You may be lazing by the pool after a visit or two to the swim-up bar, but parts of your brain are always on duty — ready to leap into action should a stressful event require attention.
This skeleton crew of sorts is called the default-mode network.
It includes one of the busiest and most important structures in the entire brain, the hippocampus, which is responsible for processing memories. "Whenever you have to look something up or file something away, you ask your hippocampus to do it," says Jens Pruessner, an associate professor in the departments of psychology, psychiatry, neurology and neurosurgery at McGill University in Montreal.
It also includes the medial orbitofrontal cortex, an area involved with self-referential thoughts, and the anterior cingulate, which Pruessner calls an "error monitor.... Its job is to stay on guard for mismatches between what you expect to happen and what actually happens."
So, for example, suppose it's the first night of your vacation and you're at a karaoke bar, and your medial orbitofrontal cortex is busy thinking that maybe you should get up and sing a little. But then your killjoy hippocampus dredges up some memories of the last time you sang karaoke, causing your medial orbitofrontal cortex to start having second thoughts.
But then, before anything gets decided about your performing career, a stranger passing by trips over your chair and spills his drink on your head. Your anterior cingulate was not expecting that! So it quickly files an error report with your stress-response system and turns itself off temporarily so all your brainpower can concentrate on this critical situation. And your medial orbitofrontal cortex realizes that your brain has better things to do right now than to think about your musical talents, or lack thereof, so it turns itself off temporarily too.
Meanwhile, your hippocampus, as part of its endless job of filing away new information, stores the information that something wet has just fallen on your head. But because this is a little out of the ordinary and your hippocampus recognizes it as a possible source of stress, it puts on another of its many hats. "It's the secretary for all things memory," Pruessner says, "but it also has its hand on the button to turn on your stress response."
As soon as it pushes that button, the hippocampus also shuts itself down and waits while the response system arranges for your adrenal glands to release some cortisol. "The cortisol gives you additional energy to withstand this new stress," Pruessner says. As soon as there's enough of the hormone in circulation, the hippocampus turns itself back on.
Still, for the short time that it's offline, from seconds to minutes, you may not be quite as clever as usual. Which is why, although the perfect witty remark to make to the drink-spiller will surely come to you, that may not happen until you're back in your hotel room.