Magnetic-Stimulation-Related Physiological Artifacts in Hemodynamic Near-Infrared Spectroscopy Signals

Abstract

Hemodynamic responses evoked by transcranial magnetic stimulation (TMS) can be measured with near-infrared spectroscopy (NIRS). This study demonstrates that cerebral neuronal activity is not their sole contributor. We compared bilateral NIRS responses following brain stimulation to those from the shoulders evoked by shoulder stimulation and contrasted them with changes in circulatory parameters. The left primary motor cortex of ten subjects was stimulated with 8-s repetitive TMS trains at 0.5, 1, and 2 Hz at an intensity of 75% of the resting motor threshold. Hemoglobin concentration changes were measured with NIRS on the stimulated and contralateral hemispheres. The photoplethysmograph (PPG) amplitude and heart rate were recorded as well. The left shoulder of ten other subjects was stimulated with the same protocol while the hemoglobin concentration changes in both shoulders were measured. In addition to PPG amplitude and heart rate, the pulse transit time was recorded. The brain stimulation reduced the total hemoglobin concentration (HbT) on the stimulated and contralateral hemispheres. The shoulder stimulation reduced HbT on the stimulated shoulder but increased it contralaterally. The waveforms of the HbT responses on the stimulated hemisphere and shoulder correlated strongly with each other (r = 0.65–0.87). All circulatory parameters were also affected. The results suggest that the TMS-evoked NIRS signal includes components that do not result directly from cerebral neuronal activity. These components arise from local effects of TMS on the vasculature. Also global circulatory effects due to arousal may affect the responses. Thus, studies involving TMS-evoked NIRS responses should be carefully controlled for physiological artifacts and effective artifact removal methods are needed to draw inferences about TMS-evoked brain activity.

Non-invasive brain stimulation and the autonomic nervous system

“…In addition, the ANS has been increasingly used as an outcome measure in NIBS studies in order to understand the broad effects of these techniques including their safety profile. However, while there is clear evidence of autonomic effects of brain stimulation on animals (Yasui et al, 1991;Sequeira et al, 1995;Tavares et al, 2004), recent articles found conflicting results on the relationship of NIBS and ANS (Lai et al, 2010;Näsi et al, 2011;Vandermeeren et al, 2010;Brunoni et al, 2013). Therefore, it is important to consider the two-way relationship between central modulation and ANS function that can be used to explore both NIBS mechanisms and ANS function.…”

Concurrent application of TMS and near-infrared optical imaging: methodological considerations and potential artifacts

“…A second approach for designing a concurrent TMS-optical montage is to reorient optodes so that they may be mounted in a low-profile optical patch directly beneath a TMS coil (Figure 2B). Creating a low-profile optical montage can be accomplished either by bending optodes, reorienting their tips at a perpendicular angle (Figure 2C; Noguchi et al, 2003; Mochizuki et al, 2006) or by using prisms to redirect light at a perpendicular angle (Figure 2C; Näsi et al, 2011; Parks et al, 2012). The advantage of the low-profile approach is that a montage may be designed with freedom, arranging optodes in any desired configuration.…”

“…However, TMS induces electrical currents not only in the neural tissue of the cerebral cortex but also in superficial tissue, musculature, and nerves of the scalp and skull. Thus, TMS-induced changes in near-infrared absorption and scattering may also occur in superficial tissue, including changes in superficial blood flow and oxygenation (Figure 3C) (Näsi et al, 2011). Such superficial tissue artifacts , if present, are not easily disentangled from cortical activations and present the greatest challenge for combined TMS and optical imaging experiments (Näsi et al, 2011).…”

“…Though short channels provide no measure of neural activity, they form a powerful method for identifying TMS-related artifacts. This is accomplished by using short channels to yield a measure of near-infrared absorption and scattering that is confined to superficial tissue of the scalp and skull (Firbank et al, 1998; Medvedev et al, 2008; Näsi et al, 2011; Takahashi et al, 2011; Parks et al, 2012; Fabiani et al, 2013). The presence of electromagnetic, movement, and superficial tissue artifacts will be apparent in these channels.…”

“…The presence of electromagnetic, movement, and superficial tissue artifacts will be apparent in these channels. The absence (or presence) of effects in short-distance channels as compared to long-distance channels can be used to eliminate (or confirm) the contribution of artifacts as an explanation of observed TMS-evoked fNIRS or EROS activations (Näsi et al, 2011; Parks et al, 2012). Presently, short channels provide the only means of evaluating the contribution of superficial tissue artifacts.…”