Stop right there: Why we freeze when scared

How a new study found the answer behind the “deer in the headlights” reaction

If you’ve ever had the misfortune of being scared out of your wits by a friend, coworker, or enemy, then you know that frozen feeling you get immediately after nearly jumping out of your seat. Your muscles tighten up, your eyes bulge out of your head, and for a split second, you just can’t move. A new study now tells us why our bodies have that reaction when we’re scared and what it means for us.

Recently, a Columbia University study found that Serotonin triggered certain reactions in fruit flies — reactions very similar to humans when they are startled. The study found that when fruit flies were introduced to changes in the environment, such as sudden vibrations, the flies would stop in their tracks momentarily, thanks to a release of serotonin in their bodies.

These findings from the study, now published in Current Biology, offer a broader insight into the inner workings of the startle response and how it works biologically.

“Imagine sitting in your living room with your family and — all of a sudden — the lights go out, or the ground begins to shake,” said Richard Mann, PhD, a principal investigator at Columbia’s Mortimer B. Zuckerman Mind Brain Behavior Institute and the paper’s senior author. “Your response, and that of your family, will be the same: You will stop, freeze and then move to safety. With this study, we show in flies that a rapid release of the chemical serotonin in their nervous system drives that initial freeze. And because serotonin also exists in people, these findings shed light on what may be going on when we get startled as well.”

If you don’t know what serotonin is, it is the chemical in our bodies that is closely related to regulation of moods and emotions. In other words, we can call it the “happy chemical”.

Interestingly enough, scientists in the past have found that serotonin can also affect animals speed and range of motion.

Researchers at Columbia analyzed the movement speed of frut flies using a special apparatus called the FlyWalker, which measures the steps and speed of the flies on a special type of glass.

The team introduced different levels of chemicals such as serotonin and dopamine into the fly and reviewed the results. The initial report revealed that activating neurons responsible for producing serotonin slowed the fly down, while silencing them sped the fly up.

“We witnessed serotonin’s biggest effects when the flies experienced rapid environmental changes,” said Clare Howard, PhD, the paper’s first author. “In other words, when they were startled.”

To test the fly further, the team devised two more scenario’s for the test subject: putting through both a blackout and a simulated earthquake.

Using a specialized miniature arena set atop vibrating motors, the team then set it in motion, observing the fly’s actions when put into a constantly changing environment.

“We found that when a fly is startled in these scenarios, serotonin acts like an emergency brake; its release is needed for them to freeze, and that part of this response may be a result of stiffening both sides of the animal’s leg joints,” said Dr. Mann, who is also the Higgins Professor of Biochemistry and Molecular Biophysics (in Systems Biology) at Columbia’s Vagelos College of Physicians and Surgeons. “This co-contraction could cause the brief pause in walking, after which the insect begins to move.”

Although the fly’s response in both simulated circumstanced was to freeze, their subsequent walking speeds differed greatly.

“After being startled in the blackout scenario, the fly’s gait was slow and deliberate,” said Dr. Howard. “But the earthquake caused the flies to walk faster after the initial pause.”

While these findings are specific for fruit flies, the biological and chemical reaction found in fruit flies could translate to more complex species when startled just the same.

From these new findings, the research team at Columnia hopes to further investigate serotonin’s role in movement and if there are any other factors in play in similar situations.

“As we and others continue to investigate, we hope to develop a detailed, molecular blueprint for locomotion that can be applied broadly to other animals, perhaps even people,” said Dr. Mann.