Abstract
BackgroundNeuroimaging technology is being developed to enable non-invasive mapping of the latency distribution of cortical projection pathways in white matter, and correlative clinical neurophysiological techniques would be valuable for mutual verification. Interhemispheric interaction through the corpus callosum can be measured with interhemispheric facilitation and inhibition using transcranial magnetic stimulation. ObjectiveTo develop a method for determining the latency distribution of the transcallosal fibers with transcranial magnetic stimulation. MethodsWe measured the precise time courses of interhemispheric facilitation and inhibition with a conditioning-test paired-pulse magnetic stimulation paradigm. The conditioning stimulus was applied to the right primary motor cortex and the test stimulus was applied to the left primary motor cortex. The interstimulus interval was set at 0.1 ms resolution. The proportions of transcallosal fibers with different conduction velocities were calculated by measuring the changes in magnitudes of interhemispheric facilitation and inhibition with interstimulus interval. ResultsBoth interhemispheric facilitation and inhibition increased with increment in interstimulus interval. The magnitude of interhemispheric facilitation was correlated with that of interhemispheric inhibition. The latency distribution of transcallosal fibers measured with interhemispheric facilitation was also correlated with that measured with interhemispheric inhibition. ConclusionsThe data can be interpreted as latency distribution of transcallosal fibers. Interhemispheric interaction measured with transcranial magnetic stimulation is a promising technique to determine the latency distribution of the transcallosal fibers. Similar techniques could be developed for other cortical pathways.
Highlights
A fundamental issue in understanding connectivity between neurons in different cortical areas is the distribution of latencies in white matter pathways that connect these areas
Post hoc paired t-test confirmed that conditioned motor evoked potential was larger than the test motor evoked potential at interstimulus intervals of À1.0 to À0.6 ms (P < 0.05) and À0.5 to 0.5 ms (P < 0.01) around the interval for maximal facilitation
Post hoc t-test confirmed that significant interhemispheric inhibition occurred at interstimulus intervals of À1.2 to À0.6 ms (P < 0.05), À0.5 to 0.1 ms (P < 0.01) and 0.2 to 0.5 ms (P < 0.05) around the interval for maximal inhibition
Summary
A fundamental issue in understanding connectivity between neurons in different cortical areas is the distribution of latencies in white matter pathways that connect these areas. Neuroimaging technology is being developed to enable non-invasive mapping of the latency distribution of cortical projection pathways in white matter, and correlative clinical neurophysiological techniques would be valuable for mutual verification. Objective: To develop a method for determining the latency distribution of the transcallosal fibers with transcranial magnetic stimulation. Methods: We measured the precise time courses of interhemispheric facilitation and inhibition with a conditioning-test paired-pulse magnetic stimulation paradigm. The proportions of transcallosal fibers with different conduction velocities were calculated by measuring the changes in magnitudes of interhemispheric facilitation and inhibition with interstimulus interval. The latency distribution of transcallosal fibers measured with interhemispheric facilitation was correlated with that measured with interhemispheric inhibition. Interhemispheric interaction measured with transcranial magnetic stimulation is a promising technique to determine the latency distribution of the transcallosal fibers.
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