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The area of the brain known as the limbic system is made up of a number of structures which are linked together by fibre pathways and control such behaviour as emotional expression and memory storage and recall.
Studies of rats indicate that the circuit through the hippocampus and thalamus is involved in spatial memories while the one through the amygdala and thalamus is involved in emotional memories. Nuerosurgeons tracing the brain circuitry involved in the fear response are focusing on the lateral nucleus, a portion of the amygdala, which appears to have a key role in fear conditioning. Fear seems to send nerve impulses along the pathway from the ear to the thalamus and either directly to the amygdala or, more desirably, into the auditory portion of the sensory cortex, where they can be analysed before being sent to the amygdala. When the amygdala receives nerve signals indicating a threat, it sends out signals that trigger such reactions as rapid heartbeat and raised blood pressure.
Studies made by Joseph LeDoux, Ph.D., a neuroscientist at New York University, show that all the ingredients necessary for fear conditioning are in place in the lateral nucleus of the amygdala a rich supply of nerve cell extensions connecting it to the thalamus and other parts of the amygdala and the cortex; rapid response to stimuli; high threshold for stimulation and high frequency preference. The various portions of the amygdala communicate with each other through internal nerve cell connections. Once fear conditioning has taken place, these interior circuits continue to respond to the fear stimulus. Therefore, a person with a phobia may appear to be cured after behavioural treatment only to have the phobia return during an episode of high stress. LeDoux suggests that this is because the signal pathways from the thalamus to the amygdala and sensory cortex have been normalized, but the internal circuits in the amygdala have not. One reason for the difficulty in exerting conscious control over fear could be because there are far more cell circuits leading from the amygdala to the prefrontal cortex, the area of the brain most responsible for reasoning, than there circuits going the other direction.
Functional magnetic resonance imaging scans of brains in human beings are beginning to show that as in the rats the amygdala is the central site of fear conditioning. Fear conditioning is believed to play a role in anxiety disorders such as phobias, post-traumatic stress disorder, and panic disorder. If, as research suggests, the memories stored in the amygdala are relatively indelible, Dr. Le Doux suggests that the aim of therapy for anxiety disorders should be to increase cortical control over the amygdala. He sees a need for more behavioural and neuroscientific research to increase understanding of how multiple memory systems work together in fear conditioning and other emotional responses. With more scientists focusing on the emotional brain, the unravelling of the memory functions of the amygdala will favourably impact on treatment for anxiety disorders.
Copyright Jean Jardine Miller.