BACKGROUND - Applied over the skull, transcranial magnetic stimulation (TMS) non-invasively stimulates neurons using time-varying magnetic fields. Direct knowledge of TMS brain effects has been limited, with most information coming from observations of peripheral effects. Many have assumed that TMS causes dramatic changes in brain activity, only some of which are seen with peripheral measures.
METHODS - By interleaving TMS with functional imaging (echo planar BOLD functional Magnetic Resonance Imaging (fMRI)), we directly observed the relationship between TMS induced brain activation and normal brain activity.
RESULTS - TMS induced thumb movement at one pulse per second activated approximately the same brain region as volitional movement with the same magnitude and timing of blood flow response. Thus, under these conditions, TMS induced brain activity over motor cortex closely parallels changes associated with normal brain activity.
Figure 1 METHOD-Diagram of the relative timing of the interleaved TMS stimulus and scanning used in the study.
Figure 2
RESULTS - Coronal MRI images of one subject directly below the TMS coil, a) TMS minus rest and b) volition minus rest. An expanded view of ipsilateral motor cortex activation is shown in (c).
Figure 3
Bar graph of activated brain volume for all 7 subjects by region for the two comparisons.
Figure 4
Cycle averaged time activity curve of BOLD signal for all subjects.
Figure 5
(a) TMS and volitionally activated cluster compared by projecting clusters into the same plane. Centers of clusters calculated as described in the text. The arrow is the displacement from the center of the volition cluster to the center of the TMS cluster. (b) Displacement vectors for five subjects.
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