The effect of magnetic coil orientation on the latency of surface EMG and single motor unit responses in the first dorsal interosseous muscle.

We examined the effect of the orientation of a figure-of-eight coil on the latency of surface electromyographic (EMG) responses and the firing pattern of single motor units evoked in the first dorsal interosseous muscle by transcranial magnetic brain stimulation. Two coil positions were used: the coil held on a parasagittal line either with the induced current in the brain flowing in a postero-anterior direction (PA) or with the current flowing latero-medially (LM). The results were compared with those observed after anodal electrical stimulation. LM stimulation produced surface and single unit responses which occurred 0-3 msec earlier than PA stimulation. In many cases responses to LM stimulation had the same latency as those produced by anodal electrical stimulation. Responses evoked by LM stimulation were less affected by changes in motor cortical excitability (cortico-cortical inhibition and transcallosal inhibition) than those to PA stimulation. We suggest that LM stimulation can sometimes stimulate corticospinal fibres directly, at or near the same site as anodal stimulation. In contrast, PA stimulation tends to activate corticospinal fibres trans-synaptically. The difference in stimulation sites may make a comparison of PA and LM stimulation a useful method of localising changes in corticospinal excitability to a cortical level.