Fig. 1.

Latency distribution of the transcallosal fibers tested with interhemispheric interaction. (A) Experimental setup. IHF and IHI from the right primary motor cortex to the left primary motor cortex tested with a paired-pulse transcranial magnetic stimulation paradigm were used to verify the latency distribution of transcallosal fibers with different diameters (color lines). The first CS was given to the right primary motor cortex. The second TS was given to the left primary motor cortex. The small arrows close to the coil show the induced current direction in the brain. (B) Hypothesis. CS activates multiple pyramidal neurons (triangles with different sizes) in the right primary motor cortex. These pyramidal neurons are connected to the transcallosal fibers with different diameters (color lines with different thickness). Pyramidal neurons (black triangle) in the left primary motor cortex are directly influenced by transcallosal inputs, leading to IHF. Local inhibitory interneurons (black rhombus) in the left cortex are also activated by transcallosal inputs, leading to IHI with longer latency. Dashed line indicates the mid-sagittal line. Facilitatory and inhibitory interactions are marked with small open and filled circles, respectively. (C) Example recordings. Average of 10 trials. All trials included a TS (vertical line). TS alone generated a MEP of about 0.5 mV in amplitude (first row). A preceding CS produced IHF at short ISIs (2–6 ms, second to sixth rows). IHI was induced at longer ISIs (8–14 ms, bottom rows). (D) IHF and IHI (N = 12) tested at ISIs of 1–15 ms with 1 ms resolution. Abscissa indicates the ISI. Ordinate indicates MEP amplitude induced by the paired-pulse stimulation. It is expressed as a percentage value of the mean MEP amplitude evoked by CS-TS to that evoked by TS alone (dashed line). Values above 100% indicate IHF (open circle) and values below 100% indicate IHI (filled circle). ∗P < 0.05, ∗∗P < 0.01, post hoc paired t-test with Bonferroni’s correction comparing MEP with paired-pulse stimulation to that with TS alone. CS = conditioning stimulus; IHF = interhemispheric facilitation; IHI = interhemispheric inhibition; IN = local inhibitory interneuron in the left primary motor cortex; ISI = interstimulus interval; MEP = motor evoked potential; TS = test stimulus. . (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)