No surprise here: You make snap judgments about new people based on their facial features. But parts of your brain may start reacting to those subtle facial cues even before you consciously “see” the person.

In a new study published in The Journal of Neuroscience, researchers report that the amygdala — a part of the brain associated with decision making, memory and emotion — plays a part in telling us who to trust almost instantly.

The researchers created a spectrum of simulated faces that had different degrees of trustworthiness written into their features. That’s not hard to do, since multiple studies have pinpointed which features stick out to humans as such: Upturned eyebrows and lips tend to do the trick. (For a visual example, check out this video from Princeton researchers).

In the new study, participants were placed in a brain scanner and shown these faces, but only for 33 milliseconds. The flash of face was followed by 167 milliseconds of a neutral mask image, to be sure that the subjects couldn’t continue to process the first image.

“The amygdala showed highly sensitive responses,” Jonathan Freeman, an assistant professor in New York University’s Department of Psychology and the study’s senior author, said. “Even though, objectively, the face wasn’t seen.” In other words, the participants probably couldn’t have said for sure that they’d even been shown a human face, but part of their brain had already begun to process that human’s intentions.

Of course we can’t say that a single region of the brain is “reacting” to or “judging” anything. That’s what the mind does as a whole. “But it means that the brain contains mechanisms that can decode these facial cues outside of conscious awareness, without perception,” Freeman said. “This is just one small step in understanding how the brain responds to social cues outside of awareness.”