Rethinking Facial Expressions

Our lab doesn’t just focus on speech and language, but also facial expressions and other “paralinguistic” aspects of communication that layer emotion onto the words people say. As we do so, we go into further depth than is often included in even neurological education.  

For example, classic organization of movement in the brain shows a homunculus, or little human, laid out along the outside of the brain’s surface in regions called Brodmann areas. Relevant to this discussion, Broadmann areas 4 and 6 direct facial expression. 

Curiously, we are often taught that if just the lower part of the face, i.e. the mouth, is unable to move on one side, it’s potentially more serious than if the upper and lower are both paralyzed. That’s because control of the eyes and forehead actually comes from both sides of the brain, further from conscious control. This may be why it’s harder to control each eyebrow separate from the other– can you raise the left without pushing down the right, and also raise the right without pushing down the left?

But even most neurologists aren’t taught where in the brain that bilateral control happens. 

In the early 2000s, Dr. Morecraft and his colleagues decided to take a closer look at rhesus monkeys (our close cousins in the primate world), and found some surprises.  While the classic areas are still important, Morecraft found some new players in the facial expression game:

• The supplementary motor area (let’s call it M2).
• The rostral cingulate motor cortex (M3).
• The caudal cingulate motor cortex (M4).

These areas, especially M3 and M4, have strong connections to the emotional centers of the brain.

M3 helps control the upper face on both sides pretty evenly.

M4 is more focused on the opposite side of the face and is great at expressions like snarling and disgust.

These regions are called “perilimbic” meaning that they’re a bit further away from our conscious control. This may help explain the so-called “Duchenne” smile, which involves crinkles around the eyes, as opposed to a smile where the lips move but the eyes are unchanged, which may be less sincere. 

Our faces are even more amazing and complex than we imagined. It’s not just about simple muscle movements – it’s a sophisticated dance between different brain areas, each playing their part in showing our emotions to the world. Next time you’re trying to read someone’s expression, remember – there’s a whole lot of brain science happening behind that look!