Influence of transcranial magnetic stimulation on the execution of memorised sequences of saccades in man.

Memorised sequences of saccades are cortically controlled by the supplementary motor area (SMA), as shown in animal experiments and in humans with isolated SMA lesions. We applied transcranial magnetic stimulation (TMS) in eight healthy subjects executing memorised sequences of saccades. Sequences of three targets were presented. Then, upon a go-signal, the subjects had to execute the appropriate sequences. Ten to fifteen sequences were performed in each experiment, and the number of errors were counted. The number of errors increased significantly if TMS was given 80 ms before or 60 ms after the go-signal, with the stimulation coil overlying the SMA. There was no significant increase in errors if different stimulation intervals were chosen (160 ms and 120 ms before the go-signal; 100 ms, 140 ms or 240 ms after the go-signal), if the coil was positioned inappropriately (e.g. over the occipital cortex), or if the stimulator output was too low. We conclude that TMS can interfere specifically with the function of the SMA during a critical time interval close to the go-signal.