In short, Botox not only changes one's appearance, it also changes real emotions.
Botox weakens or paralyzes facial muscles. Those muscles have less movement, so they're providing less feedback to the brain.
"With Botox, a person can respond otherwise normally to an emotional event like a sad movie scene, but will have less movement in the facial muscles that have been injected, and therefore less feedback to the brain such as facial expressivity," said Joshua Davis, a co-author of the study and a psychology professor at Barnard College.
"It thus allows for a test of whether facial expressions and the sensory feedback from them to the brain can influence our emotions," Davis added.
The study was published in the June issue of the journal "Emotion."