To understand the expressive range of the human face， nothing beats watching a colleague scream his head off in slow motion. When my lab began to study protective reflexes in the early 2000s， the video cameras came out and the place became a scare factory. Graduate students took to lurking in hidden corners and lunging out with Velociraptor shrieks. Sundry plastic bugs and a pair of taxidermized monkey arms found their way inside the lunch refrigerator. I confess， I once took a cow eyeball from a dissection class， wrapped it in foil， and gave it to a colleague as a chocolate truffle.
By filming the reactions and reviewing the videos frame by frame， we began to realize that the startle reflex might be an evolutionary point of origin for many of our most common human emotional expressions.
When you look at still frames of a startle reaction， two features stand out: the pursing of skin around the eyes and the flashing of teeth. As the face scrunches， the upper lip pulls up， baring the upper teeth in a way that looks like a fleeting smile or a laugh. A lot of guesses have been floated about the purpose of this part of the movement. If you're about to be attacked， maybe it's good to appear as if you're ready to bite. But a close look at the movement， especially if you measure muscle activity in the face， suggests a different function: eye protection. If you expose your teeth to bite a hamburger，you recruit a set of muscles that ring the mouth. In contrast， the startle reflex recruits muscles around the eyes and in the cheeks. The forehead is mobilized downward and the cheeks are mobilized upward， dragging the upper lip with themand shielding the eyes in wrinkles.
You'll know what I mean if you've ever walked from a dark indoor space into a bright， sun-saturated summer day. Your whole face contracts into a kind of sun smile， or maybe a sun grimace， exposing your upper teeth， bunching your cheeks upward， and wrinkling the skin around your eyes to protect them from the excess light.
They say that the eyes are the windows to the soul. But the eyeballs themselves aren't really the windows. If you could look at a pair of eyeballs minus the rest of the face， you'd only learn where the eyes are looking and how dilated the pupils are. For reading another person，it's everything surrounding the eyes that matters the most. The windows to the soul are the eyelids that can narrow skeptically or open wide， the eyebrows that move and shape expressively， the sly wrinkles at the outer corners or on the bridge of the nose， the upward bunching of the cheeksthe many tensions and relaxations rioting around the center.
Ask any professional portraitist and you will be told that although the eyes are the most important part of the portrait， the eyeballs themselves are not so interesting to paint. They're merely bluish-white ovals with a black dot and a reflection. The emotional expression， and the challenge to the painter， lies in all the subtleties crowding around the eyeballs.
It may not be a coincidence that those true windows to the soul correspond so exactly to the epicenter of the startle reflex. The startle has a side effect that has nothing to do with self-protection: It broadcasts personal information well beyond the simple fact that you are startled. The reason is that your moods and thoughts strongly influence how your startle reflex manifests itself in any given moment. As much as the reflex relies on primitive pathways through the base of the brain that evolved before any higher thought or emotion， it is nonetheless influenced by networks all throughout the brain. Emotions，attention， and expectation sift through the cerebral circuitry， affect the startle reflex， and shape the way it looks to others.
The startle reflex is not， itself， a social gesture. It's not a smile， a laugh， or a frown. It's simply a reflex that evolved to protect the body. But it puts you at risk，too. By leaking signs about your inner state to the rest of the world， it becomes an exploitable data breach. And it's not subtle. You might as well put a neon billboard on your head.