Background. Motor function is deficient in many patients with obsessive–compulsive disorder (OCD), especially in the face. To investigate subtle motor dysfunction, kinematical analysis of emotional facial expressions can be used. Our aim was to investigate facial movements in response to humorous film stimuli in OCD patients.

Method. Kinematical analysis of facial movements was performed. Ultrasound markers at defined points of the face provided exact measurement of facial movements, while subjects watched a humorous movie (‘Mr Bean’). Thirty-four OCD patients (19 male, 15 female; mean (S.D.) age: 35·8 (11·5) years; mean (S.D.) total Y-BOCS score: 25·5 (5·9)) were studied in unmedicated state and after a 10-week treatment with the SSRI sertraline. Thirty-four healthy controls (19 male, 15 female; mean (S.D.) age: 37·5 (13·1) years) were also investigated.

Results. At baseline, OCD patients showed significantly slower velocity at the beginning of laughing than healthy controls and a reduced laughing frequency. There was a significant negative correlation between laughing frequency and severity of OCD symptoms. Ten weeks later a significant increase of laughing frequency and initial velocity during laughing was found.

Conclusions. Execution of adequate facial reactions to humour is abnormally slow in OCD patients. Susceptibility of OCD patients with regard to emotional stimuli is less pronounced than in healthy subjects. This phenomenon is closely correlated to OCD symptoms and is state-dependent.