Using simultaneous repetitive Transcranial Magnetic Stimulation/functional Near Infrared Spectroscopy (rTMS/fNIRS) to measure brain activation and connectivity

Abstract

Introduction: Simultaneously acquiring functional Near Infrared Spectroscopy (fNIRS) during Transcranial Magnetic Stimulation (rTMS) offers the possibility of directly investigating superficial cortical brain activation and connectivity. In addition, the effects of rTMS in distinct brain regions without quantifiable behavioral changes can be objectively measured.

Methods: Healthy, nonmedicated participants age 18-50 years were recruited from the local community. After written informed consent was obtained, the participants were screened to ensure that they met inclusion criteria. They underwent two visits of simultaneous rTMS/fNIRS separated by 2 to 3 days. In each visit, the motor cortex and subsequently the prefrontal cortex (5 cm anterior to the motor cortex) were stimulated (1 Hz, max 120% MT, 10 s on with 80 s off, for 15 trains) while simultaneous fNIRS data were acquired from the ipsilateral and contralateral brain regions.

Results: Twelve healthy volunteers were enrolled with one excluded prior to stimulation. The 11 participants studied (9 male) had a mean age of 31.8 (s.d. 10.2, range 20-49) years. There was no significant difference in fNIRS between Visit 1 and Visit 2. Stimulation of both the motor and prefrontal cortices resulted in a significant decrease in oxygenated hemoglobin (HbO(2)) concentration in both the ipsilateral and contralateral cortices. The ipsilateral and contralateral changes showed high temporal consistency.

Discussion: Simultaneous rTMS/fNIRS provides a reliable measure of regional cortical brain activation and connectivity that could be very useful in studying brain disorders as well as cortical changes induced by rTMS.

Figures

Figure 1. Location of the rTMS Coil and fNIRS Sources and Detectors

Location of the Transcranial Magnetic Stimulation coil (light grey figure 8) with respect to functional Near Infrared Spectroscopy (sources are circles and detectors are squares) on the participant’s head. The fNIRS probe comprised of 28 source-detector pairs (marked with solid lines between sources and detectors) in total with 14 source-detector pairs on each hemisphere.

Figure 2. Functional brain changes under the coil and in the contralateral region

Averaged time course of deoxygenated hemoglobin (Hb) and oxygenated hemoglobin (HbO2) signals for the motor cortex and the prefrontal cortex. Measurements were made under the coil and in the corresponding contralateral cortex. One hertz stimulation was performed for 10 seconds followed by 80 seconds of rest for 15 epochs. The x-axis represents the time from 0 to 90 seconds in the epoch, and the y-axis represents the mean and 95% confidence interval for hemoglobin (Hb or HbO2) concentration in μM/L. (A) is the ipsilateral and contralateral Motor Cortex and (B) is the ipsilateral and contralateral Prefrontal Cortex.

Figure 3. Spatial distribution of functional brain changes associated with stimulation from rTMS

Spatial distribution related to Transcranial Magnetic Stimulation of Motor and Prefrontal Cortex measured with functional Near Infrared Spectroscopy. Concentration changes in oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) were measured from the 6th to the 10th seconds of stimulation. The concentration changes of HbO2 and Hb were in μM/L with the color-coded scale to the right of the image. The x-axis and y-axis were in cm (the probe size was 20×6 cm2). PFC – prefrontal cortex, MC – motor cortex.