What type of literary device is this?

I agree with Little Eva's answer, but I'll add one more thing:

The expression could be taken as an example (though perhaps not a perfect one) of a literary device called Synecdoche.

Synecdoche is when you use part of something to refer to the whole.

"I can see pain in your eyes."

When I say I can see pain in your eyes, I mean I see the signs of emotional pain not in your literal eyeballs, but in your whole aspect (the muscles around your eyes, your forehead, your mouth, even your posture).