The inappropriate smile and zygomatic muscle activity.

Long-term recording of activity of the zygomatic muscle, the most important mimic muscle involved in smiling, was performed in 22 disorganized type schizophrenic patients with inappropriate smiles and 15 normal subjects in two separate experiments. During inappropriate smiles, the zygomatic muscle exhibited waxing and waning bursts of activity with an amplitude of 75 to 120 microV, which could not be distinguished from the activity observed during usual smiling in normal subjects. However, the duration of such activity tended to be longer when compared to that in usual smiling by normal subjects as well as by schizophrenic patients.