Brain under threat

What causes your brain to switch from the quiet focus needed to read (or write) these words to the frantic, goggle-eyed arousal needed to confront a frothing dog or rabid boss?

That hyper condition, popularly called the fight-or-flight response, is a hormonally inflicted surge of stress that puts all systems on alert, raises the heart rate and blood pressure, and shifts blood from the gut to the muscles.

This is not when you want to be translating Latin or solving equations, but fight-or-flight certainly fulfills its evolutionary role of allowing the body and brain to survive threatening circumstances.

After the transition, the brain regulates attention differently: A person studying Japanese woodcuts is unlikely to notice someone prowling on the other side of the art library. A person cranked up on stress hormones is unlikely to miss the lurker.

Neuroscientists long ago fingered two “stress” hormones — cortisol and noradrenaline — as playing key roles in fight-or-flight and today, a study in Science helps confirm that noradrenaline, not cortisol, triggers the transition to a different level of attention. “Many people thought cortisol would have an effect on the attention process in the early phase, but our study shows cortisol probably is not as important” as noradrenaline, says first author Erno Hermans, of the Donders Institute for Brain, Cognition and Behaviour at Radboud University Nijmegen Medical Center in Holland.

Putting the stress on stress

Image: Irréversible

According to some film critics, Irréversible was one of the most disturbing films of 2002. No wonder it stressed-out the study subjects!

To study the mental effects of stress, Hermans and colleagues put 80 subjects in a magnetic resonance imager and tracked the usage of oxygen in the brain to show which structures were active at any moment. Then the subjects watched parts of a French movie containing what Hermans calls “particularly horrific” scenes of violence.

The scans revealed changes in what’s called the salience network, which “is active in a general state of hyper-arousal, vigilance,” Hermans says. “It scans the environment for things that might be important, and allows you to redirect your attention.” The result is not just a change of focus, “but a switch to a state where a change of your focus becomes more likely.”

To confirm that the violent movie clip was triggering the stress response, the researchers measured heart rate and chemicals in the saliva.

Counting on cortisol

Long-term stress can lead to many problems, including the disabling post-traumatic stress disorder, and cortisol, which makes memories more vivid and plays a major role in the constant arousal and intrusive memories of PTSD, has long been considered a major player in stress in general.

“Stress research in humans has been very focused on cortisol for very good reason,” says Hermans, “as it’s linked to a number of very important features of stress in the body and also in the brain.”

In a second phase of the experiment, Hermans and his colleagues used drugs to block either cortisol or noradrenaline. Blocking cortisol did not prevent the changes in brain networks, but blocking noradrenaline did. “Because blocking noradrenaline results in a reduction in the salience network, this shows that noradrenaline is important for this reorganization of the brain,” Hermans says.

Stress or distress?

The new study helps explain our world, says Christopher Coe, a professor of psychology at the University of Wisconsin-Madison and an expert in cortisol and stress. “As we all have subjectively experienced, a fearful stimulus can exert a galvanizing influence on us. It can reorient our attention and, when sufficiently provocative, make us feel more alert, energized and focused. This change in state is facilitated by the type of coordinated brain reaction described in this Science paper. We and our brains are mobilized in order to better analyze the situation, to quickly interpret and utilize incoming information … and to respond adaptively.”

Coe adds that although “it is reasonable to conclude” that cortisol is not initiating the change in salience, “nevertheless, because of cortisol’s widespread effects and potency, if its release into the blood stream is sustained, it may ultimately exert a more protracted effect on both the brain and other physiological functions.”

Changes in the mode of attention are a fact of life, Hermans says. “We are really selective about accepting information while doing a focused task,” but a threat “requires a switch so your brain can respond to significant things in the surroundings. The brain becomes more responsive to stimuli, the eyes are wide open, the pupils become larger, everything is focused on having more sensory intake.”