The use of non-invasive brain stimulation (NIBS) in human neuroscience is a means of providing causal evidence for a brain structure in a basic neurophysiological, behavioural or cognitive process. One challenge of such an approach is ensuring that the correct site is being stimulated in a group of subjects, where slight heterogeneity exists in brain anatomy. An additional challenge is ensuring that the same site is returned to as a target in the same subject across experimental/treatment sessions. The use of neuronavigation technology provides a means of addressing these two problems using an infrared camera in conjunction with an MRI scan to monitor coil position on a reconstruction of the participants head and/or brain.
Neuronavigation systems, such as Brainsight TMS Navigation, enable the position of a TMS coil to be tracked in real time. The position of the TMS coil can be tracked relative to a target or the position of the TMS coil can be tracked relative to an anatomical area of interest. Brainsight not only tracks the position of the coil but also the orientation of the coil handle relative to the hotspot where the magnetic field is greatest. The orientation of coil handle is a critical determinant of the neutrons that are recruited by the TMS pulse (Mills et al., 1992). As a result of this, any change in the orientation of the coil handle can affect the cortical response to TMS. Once a hot spot has been found, the orientation of the coil handle when such a hot spot was found can be recorded, ensuring that the coil orientation and hot spot position are kept constant throughout an experimental session.
Targets can be defined in two ways. One method is to identify a hot spot, such as a site most likely to evoke a motor evoked potential (MEP) or a phosphene, and then set this hot spot as a target. This will ensure that a location most likely to evoke a desired response is consistently stimulated in an experiment. An alternative method is to use a set of standardised co-ordinates, such as Montreal Neurological Institute (MNI) space, as a target. Such an approach is popular when sites derived from BOLD contrasts from fMRI are obtained for each participant, enabling subject-specific targets (e.g. Rahnev et al., 2016). Brainsight TMS Navigation can also be used to transform a subject’s MRI from native space to a standardised co-ordinate space. This transformation will identify the MNI co-ordinates within a participant’s MRI scan, enabling these MNI co-ordinates to be used a participant-specific basis.
A recent approach is to incorporate current flow modelling into NIBS paradigms, which reveals how the electric field induced by transcranial magnetic stimulation (TMS) or transcranial electric stimulation (tES) is shaped by how the cortex folds in proximity to the externally applied field (e.g. Opitz et al., 2011). Use of SimNIBS has successfully clarified the the regions that are most likely to be affected by a TMS pulse, and how stimulation of these regions affects behavioural phenomena (Thielscher et al., 2010). Similarly, current flow models have also been successful in verifying whether tES montages are affecting the cortex in the way that is expected (e.g. Saturnino et al., 2017). Such tools are now integrated with Brainsight TMS Navigation, enabling targets to be verified with current flow modelling prior to the participant arriving in the laboratory. This enables electric field strength to be examined whilst setting up a target to ensure that the TMS coil location is positioned in a way that maximises the induced electric field in a target of interest.
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