I’ve never been a fan of scary movies. Chainsaw-wielding maniacs, nasty men in hockey masks and children’s toys possessed by evil spirits may give some people a campfire thrill, but for me their capacity to spook is just a little too effective.
A few years ago, for example, there was the embarrassing occasion when an old episode of Twin Peaks, that moderately creepy television series from the early 1990s, caused me to go to bed with the lights on. My wife, however, is made of tougher stuff so when there’s a mysterious bump in the night these days, it’s she who stomps downstairs to confront the unknown.
Researchers have shown that the neurological signals responsible for fear centre around an area deep inside the brain known as the amygdala. This almond-shaped structure gathers neural messages about the outside world, assesses the significance of those signals and triggers mental and physical changes -the worried thoughts, sweaty palms, pounding heart loved by movie makers.
The amygdala is an ancient system that seems to be at the heart of many of our most fundamental emotional responses. It appears to have been retained throughout much animal evolution and studies in rats, birds, reptiles, fish and tadpoles have shown that the amygdala plays similar roles to those it takes in humans. Tadpoles obviously aren’t afraid of horror films, but the way their brains deal with danger seems to be similar to our own.
From an evolutionary perspective, you can see why a fear response would be worth holding on to. Faced with a dangerous situation, all animals need a mechanism to encourage them to react appropriately. But the system sometimes spirals out of control and conditions such as autism, depression, post-traumatic stress disorder, and phobias may be linked to abnormal amygdala function.
My dislike of horror films may not prevent me from leading a normal life, but for those people in whom fear gets out of hand, dealing with the consequences can be difficult. In the US alone, about 19 million people are estimated to suffer from fear-related disorders.
The importance of the amygdala in anger and fear has been appreciated for decades, but the exact biological mechanisms behind its action have until recently remained obscure. Last month, however, neuroscientist Ron Stoop, from the University of Lausanne, Switzerland, and his colleagues Daniel Huber and Pierre Veinante showed how two important brain signals affect nerve cells at the centre of the amygdala.
They reported in the journal Science that a chemical called vasopressin, which increases anxiety and stress, stimulated neurons in one part of the central amygdala, while another called oxytocin, with calming effects, triggered nerves in a different part.
These results suggest that an individual’s fear response depends on the relative number of nerve cells in the amygdala that are sensitive to vasopressin and oxytocin. That balance is determined by the presence of “receptors” on the surface of those cells that can detect the two chemicals.
”The balance between these two receptors may be important for setting the threshold of activation of the central amygdala in response to external stimuli,” Stoop says, adding that although the latest experiments were conducted in rodents, the findings have the potential to explain why some people are more anxious than others.
Scientists already know that in some animals the levels of the receptors for oxytocin and vasopressin can be affected by experiences in early life. “The question is whether we can change that balance in later life, perhaps by psychotherapy or drugs,” says Stoop.
The research has interesting implications for our understanding of how fear passes from one generation to another. Feelings of anxiety and fear can directly affect parental care, Stoop says, which in rodents can alter the expression of oxytocin and vasopressin receptors in their offspring, and on through the generations. “It’s kind of transmitted from mother to child,” he explains. “It’s not really passed down through the genes but through behaviour.”
In the context of my own filmic phobia, it is comforting to blame any fear on the amygdala rather than a personality failure. But as another leading neuroscientist, Joseph LeDoux, warned me, “be careful of the amygdala bandwagon”.
”These days some people are turning to the amygdala as the answer to all questions about emotions,” LeDoux said. But “easy answers like this are likely to be wrong. We need a lot more work on a lot more kinds of emotional behaviours before we truly understand what’s going on.”
The amygdala is clearly not the whole picture of human emotion. Apart from anything else, our capacity to reason means that many layers of cognition are intertwined with the ancient amygdala fear trigger.
So in the absence, for now, of a pill to turn off the fear response, I’ll just have to rely on that part of the brain responsible for another effective anxiety-reduction tool-the finger that turns off the television.
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