Trascranial Magnetic Stimulation (TMS) with interhemispheric paired pulses: effect of handedness.

The study assesses the effect of hand preference on the Transcal- losal Inhibition (TI) as measures by the Inter-Hemispheric Paired Pulse (IHPP) paradigm with Transcranial Magnetic Stimulation. IHPP has been recorded in 32 normal subjects [16 right-hand- ers and 16 left-handers (8 M and 8 F for each group)], with stim- ulation intensity set to 120% of Motor Threshold (MT) for each subject in both stimuli (conditioning and test). The Inter-Stimulus Intervals (ISIs) were: 2, 4, 6, 8, 10, 12, 14, 16, 18, and 20 ms for both hemispheres. The Motor Evoked Potential (MEP) responses were recorded from the abductor digit minimi (ADM) muscles. The variables analyzed were the MTs and the ratio between con- ditioned responses (preceded by conditioning pulse) divided by unconditioned responses (in which only the pulse has been admin- istered). Results on the amplitude ratio showed only a main effect for the ISI factor without any interaction involving Handedness. Therefore, TI does not seem different between handedness groups, and callosal conductivity could be not affected by which hemi- sphere controls the dominant hand. On the other hand, results on MTs showed a significant Hand- edness × Hemisphere interaction, pointing to a higher threshold on the hemisphere contralateral to non-dominant hand.