Researchers used a fMRI scanner to measure responses to various types of sounds in the brains of study participants. They played positive sounds such as laughter or triumphant shouting; they played negative sounds like screaming or throwing up or groaning. All the sounds triggered a response in the premotor cortical region of the brain. This part of the brain prepares facial muscles to respond to sounds. The response to positive sounds, especially laughter, was greater than the response to negative sounds, suggesting that the positive sounds were more contagious than negative ones. This may be one reason humans tend to respond to laughter or cheering or even smiles with an involuntary smile. It also may be the science behind using “laugh tracks” with sitcoms or other programs.