Interhemispheric Effect of Parietal TMS on Somatosensory Response Confirmed Directly with Concurrent TMS–fMRI

Abstract

Transcranial magnetic stimulation (TMS) has been used to document some apparent interhemispheric influences behaviorally, with TMS over the right parietal cortex reported to enhance processing of touch for the ipsilateral right hand (Seyal et al., 1995). However, the neural bases of such apparent interhemispheric influences from TMS remain unknown. Here, we studied this directly by combining TMS with concurrent functional magnetic resonance imaging (fMRI). We applied bursts of 10 Hz TMS over right parietal cortex, at a high or low intensity, during two sensory contexts: either without any other stimulation, or while participants received median nerve stimulation to the right wrist, which projects to left primary somatosensory cortex (SI). TMS to right parietal cortex affected the blood oxygenation level-dependent signal in left SI, with high-versus low-intensity TMS increasing the left SI signal during right-wrist somatosensory input, but decreasing this in the absence of somatosensory input. This state-dependent modulation of SI by parietal TMS over the other hemisphere was accompanied by a related pattern of TMS-induced influences in the thalamus, as revealed by region-of-interest analyses. A behavioral experiment confirmed that the same right parietal TMS protocol of 10 Hz bursts led to enhanced detection of perithreshold electrical stimulation of the right median nerve, which is initially processed in left SI. Our results confirm directly that TMS over right parietal cortex can affect processing in left SI of the other hemisphere, with rivalrous effects (possibly transcallosal) arising in the absence of somatosensory input, but facilitatory effects (possibly involving thalamic circuitry) in the presence of driving somatosensory input.

On the Role of Prestimulus Alpha Rhythms over Occipito-Parietal Areas in Visual Input Regulation: Correlation or Causation?

“…Because each processor controls attention toward the opposite hemi-field and because reciprocally interacting through inhibitory cross talk, the model posits that damage to one processor will suppress perception contralaterally and release it ipsilaterally, the latter effect being the result of disinhibition of the unaffected processor from the disrupted, contralateral counterpart. Corresponding results have been obtained previously with parietal TMS using so-called disruptive ("virtual lesion") TMS protocols (Seyal et al, 1995;Hilgetag et al, 2001;Blankenburg et al, 2008), some of which include 10 Hz TMS over a short interval (online rTMS protocol). We show for the first time that the potential of behavioral suppression (and associated "paradoxical" enhancement) of online rTMS depends on the frequency applied (maximum for alpha TMS) in line with the inhibitory alpha hypothesis, at least for stimulation over alpha oscillating sites at short trains.…”

Causal evidence for frontal involvement in memory target maintenance by posterior brain areas during distracter interference of visual working memory

“…High-resolution T1-weighted images (176 sagittal slices; 1 × 1 × 1 mm within a 256 × 240 matrix) were obtained in all participants. A multislice gradient echo echoplanar imaging sequence was used to acquire the BOLD signal (repetition time of 3,000 ms, echo spacing of 50 ms; 64 × 96 matrix; 33 axial slices, 3 × 3 × 2.5 mm, 1.25-mm gap; 50% phase oversampling in the phase encoding direction was applied to shift any Nyquist ghost artifact due to presence of the TMS coil outside the volume of interest) (16,17,20,21,42). Each participant completed five functional runs, each comprising 184 volumes, including 5 initial volumes discarded to allow for steady-state tissue magnetization.…”

“…For further details on the setup and implementation of TMS-fMRI in this scanning environment with similar parameters, see refs. 16,17,20,21, and 42.…”

“…After reconstruction, slices spoiled by TMS pulses (one slice per pulse; see above) were removed by means of an in-house routine, in which slices with a signal magnitude of >3 SD from the mean were replaced by the mean of the same slices from the preceding and proceeding volumes (17,21). Next, the first five volumes were discarded, and images were realigned to the middle of the series.…”

“…Applying TMS online during fMRI scanning has revealed blood oxygen level-dependent (BOLD) changes that occur not only at the site of stimulation, but also in anatomically remote but functionally coupled regions. Such observations suggest that TMS induces local activity, which propagates to areas communicating with the stimulated site in particular task conditions (15)(16)(17)(18). Moreover, such communication (or functional coupling) of the stimulated region with other brain areas can change in a dynamic fashion, thereby permitting assessment of task context-dependent activity propagation within brain networks (19).…”