[3] Studies have found that activity of the corrugator muscle, which lowers the eyebrow and is involved in producing frowns, varies inversely with the emotional valence of presented stimuli and reports of mood state[citation needed].

Facial EMG has been used as a technique to distinguish and track positive and negative emotional reactions to a stimulus as they occur.

[4] A large number of those experiments have been conducted in controlled laboratory environments using a range of stimuli, e.g., still pictures, movie clips and music pieces.

[5] Although commonly used as an index of emotional responses, facial muscle activity is also influenced by the social context in which it is measured.

The authors considered facial EMG could effectively monitor loss of understanding or of situation awareness.