You’re walking with a friend on a trail beside the gorge and you see a stick spread across the path. You recoil, thinking it’s a snake, but your friend stays calm. Why do two people react differently to the same stimulus?

“There’s a difference between sense data and perception,” Bernard Tarr, grad, said. “That we all receive the same physical information in terms of sense data … does not necessarily imply that we therefore must perceive those things the same.”

Both people in the example see the same stick, but they perceive it differently based on past experience and genetics. A traumatic event or a family history of anxiety problems can shape the way the brain responds to sudden stimuli.

Allostasis is a term neuroscientists use to describe the process through which the body maintains homeostasis by reacting to daily events. Stress is a natural part of that system. Sudden events trigger the release of “stress hormones,” like adrenalin and glucocorticoids, that cause heart rate and blood pressure to increase, preparing the body for a fight-or-flight response. Ancient humans with the stress response were more likely to survive and pass on the trait.

Although stress might seem more harmful than helpful in the modern world, Tarr pointed out that a snake today can pose just as much of a threat to us as it did to ancient humans.

“Anxiety is a part of life,” Matt Boone, coordinator of Let’s Talk at Gannett, said. “We have to have anxiety in order to function. There’s an optimal amount of anxiety that will help us complete tasks, such as study for a test.”

Too much stress, however, results in “allostatic overload,” which is roughly equivalent to being “stressed out.” In allostatic overload, the stress response doesn’t work properly. It might stay on longer than it needs to, causing unnecessary wear and tear on the system or do the opposite and not activate at a time when it is needed. Sometimes this state is triggered by behavior. Sleep deprivation, lack of exercise, excessive eating and smoking can all lead to allostatic overload.

One of the most important stress-centers of the brain is the hippocampus. Research has shown that people with a small hippocampus tend to feel more anxious than people with a larger one. War veterans suffering from post-traumatic stress disorder tend to have small hippocampi. Researchers without pre-war data have trouble determining whether a small hippocampus led to post-traumatic stress disorder or whether the stress caused irreversible shrinkage to the hippocampus.

A recent Cornell experiment looked at the hippocampus of adult black-capped chickadees. Several of the birds were captured, and half were released into the wild while half were held in cages. Four to six weeks later, the hippocampus of the captured birds was shown to be on average 23 percent smaller than the hippocampus of the wild birds. The researchers concluded that the size of the hippocampus had changed because of captivity and speculated that the stress of being in a cage was one factor that led to this change.