BACKGROUND: Tics are the core symptom of patients with Gilles de la Tourette syndrome, yet the spatial-temporal dynamics of neural activity causing a tic remains to be determined.

OBJECTIVE: Identification of cortical events preceding tic onset.

METHODS: In twelve patients with Tourette syndrome we performed magnetoencephalography to trace the time course of beta oscillations (15-30 Hz) in motor cortical areas before tic onset.

RESULTS: Patients showed a biphasic modulation of cortical beta activity during the second before tic onset. We observed an initial increase of beta power over the left-hemispheric channels overlying the motor cortex. This increase was subsequently replaced by a decrease in beta power. The beta decrease close to tic onset resembled the typical pattern accompanying preparation of voluntary movements. Only the initial increase in beta power positively correlated with the intensity of motor urges preceding tics.

CONCLUSIONS: The spatial-temporal dynamics of cortical activity suggests a voluntary component of tics that might be triggered by a failure of compensatory motor inhibitory mechanisms.