1. Intracortical inhibition (ICI) and facilitation (ICF) of the human motor cortex can be induced by paired transcranial magnetic stimulation (TMS). Although demonstrated in experimental animals, the existence of intracortical inhibitory and excitatory circuits in parietal sensory cortex has not been documented in humans. The aim of this study was to investigate the effects of paired TMS of the parietal cortex on contralateral tactile perception. 2. Fifteen healthy subjects were involved in a task of discrimination of electrical stimuli delivered at near-threshold intensity of sensory perception over the left thumb. Paired TMS was delivered with a focal coil on the right posterior parietal lobe after various delays from the presentation of finger stimuli. The effects of different interstimulus intervals (ISI: 1,3,5, 7, 10 and 15 ms) between the conditioning and the test TMS stimulus on tactile perception were studied. The conditioning stimulus intensity was set at 70% of motor threshold, while test TMS intensity was 130% of motor threshold. 3. Single pulse suprathreshold TMS interfered with the perception of finger stimuli, while subthreshold stimuli such as the 'conditioning' stimuli had no effect on sensory perception. Paired TMS differentially influenced the performance depending on the ISI. At an ISI of 1 ms, paired TMS stimuli induced a significant worsening of the performance compared with single pulse TMS; at an ISI of 5 ms, paired TMS stimuli induced a significant facilitation of the performance compared with single pulse TMS, restoring baseline performance levels. 4. These results suggest that paired TMS can reveal a selective pattern of ICI and ICF in the human parietal cortex.
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