS) or via the visual thalamic nuclei to the anterior thalamus (LS). Activation of the LS circuits at this time presumably stimulates the subjective experience of pleasure/good feelings. The motor responses of smiling can be activated via the hypothalamic outflow to the brain stem nuclei serving in control of muscles of expression. The hypothalamus also serves as the LS center / output for such autonomic motor responses as changes in heartbeat. To touch/pick the rose, voluntary motor areas of the cortex are accessed via the cingulate gyrus or hippocampus. Similar connections to frontal lobes and temporal lobes evoke the higher aspects of feelings (e.g., love) and fond memories respectively.
Daryl Conant, M.Ed.