Abstract
Applying a precisely timed pulse of transcranial magnetic stimulation (TMS) over the right posterior parietal cortex (PPC) can produce temporary visuo-spatial neglect-like effects. Although the TMS is applied over PPC, it is not clear what other brain regions are involved. We applied TMS within a functional magnetic resonance imaging (fMRI) scanner to investigate brain activity during TMS induction of neglect-like bias in three healthy volunteers, while they performed a line bisection judgment task (i.e., the landmark task). Single-pulse TMS at 115% of motor threshold was applied 150 ms after the visual stimulus onset. Participants completed two different TMS/fMRI sessions while performing this task: one session while single-pulse TMS was intermittently and time-locked applied to the right PPC and a control session with TMS positioned over the vertex. Perceptual rightward bias was observed when TMS was delivered over the right PPC. During neglect-like behavior, the fMRI maps showed decreased neural activity within parieto-frontal areas, which are often lesioned or dysfunctional in patients with left neglect. Vertex TMS induced behavioral effects compatible with leftward response bias and increased BOLD signal in the left caudate (a site which has been linked to response bias). These results are discussed in relation to recent findings on neural networks subserving attention in space.
Highlights
In cognitive neuroscience, the non-invasive technique of transcranial magnetic stimulation (TMS) can be used in healthy participants to temporarily disrupt the activity of a focal brain region and test its function (Pascual-Leone et al, 2000)
Within each task instruction, the participants showed a tendency to underestimate the left segment during active TMS with respect to baseline trials
For the posterior parietal cortex (PPC) condition during task execution (Figure 4C and Table 3), significant TMS-induced BOLD signal decrease was found in the right angular gyrus (AG), supramarginal gyrus (SMG), inferior parietal lobule (IPL), supplementary motor area (SMA), MFG, and superior frontal gyrus (SFG)
Summary
The non-invasive technique of transcranial magnetic stimulation (TMS) can be used in healthy participants to temporarily disrupt the activity of a focal brain region and test its function (Pascual-Leone et al, 2000). TMS delivered over the right PPC 150 ms after the visual stimulus onset induced transitory rightward neglect, while right frontal TMS at these intervals did not affect the participants’ behavior In this preliminary study, we used a single-pulse TMS protocol very similar to the one used by Fierro et al (2001) in three healthy participants, to test the feasibility of applying TMS within an MRI scanner to measure the patterns of brain activity that occur during induction of a visuo-spatial bias. In Heinen and colleagues’ study (Heinen et al, 2011) TMS of the right angular gyrus (AG) facilitated reorienting to invalidly cued right visual targets, during an exogenously cued visuo-spatial attention task, and enhanced BOLD signal in the left AG and left retinotopic cortex In this initial study we used the interleaved TMS/fMRI technique in three healthy volunteers to directly investigate the specific impact of right PPC single-pulse TMS on brain activity changes during induction of neglect-like attentional rightward bias. Results from this pilot study may lay the groundwork for follow up investigations to determine brain activation changes occurring during induction of TMS behavioral effects in relation to other visuo-spatial tasks or brain areas thought to be involved in the neural circuitry of spatial attention and representation
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