Intracortical inhibition after paired transcranial magnetic stimulation depends on the current flow direction.
OBJECTIVE
To assess whether cortico-cortical inhibition (CCI) induced by paired-pulse transcranial magnetic stimulation (TMS) is influenced by 'preferential' or 'non-preferential' activation of the motor cortex.


METHODS
Paired-pulse TMS (conditioning-test paradigm with interstimulus intervals 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 of CCI in the opponens pollicis and first dorsal interosseus muscles was determined.


RESULTS
When a clockwise current was induced in the brain (side A of the coil uppermost) a 'preferential' activation of the left hemisphere (right hand muscles) was observed, but the suppression of the test response by the conditioning stimulus (i.e. the CCI) was significantly greater in the left hand 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 conditioning stimulus strength.


CONCLUSIONS
CCI is more effective in the 'non-preferentially' stimulated hemisphere, and the neural elements generating the indirect I3 wave are more sensitive to intracortical inhibition than those generating the I1 wave.
Trompetto
C
C
Department of Neurological Sciences and Neurorehabilitation, University of Genoa, Italy.
Assini
A
A
Buccolieri
A
A
Marchese
R
R
Abbruzzese
G
G
D000328:Adult; D000704:Analysis of Variance; D004576:Electromyography; D005260:Female; D007839:Functional Laterality / Q000502:physiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D050781:Transcranial Magnetic Stimulation