Nonphysiological factors in navigated TMS studies; Confounding covariates and valid intracortical estimates

Abstract

It is necessary to partition physical and physiological variance in TMS studies to make confounded data interpretable. The spatial resolution of nTMS is <5 mm and the EF-estimates are valid.

A low-cost system for coil tracking during transcranial magnetic stimulation

“…During a TMS session where the user placed the coil over the scalp using a virtual interface that displayed the system’s output (x, y, z, roll, pitch, and yaw) as feedback, we were able to locate the coil over the target area with high accuracy, which was verified using a commercial 3D camera system. The amount of error seen while using our system was similar to that seen in the literature for other frameless stereotaxic systems, which are estimated to be up to 5mm (Forster et al, 2014; Julkunen et al, 2009; Schmidt et al, 2015; Sparing et al, 2008). Additionally, we found that when using our low-cost system during TMS on human subjects, the measured MEPs were larger than those collected by placing the coil without any feedback (non-navigated).…”

“…It has been recognized as an effective tool for quantifying corticospinal excitability in cognitive and clinical neuroscience using single or dual pulse techniques (Bestmann and Krakauer, 2015; Chen et al, 2008; Edwards et al, 2013), and for neurorehabilitation of many neuromuscular and psychiatric disorders by means of repetitive TMS (Hallett, 2007; Hoogendam et al, 2010; Levkovitz et al, 2015; Saba et al, 2015; Sun et al, 2012; Wang et al, 2014). However, outcomes of TMS experiments can be confounded by the accuracy of coil positioning over the target area (Bestmann and Krakauer, 2015; Burke et al, 1995; Kiers et al, 1993; Schmidt et al, 2009), as slight changes in stimulator coil location/orientation could alter current flow across neurons (Di Lazzaro et al, 2001; Schmidt et al, 2015; Werhahn et al, 1994). …”

“…Frameless stereotaxic systems (FSS) exist to track coil position/orientation and locate the target area to deliver stimulations during TMS with high accuracy (Cincotta et al, 2010; Fleming et al, 2012; Freundlieb et al, 2015; Julkunen et al, 2009; Schmidt et al, 2015). FSS can also sync patient-specific or generic magnetic resonance imaging (MRI) data for stereotaxic placement of the TMS coil (Andoh et al, 2009; Cancelli et al, 2015; Schmidt et al, 2010).…”

“…These systems find the target area relative to brain cortex geography or scalp, and remember the exact location for repeatability (Forster et al, 2014; Peterchev et al, 2012). Some systems are also capable of providing estimates of electric field gradients on the cortex (Schmidt et al, 2015). However, they are expensive and not feasible for all TMS users.…”

TMS-EEG: first steps towards a clinical application in epilepsy

“…Currently, increasingly more attention is being paid to assessing the variability and repeatability in TMS studies (Beaulieu et al 2017;Goldsworthy et al 2016;Guerra et al 2017;Schmidt et al 2015), which is needed to establish the reliability of these excitability measures. The large inter-individual variation seems to be a common aspect in both TMS-EMG and TMS-EEG that hampers not only the clinical use in epilepsy, but also its potential application in other diseases and neuroscience research in general.…”