THE LIMBIC SYSTEM
This structure, also referred to as the paleocortex, lies on the border of the brain stem and below the neocortex.
Role of the Limbic System
The phylogenetically old structures of the brain stem are responsible for many complex behaviors that are necessary for the survival of organisms.
However, their ability is limited to instinctive, inflexible behaviors. These structures are incapable of developing adaptive behaviors.
It is the development of the paleocortical and neocortical structures that helped the higher animals to adapt effectively and intelligently to the changing environmental conditions.
Paul D. MacLean gave a comprehensive three-level model of the functions of the limbic system.
1. He identified the first circuit at the amygdala. It is concerned with emotional behaviors concerned with ‘self-preservation’ (e.g. feeding, fighting, self-protection).
2. The second circuit lies in the septal area and is concerned with sociability and sexuality.
3. The third circuit connecting the mammillary bodies with the anterior thalamus and the cingulate gyrus. He suggested this circuit is absent in lower animals and largest in humans.
Role of the Limbic System
Another evidence for the role of the limbic system in motivational-emotional behavior came from the study of the ‘Kluver-Bucy Syndrome’
Terzian & Dalle also reported symptoms similar to the Kluver – Bucy Syndrome in a 19 year old man, whose temporal lobes were removed to treat temporal lobe epilepsy.
Later, more restricted lesion studies focusing only on the amygdala was found to reduce aggressive and rage behaviors in humans, without giving rise to other dramatic behaviors as observed in the Kluver-Bucy Syndrome.
Mark and Ervin suggested the use of electrodes for more precise therapeutic lesions.
Role of the Environmental Factors in the functioning of Limbic System
Thus, primary evidence shows that stimulation in the area of the amygdala often produces rage and fear, whereas lesions in this area makes the animal tame.
However, environmental factors are very important in influencing the effects of these stimulation and lesions.
Adams reported that defense-attack can become a submissive act depending upon the environmental cues.
Von Holst & Von Saint Paul studied the behavior of roosters who were stimulated, in different conditions.
The Septal Circuit
Lesions and stimulation of the septal region has effects that are completely different from that of the amygdala.
In rats, septal lesions produced temporary viciousness and increased reactivity to stimulation.
Evidence shows that septal lesions have ‘disinhibitory’ effects, thus making behavior impulsive.
Gorenstein & Newman pointed out that these symptoms in animals are similar to ‘disinhibitory psychopathology’ in humans.
This symptomolgy that they labelled ‘Septal Syndrome’, is characterized by the following –
• No inclination to inhibit punished responses
• Feared stimuli inhibit behavior only if they are actually present
• Inability to delay gratification
• High need for gratification
• Stimulation of the septal areas usually leads to positive, pleasurable subjective experience.
• These experiences usually have a sexual reference and may also be followed by grooming behavior even in rats.
• In humans, ‘feel-good’ self-reports were observed, again with sexual overtones. Heath reported drastic change in mood and conversations of patients stimulated in the septal region.
The hippocampus works closely in association with the amygdala and the septal area.
The hippocampus is also sensitive to hormonal activity in the cerebrosoinal fluid and blood.
It plays a role in complex systems of emotion, memory consolidation and retrieval, attention, arousal, movement, behavior inhibition, orienting, stimulus analysis, spatial mapping and anxiety.