Remember: Be Afraid--Be Very Afraid, Monsieur Amygdala

The amygdala is involved in fear and fear based memory. A recent article at the Neurophilosopher Blog provides a fascinating discussion of the general role of the amygdala, along with some very interesting recent research findings into gene expression within the amygdala.

The amygdala receives its main inputs from the visual, auditory and somatosensory cortices; it couples these sensory stimuli (something scary) to the fight-or-flight response and, therefore, its main outputs are to the hypothalamus, which controls hormone production and homeostasis, and autonomic centres in the brainstem.

Recent work by Gleb Shumyatsky and his colleagues at Rutgers University in New Jersey has led to the discovery of several genes that are enriched in the amygdala and appear to be involved in the formation of fearful memories. One of these proteins, stathmin (also known as oncoprotein 18) is now known to be involved in mediating the formation of memories of of both conditioned and unconditioned fear; there is a high level of expression of the stathmin gene, and a corresponding high concentration of stathmin protein, in the amygdala, but not in the adjacent hippocampus.

Mutant mice lacking the stathmin gene were unable to learn new fears or to act instinctively in a fearful situation, i.e. they had weaker memories of fearful experiences. The stathmin knockout mice also showed less anxiety when presented with new mazes to explore or with potentially dangerous situations. Upon further examination, it was observed that mice lacking the stathmin gene had a less dynamic microtubule network than wild type (normal) mice. Memories are formed by the establishment of new synaptic connections, which require a re-arrangement of microtubules. In the absence of the stathmin protein, microtubules aren’t re-arranged so easily, and, as a consequence, the synapses that would normally be modified during memory formation are not as plastic as they should be.

More recently, researchers have used functional magnetic resonance imaging (fMRI) to show that the rostral anterior cingulate cortex(rACC) modulates activity in the amygdala, effectively acting as an ‘on-off’ switch. Joy Hirsch and her colleagues at Columbia University used a variant of the Stroop test, which involves presenting words for colours, which are printed in a colour that differs from that of the meaning of the word (e.g. green). This discrepancy leads to a delay in the processing of the visual information, reducing the reaction time taken to perform the task.

Instead of using words, Hirsch’s team presented participants in their experiments with photographs of happy or scared faces with the word ‘HAPPY’ or ‘FEAR’ written across them. It was found that the amygdala was activated before the rACC when the participants were presented with a happy face with the word ‘FEAR’ printed across it. Soon afterwards, though, the activity in the amygdala would be reduced. That is, initially, the amygdala processes the word ‘FEAR’, but, soon afterwards, the rACC processes the happy face, and inhibits activity in the amygdala to reduce the fear response. In contrast, when participants were presented with a photograph of a scared face which had the word ‘FEAR’ written across it, the rACC remained inactive, while activity in the hippocampus persisted for longer.

More at Source.

In twin studies, some aspects of phobia show a heritability of around 45% or higher.

Although many social scientists still deny the importance of heritability in cognitive and emotional function, people who actually work in the field know better. In fact, it would not be surprising to find that heredity influences cognitive styles that would predispose individuals to particular styles of political or ideological belief, within the broad spectrums of human political/ideological behaviour.

When one contemplates the increasing likelihood that different attitudes toward morality are at least partially inherited, it is not a great leap to expect that heredity plays a part in philosophical, ideological, or political predisposition. Teasing out all the possible mechanisms will be beyond complex.