Objectives: To directly compare the volleys evoked by figure-of-eight and circular magnetic coil stimulation of the motor cortex and to correlate the descending volleys with the EMG responses in distal hand muscles.
Methods: Descending corticospinal volleys were recorded from an electrode inserted into the cervical epidural space of two conscious human subjects after transcranial stimulation of the hand area of the motor cortex. We compared volleys evoked by stimulation with (a) a figure-of-eight coil inducing posterior-anterior or latero-medial currents in the brain, (b) a large circular coil centred at the vertex inducing clockwise currents in the brain, and (c) anodal electric pulses.
Results: For a given amplitude of EMG response in the first dorsal interosseous muscle, volleys were larger after stimulation with a circular than a figure-of-eight coil. In addition, the D wave evoked by circular coil stimulation had a longer latency than the anodal D wave, and increased in amplitude when stimulation was given during voluntary contraction.
Conclusions: We conclude that stimulation with a large circular coil activates descending outputs less selectively than figure-of-eight coil stimulation and that it is capable of activating pyramidal neurones at the initial segment region.