Researchers recently found that yawning isn’t only catching among people; it is also among chimpanzees. (Click here for a brief video from this research.) No one has devised a fully convincing explanation of why.

Compounding the mystery is the odd way in which the contagious power of yawning is largely unconscious. We can see someone yawn, yearn to replicate the action ourselves, and do it, all without thinking about it. Other times we’re aware it is happening, though it still floats somewhere beneath the realm of reason and of purposeful actions.

So what gives? In an effort to find the answer, the Finnish government recently funded a brain scanning study. The results turned up some hard-to-interpret, possible clues. It also confirmed the obvious: yawn contagion is largely unconscious. Wherever it might affect the brain, it bypasses the known brain circuitry for consciously analyzing and mimicking other people’s actions.

This circuitry is called the “mirror-neuron system,” because it contains a special type of brain cells, or neurons, that become active both when their owner does something, and when he or she senses someone else doing the same thing. 

Mirror neurons typically become active when a person consciously imitates an action of someone else, a process associated with learning. But they seem to play no role in yawn contagiousness, the researchers in the new study found. The cells are have no extra activity during contagious yawning compared with during other non-contagious facial movements, they observed.

Brain activity “associated with viewing another person yawn seems to circumvent the essential parts of the MNS [mirror neuron system], in line with the nature of contagious yawns as automatically released behavioural acts—rather than truly imitated motor patterns that would require detailed action understanding,” wrote the researchers, with the Helsinki University of Technology and the Research Centre Jülich, Germany. The findings are published in the February issue of the research journal Neuroimage.

But if seeing someone yawn doesn’t activate these centers, what does it do to the brain? The researchers found that it appears to strongly activate at least one brain area, called the superior temporal sulcus. But this activation was unrelated to any desire to yawn in response, so it may be irrelevant to the contagion question, the researchers added.

Possibly more significant, they wrote, was the apparent deactivation of a second brain area, called the left periamygdalar region. The more strongly a participant reported wanting to yawn in response to another person’s yawn, the stronger was this deactivation. 

“This finding represents the first known neurophysiological signature of perceived yawn contagiousness,” the researchers wrote.

Exactly what the finding means is less clear, they acknowledged. The periamygdalar region is a zone that lies alongside the amygdala, an almond-shaped structure deep in the brain in the area of the side of the head. The periamygdalar region has been linked to the unconscious analysis of emotional expressions in faces. Why it would be deactivated in tandem with yawn contagion is unclear, the researchers said.

One thing seems clear from the study is that “contagious yawning does not rely on brain mechanisms of action understanding,” wrote one of the researchers, Riitta Hari of the Helsinki University of Technology, in a recent email. Rather, she continued, it seems to be an “‘automatically’ released (and most likely very archaic) motor pattern,” or sequence of physical actions.

In the study, volunteers looked at videos of actors yawning or making other mouth movements. Meanwhile their brains were scanned using functional Magnetic Resonance Imaging, a system that shows the amount of activity or work going on in various brain areas based on the amount of oxygen being used up there. The volunteers were later asked how strongly they had been tempted to yawn while viewing the pictures.

Apart from the physical brain mechanisms of yawn contagiousness, researchers have offered different reasons as to why it exists. Some have proposed that in early humans, yawn contagiousness might have helped people communicate their alertness levels to each other, and thus coordinate their sleep schedules.

This might be part of a more general phenomenon of unconscious signals that serve to synchronize group behavior, the authors of the Neuroimage paper wrote. “Such synchronization could be essential for species survival and works without action understanding, like when a flock of birds rises to the air as soon as the first bird does so—supposably as it notices a predator.”