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Assessing low-frequency repetitive transcranial magnetic stimulation with functional magnetic resonance imaging: a case series.

BACKGROUND AND PURPOSE
This case series assesses the effects of five consecutive days of low-frequency (1 Hz) repetitive transcranial magnetic stimulation (rTMS) with and without a 6-Hz primer. Although this paper studies able-bodied individuals, similar rTMS protocols are used to facilitate motor recovery in patients with hemiplegia following stroke. However, the cortical mechanisms associated with repeated daily doses of rTMS are not completely understood.


CASE DESCRIPTION
Four right-handed healthy volunteers (two men, aged 20-50 years) participated in a double-blind case series of primed and unprimed rTMS. Functional magnetic resonance imaging was used to compare task-related haemodynamics during a simple motor task and resting-state cortical connectivity. Understanding the mechanisms of repeated rTMS sessions may serve as a precursor to development of rTMS paradigms involving motor cortex stimulation in patients with a range of neurologic dysfunction.


OUTCOMES
Following five consecutive days of rTMS, all subjects had reduced task-related haemodynamics. Resting-state brain connectivity between motor regions was reduced only after primed rTMS.


DISCUSSION
This is the first study to indicate that resting-state brain connectivity can distinguish the effect of primed and unprimed rTMS to a greater extent than task-related haemodynamics. Furthermore, priming may inhibit the connectivity between the area of the cortex underlying the rTMS site and remote brain regions.


SIGNIFICANCE
These findings benefit rTMS rehabilitation studies by examining haemodynamics on repeated days of stimulation and incorporating resting-state brain connectivity analysis to further understand underlying neural mechanisms. Furthermore, this work encourages the utilization of resting connectivity in future rTMS studies.