EEG events prior to simple tics have been examined in six patients with Gilles de la Tourette's syndrome, and compared to EEG changes occurring when the same subjects voluntarily mimicked their own tics. Voluntary jerks were prefaced by a premovement negative potential commencing about 500 ms prior to the muscle EMG discharge and reaching an amplitude of about 7 muV. No such premovement potential was evident in the EEG prior to spontaneous tics in five of the six patients; a very small event was seen in the sixth patient but it was only about one-tenth of the size of the premovement potential seen prefacing voluntary jerks in the same subject. These data indicate that simple tics in Gilles de la Tourette's syndrome are physiologically distinct from normal self-paced willed movements.
