Intracortical inhibition after paired transcranial magnetic stimulation depends on the current flow direction

Objectives: To assess whether cortico-cortical inhibition (CCI) induced by paired-pulse transcranial magnetic stimulation (TMS) is influenced by 'pref erential' or 'non-preferential' activation of the motor cortex. Methods: Paired-pulse TMS (conditioning-test paradigm with interstimulus in tervals of 2-5 ms) with a round coil centered over the vertex was performed in 10 normal subjects using opposite current flow directions. The amount o f CCI in the opponens pollicis and first dorsal interosseus muscles was det ermined. Results: When a clockwise current was induced in the brain (side A of the c oil uppermost) a 'preferential' activation of the left hemisphere (right ha nd muscles) was observed, but the suppression of the test response by the c onditioning stimulus (i.e, the CCI) was significantly greater in the left h and muscles. The situation was reversed when an anticlockwise current (side B of the coil uppermost) was induced in the brain. These effects occurred independently of the interstimulus interval, or of the absolute conditionin g stimulus strength. Conclusions: CCI is more effective in the 'non-preferentially' stimulated h emisphere, and the neural elements generating the indirect I3 wave are more sensitive to intracortical inhibition than those generating the I1 wave. ( C) 1999 Elsevier Science Ireland Ltd. All rights reserved.