On the synchronization of transcranial magnetic stimulation and functional echo-planar imaging

Purpose: To minimize artifacts in echo-planar imaging (EPI) of human brain function introduced by simultaneous transcranial magnetic stimulation (TMS). Materials and Methods: Distortions due to TMS pulses (0.25 msec, 2.0 T) were studied at 2.0 T before and during EPI. Results: Best results were obtained if both the EPI section orientation and the frequency-encoding gradient were parallel to the plane of the TMS coil. Under these conditions, a TMS pulse caused image distortions when preceding the EPI sequence by less than 100 msec. Recordings with a magnetic field gradient pick-up coil revealed transient magnetic fields after TMS, which are generated by eddy currents in the TMS coil. TMS during image acquisition completely spoiled all transverse magnetizations and induced disturbances ranging from image corruption to mild image blurring, depending on the affected low and high spatial frequencies. Simultaneous TMS and radio-frequency (RF) excitation gave rise to T1- dependent signal changes that lasted for several seconds and yielded pronounced false-positive activations during functional brain mapping. Conclusion: To ensure reliable and robust combinations, TMS should be applied at least 100 msec before EPI while completely avoiding any pulses during imaging

New approaches to the study of human brain networks underlying spatial attention and related processes

Cognitive processes, such as spatial attention, are thought to rely on extended networks in the human brain. Both clinical data from lesioned patients and fMRI data acquired when healthy subjects perform particular cognitive tasks typically implicate a wide expanse of potentially contributing areas, rather than just a single brain area. Conversely, evidence from more targeted interventions, such as transcranial magnetic stimulation (TMS) or invasive microstimulation of the brain, or selective study of patients with highly focal brain damage, can sometimes indicate that a single brain area may make a key contribution to a particular cognitive process. But this in turn raises questions about how such a brain area may interface with other interconnected areas within a more extended network to support cognitive processes. Here, we provide a brief overview of new approaches that seek to characterise the causal role of particular brain areas within networks of several interacting areas, by measuring the effects of manipulations for a targeted area on function in remote interconnected areas. In human participants, these approaches include concurrent TMS-fMRI and TMS-EEG, as well as combination of the focal lesion method in selected patients with fMRI and/or EEG measures of the functional impact from the lesion on interconnected intact brain areas. Such approaches shed new light on how frontal cortex and parietal cortex modulate sensory areas in the service of attention and cognition, for the normal and damaged human brain