A Validation Study of the Use of Near-Infrared Spectroscopy Imaging in Primary and Secondary Motor Areas of the Human Brain

YEBEH-04302; No of Pages 8 Epilepsy & Behavior xxx (2015) xxx–xxx

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A validation study of the use of near-infrared spectroscopy imaging in primary and secondary motor areas of the human brain

Christoph Drenckhahn a,b,d, Stefan P. Koch d,JohannesDümmlere, Matthias Kohl-Bareis f, Jens Steinbrink a,d, Jens P. Dreier a,b,c,d,⁎ a Center for Stroke Research Berlin, Charité University Medicine Berlin, Berlin, Germany b Department of Neurology, Charité University Medicine Berlin, Berlin, Germany c Department of Experimental Neurology, Charité University Medicine Berlin, Berlin, Germany d Berlin Neuroimaging Center, Charité University Medicine Berlin, Berlin, Germany e Department of Anaesthesiology and Intensive Care Medicine, Christian-Albrechts University, Kiel, Germany f RheinAhrCampus, University of Applied Sciences Koblenz, Remagen, Germany article info abstract

Article history: The electroencephalographically measured Bereitschafts (readiness)-potential in the supplementary motor area Accepted 3 April 2015 (SMA) serves as a signature of the preparation of motor activity. Using a multichannel, noninvasive near-infrared Available online xxxx spectroscopy (NIRS) imager, we studied the vascular correlate of the readiness potential. Sixteen healthy subjects performed a self-paced or externally triggered motor task in a single or repetitive Keywords: pattern, while NIRS simultaneously recorded the task-related responses of deoxygenated hemoglobin (HbR) in Human motor system Functional activation the primary motor area (M1) and the SMA. fi Primary motor area Right-hand movements in the repetitive sequence trial elicited a signi cantly greater HbR response in both the Supplementary motor area SMA and the left M1 compared to left-hand movements. During the single sequence condition, the HbR response Near-infrared spectroscopy in the SMA, but not in the M1, was significantly greater for self-paced than for externally cued movements. None- Spreading depolarization theless, an unequivocal temporal delay was not found between the SMA and M1. Spreading depression Near-infrared spectroscopy is a promising, noninvasive bedside tool for the neuromonitoring of epileptic seizures Epilepsy or cortical spreading depolarizations (CSDs) in patients with epilepsy, stroke, or brain trauma because these pathological events are associated with typical spatial and temporal changes in HbR. Propagation is a character- istic feature of these events which importantly supports their identification and characterization in invasive recordings. Unfortunately, the present noninvasive study failed to show a temporal delay during self-paced movements between the SMA and M1 as a vascular correlate of the readiness potential. Although this result does not exclude, in principle, the possibility that scalp-NIRS can detect a temporal delay between different regions during epileptic seizures or CSDs, it strongly suggests that further technological development of NIRS should focus on both improved spatial and temporal resolution.

This article is part of a Special Issue entitled Status Epilepticus. © 2015 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction for up to 2 s [1]. This phenomenon was termed Bereitschaftspotential or readiness potential and is assumed to be the electrophysiological corre- In 1965, Kornhuber and Deecke discovered a slow negative electro- late of increased neural activity related to readiness, preparation, and encephalography (EEG) activity that preceded self-initiated movement execution of movement. In support of this hypothesis, the duration of the Bereitschaftspotential is reduced when the movement is externally Abbreviations: BOLD, blood-oxygen-level-dependent; CSD, cortical spreading cued [1,2]. The Bereitschaftspotential is subclassified into two compo- depolarization; DC-/AC-EEG, direct current-/alternating current-electroencephalography; fi fMRI, functional magnetic resonance imaging; HbO, oxygenated hemoglobin; HbR, nents. The rst component (Bereitschaftspotential 1, BP1, or readiness deoxygenated hemoglobin; M1, primary motor cortex; NIRS, near-infrared spectroscopy; field 1) is recorded with maximum amplitude over the midline vertex PET, positron emission tomography; rCBF, regional cerebral blood flow; ROI, region of area 2 s prior to movement onset and is assumed to originate from fron- interest; SD, standard deviation; SMA, supplementary motor area. tal medial wall areas including the supplementary motor area (SMA). ⁎ Corresponding author at: Center for Stroke Research Berlin, Campus Charité Mitte, Its amplitude correlates positively with movement complexity and Charité University Medicine Berlin, Charitéplatz 1, 10117 Berlin, Germany. Tel.: +49 30 – 450 660024; fax: +49 30 450 560932. bimanual synchronization [3 8]. About 500 ms prior to movement, E-mail address: [email protected] (J.P. Dreier). the activity becomes lateralized contralateral to the movement, which

http://dx.doi.org/10.1016/j.yebeh.2015.04.006 1525-5050/© 2015 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article as: Drenckhahn C, et al, A validation study of the use of near-infrared spectroscopy imaging in primary and secondary motor areas of the human brain, Epilepsy Behav (2015), http://dx.doi.org/10.1016/j.yebeh.2015.04.006 2 C. Drenckhahn et al. / Epilepsy & Behavior xxx (2015) xxx–xxx reflects activation of the primary motor cortex (M1). This lateralization study was approved by the Ethics Committee of the Charité, is termed Bereitschaftspotential 2, BP2, or readiness field 2 [2,9–12].The Universitätsmedizin Berlin. two components of the readiness potential demonstrate the hierarchy of the motor system, with the activation of the SMA prior to the 2.2. Experimental setup and paradigm activation of the M1, and indicate that the M1 is regulated by higher order motor areas. These EEG findings correspond with whole-scalp The investigations were performed in a dark and quiet room. Partic- magnetoencephalographic observations [13]. ipants were asked to lie comfortably in a supine position, to keep their Similarly, functional MRI (fMRI) studies demonstrated that chang- eyes closed, to breathe smoothly, and to avoid any noninstructed move- es of the blood-oxygen-level-dependent (BOLD) signal in the SMA ments. This procedure minimized fluctuations of systemic parameters precede BOLD changes in the M1 during voluntary finger movement. like blood pressure, heart rate, respiration, and galvanic skin responses For instance, Wildgruber et al. [14] found a latency of 0.8 s of the that might affect the NIRS signal [29]. In particular, participants were SMA activity prior to the M1 activity. In another study, Weilke et al. instructed to avoid head and jaw movements because these movements [15] investigated the time course of the BOLD signal referring to are generated in cortical subfields of the M1 close to the cortical subfield the (rostral) pre-SMA, (caudal) SMA proper, as well as the M1 and which generates movements of the upper limb. All experiments were demonstrated nearly constant temporal delays of 0.8 s between the carried out in one session without any changes in optode position, SMA proper and M1, and 2.0 s between the pre-SMA and M1 at half- subject position, or experimental setup. maximum activation during self-paced finger tapping. In a series of Each subject participated in four experiments, consisting of a sequen- studies, Cunnington and colleagues showed that the BOLD response tial finger-to-thumb opposition task performed either in a singular or in the SMA preceded that in the M1 for both self-initiated and exter- repetitive pattern and either in a self-paced or externally triggered nally cued activation and demonstrated that the hemodynamic delay manner. Externally triggered movements were cued by a 600-millisecond varied with the type of movements [16–18].Thesefindings were vibrotactile stimulus applied by a vibration device to the left lower leg. confirmed by a study using simultaneous measurements of fMRI and This position was chosen because the primary somatosensory representa- high-density EEG [11]. tion area of the left lower leg is located outside of the cortical area where Anatomically, the SMA and M1 are partly located on the convexity of the NIRS signal was recorded. both cerebral hemispheres, which allows investigation of their hemody- In order to activate both primary and higher-order motor areas, par- namic responses with good temporal resolution using scalp-near- ticipants performed a sequence of complex finger-to-thumb opposi- infrared spectroscopy (NIRS) [19–22]. Near-infrared spectroscopy, first tions (2-3-2-4-2-5-2-4-2-5-2-4-2-3-2-4; 2 — index finger, 3 — middle described by Jöbsis in 1977, is an optically based technique which finger, 4 — ring finger, 5 — little finger) either with one hand or, syn- captures alterations of oxygenated (HbO) and deoxygenated (HbR) he- chronously, with both hands. A complex sequence of finger-to-thumb moglobin concentrations related to changes in regional cerebral blood oppositions was chosen because cortical motor areas are more active flow (rCBF). Because near-infrared light penetrates human tissue rather during complex finger movement than during simple finger movement well, NIRS allows us to investigate task-related local oxygenation chang- tasks [7,8]. Prior to the experiment, participants practiced the finger tap- es in the human cortex through the intact skull [19,23,24]. Functional ping task until they were able to accomplish one cycle of the tapping se- cerebral activation leads to a focal increase in total hemoglobin (Hbtot) quence in 5 s. Each subject performed four experimental manipulations: and HbO, a decrease in HbR in correlation with increased rCBF, and a disproportion between blood perfusion and metabolism in functionally Experiment 1 Externally triggered, single sequence, both hands synchro- activated, i.e., neurally engaged, brain areas [19,21,25–27].Boththe nously measured NIRS signal and the fMRI-BOLD signal depend largely on the In response to the external trigger, subjects had to HbR parameter and are therefore capable of indirectly detecting regions execute the sequence of finger-to-thumb opposition of changed neural activity. movements simultaneously with both hands. This was The present study focused on the temporal pattern of hemodynamic repeated 80 times, interspersed with variable intertrial responses in the M1 and SMA during different movement execution intervals (8–15 s) without movement. tasks (self-guided vs. externally triggered and single sequence vs. mul- Experiment 2 Self-paced, single sequence, both hands synchronously tiple sequences). The following hypotheses were tested by means of Similar to experiment 1, the condition consisted of NIRS: (i) NIRS imaging is a suitable tool for identifying and differentiat- 80 repetitions of single finger-to-thumb opposition se- ing the cortical areas of the primary and secondary motor systems quences. No external trigger was applied, forcing the par- involved in voluntary movements; (ii) compared to externally cued ticipants to perform the task in a self-paced manner. movements, self-paced movements lead to stronger hemodynamic Participants were requested to vary the intertrial interval responses in the M1 and SMA; and (iii) as a vascular correlate of the in analogy to experiment 1. readiness potential, the onset of the hemodynamic response in the Experiment 3 Externally triggered, repetitive sequence vs. rest, right hand SMA precedes the response in the M1. In this condition, participants alternated 23 times between a 20-second finger-to-thumb opposition sequence and a 2. Material and method 20-second resting period with the right hand. The onset of the alterations was indicated by the vibration stimulus. 2.1. Participants Experiment 4 Externally triggered, repetitive sequence vs. rest, left hand Experiment 4 was identical to experiment 3, except that Seven male and 9 female healthy volunteers aged 20–31 years the movement was performed with the left hand. (mean: 24.7 years, SD ± 2.96 years) participated in this study. Research consents were obtained. None of the subjects suffered from migraine or 2.3. Near-infrared spectrometer any other neurological or vascular disease, and none showed evidence of an idiopathic vascular disease in the family. All participants were A NIRS imaging system was developed comprising 16 laser diodes strongly right-handed according to the Edinburgh handedness invento- and 8 avalanche photon detector modules. To optimize light intensity, ry [28]. Six of them were moderate smokers and were asked not to 16 laser drivers (based on a chip by IC-Haus GmbH, Bodenheim, smoke at least 3 h before their scheduled arrival time at the laboratory. Germany) allowed fast switching with rise and fall times of ≈3 μs. All research was conducted in accordance with the Declaration of Hel- Eight laser diodes at 760 nm (RLT7605MG, Roithner Lasertechnik, sinki. Informed consent was obtained from all participants, and the 5 mW, Vienna, Austria) and 850 nm (RLT8505MG, Roithner Lasertechnik,

5 mW) were used. The wavelengths were optimized for an assessment allowing simultaneous measurements of task-related oxygenation of Hb concentration changes [30]. Eight avalanche photon detector changes in both areas [34–38]. modules (C5460-01, Hamamatsu Photonics, Hamamatsu, Japan) served as detectors. The output voltage of these modules was further amplified 2.4. Detection of movement onset with a gain between 1 and 15 and a time constant set to 2 ms. The instrument was controlled by software written in Labview (National To detect onset and duration of movements, Ag/AgCl electrodes Instruments, Munich, Germany) and a PCMCIA data acquisition card were fixed above the flexors of the right and left forearm. The (DAQCard 6036E, National Instruments) for both laser control and compound muscle action potential (M-response) was registered using detector signal acquisition with a dynamic range of 16 bit. a BrainAmp DC amplifier (BrainProducts, Gilching, Germany) with a The lasers were switched on and off sequentially with 2 or 3 lasers sampling rate of 1000 Hz. running at the same time. The duration of one full recording cycle was 290 ms, corresponding to a sampling rate of ~3.45 Hz. Keyboard marker 2.5. Analysis of HbR response and additional analog input and digital input–output channels were used for timing and monitoring purposes. Optical bundles with fibers The four datasets were analyzed for temporal dynamics, amplitude, of 50 μm in core diameter (Loptec GmbH, Berlin, Germany) delivered and location of significant event-related changes in HbR concentration. the light to the subject's head. For efficient coupling, two laser sources We chose HbR as an analytical parameter since a recent study stressed at both wavelengths were coupled into bifurcated bundles of 1 mm in the strong confound caused by systemic effects in the HbO signal [39]. diameter. The light reflected from the head was collected by bundles Prior to this analysis, the raw HbR time course for each subject was of 3 mm in diameter. These 16 optodes were positioned as shown in low-pass filtered at 0.5 Hz to reduce the pulsatile signal component, Fig. 1 to form 22 source–detector pairs. The distance between neighbor- while a 0.02 Hz high-pass filter attenuated slow-signal drifts. The imaging optodes was 2.5 cm. The detector signals were read at a frequency of ing analysis consisted of two chronological steps: (i) identification of 2 kHz, averaged by software over the time of one laser switching se- the regions of interest for the SMA (SMA-ROI) and M1 (M1-ROI), and quence, and stored to disk. Intensity data at both wavelengths (λ1 = (ii) calculation of the mean time course for the concentration changes 760 nm, λ2 = 850 nm) at each measurement position were converted in HbR. into attenuation changes ΔΑ(λ) and, subsequently, into changes of HbO and HbR concentration ΔHbO and ΔHbR, based on a modified (i) Identification of the SMA-ROI and M1-ROI: Concentration changes Beer–Lambert law. This conversion procedure took into account the were analyzed using a General Linear Model approach similar to extinction spectra ε(λ) of HbO and HbR as well as the wavelength that used for fMRI [40]. A gamma function [41] with a peak at 5 s dependence of the optical path length Da(λ) [31,32]: was used as a model for the hemodynamic response function. By thresholding the t-maps at t N 3.17 (p b 0.05, Bonferonni- Δ ðÞ¼½ε ðÞΔ þ ε ðÞΔ ðÞ A 760 nm HbO 760 nm cHbO HbR 760 nm cHbR Da 760 nm ð Þ corrected for multiple comparisons, one-sided), the SMA-ROI Δ ðÞ¼½ε ðÞΔ þ ε ðÞΔ ðÞ: 1 A 850 nm HbO 850 nm cHbO HbR 850 nm cHbR Da 850 nm and M1-ROI were located within the reference frame of the imaging pad. This equation can be solved to obtain the requested concentration (ii) Analysis of time course: For each subject, trials of each condition changes. were first temporally segmented: For the single sequence condi- Postprocessing of data was performed in Matlab (The Mathworks, tions (experiments 1–2), we used a temporal window from −2s Natick, MA, USA) and consisted of background light subtraction, drift to 15 s relative to the onset of the movement (t = 0 s), while for reduction by high pass filtering (0.02 Hz), and low pass filtering at the repetitive sequence conditions (experiments 3–4), the time- 0.5 Hz to reduce the pulsation artifact. range was from −10 s to 30 s. For the repetitive sequence trials The NIRS grid covered an area of 12.5 × 7.5 cm (Fig. 1) and en- (experiments 3–4), the course of HbR was averaged, and the am- compassed the EEG electrode positions C3 and FCz of the international plitude of the baseline (mean of the −4-second to −2-second 10–20 EEG system [33]. These electrode positions served as anatomical window relative to movement onset) was subtracted. For the references for the hand area of the left M1 (C3) and the SMA (FCz), single sequence trials (experiments 1–2), the HbR concentration

Fig. 1. Localization of the NIRS pad and laser optodes. The lasers and detectors are arranged in an alternating manner with an interoptode distance of 25 mm. The pad covers an area of 12.5 × 7.5 cm including the positions C3 and FCz according to the international 10–20 EEG system. These positions serve as anatomical markers related to the hand area of the left M1 (C3) and the SMA (FCz).

at the onset of HbR decrease in the −2-second to 4-second win- experiments. The average reaction time during the externally triggered dow relative to movement onset was set to baseline. To obtain a single trial experiments was 650 ms (SD ± 177 ms). measure for the temporal hemodynamic delay between the SMA and M1, nadir detection (minimum in the 4–8-second range) 3.3. Amplitudes was performed on the averaged trials of the single sequence conditions. The motor-related changes in HbR were determined as the difference between HbR concentration during rest (onset of functional HbR Paired two-tailed t-tests were applied to compare the occurrence of response) and the nadir response during movement (Fig. 3). The mean a hypothesized time delay in HbR with respect to SMA-ROI and M1-ROI nadir amplitudes during self-paced tapping were −6.09 × 10−5 mol/l during self-initiated and externally cued movements. A p-value of b0.05 (SD ± 3.42 × 10−5 mol/l) in the M1 and −5.98 × 10−5 mol/l (SD ± was regarded significant. 2.47 × 10−5 mol/l) in the SMA. The analogous nadir amplitudes during externally triggered tapping were − 5.88 × 10−5 mol/l 3. Results (SD ± 1.27 × 10−5 mol/l) in the M1 and − 5.03 × 10−5 mol/l (SD ± 1.87 × 10−5 mol/l) in the SMA. While self-paced movements 3.1. Localizations of hemodynamic alterations evoked a stronger hemodynamic response compared to externally triggered movements for SMA (Δ =0.95,p =0.03),nosignificant Twelve of the 16 subjects showed a significant HbR decrease following self vs. cued differences were observed for the M1 between these two conditions the finger-to-thumb task in all experiments. Four subjects showed no (Δ =0.21,p = 0.82; see Table 1). event-related hemodynamic response in one or more of the 4 experiments self vs. cued and their datasets were excluded from subsequent analysis. In each of the subjects included, we detected a decrease in HbR at the caudal-posterior 3.4. Time course site of the NIRS pad corresponding to the supposed location of the hand and finger area of the primary motor cortex. Likewise, a second area with Fig. 3 depicts the averaged HbR time courses in the M1 and SMA decreased HbR was found at the anterior site of the NIRS pad above the across 12 subjects during self-initiated and externally triggered finger vertex corresponding to the location of the SMA. These findings were tapping. interpreted as a focal hemodynamic response to functional activation of Regarding temporal progression of the hemodynamic response, we the primary motor cortex and the SMA. In most cases, the region of HbR found differences in the onset of the HbR response between self-paced reduction exceeded the range of the NIRS pad, precluding determination and externally triggered finger tapping. For self-paced finger tapping, of the total expanse of the task-related hemodynamic changes. The region the onset of hemodynamic response in the SMA was detected 0.17 s of decrease in HbR corresponding to the SMA tended to show a more (SD ± 1.06 s) prior to movement onset, and the M1 revealed a tempo- diffuse location, i.e., data revealed a greater variety with respect to rally delayed onset, occurring 0.31 s (SD ± 0.75 s) after initiation of the location of the most significant activation. the movement. Fig. 2 illustrates the location of HbR alterations in a single subject For the externally triggered movement, HbR started to decrease at following the self-paced single sequence trial (experiment 2). We did 2.12 s (SD ± 1.02 s) and 1.83 s (SD ± 0.59 s) in the SMA and M1, respec- not perform a grand average of HbR changes across subjects because tively. This temporal asynchrony between the onset of hemodynamic of interindividual variations of the activation foci as result of the poor response in the SMA and in the M1 of 2.39 s and 1.52 s, respectively, spatial resolution of NIRS and interindividual variations in the brain was statistically significant (p = 0.02 (SMA) and p = 0.03 (M1)). anatomy, cortical activation pattern, craniocerebral correlation, and Mean nadir time of HbR for self-paced movements was 5.53 s (SD ± placement of the NIRS pad. 1.17 s) and 5.31 s (SD ± 1.02 s) for the SMA and M1, respectively. For externally triggered movements, we observed a mean nadir time of 3.2. Single trial experiments (experiments 1 and 2; n = 12 for 6.79 s (SD ± 1.24 s) for the SMA and 5.94 s (SD ± 0.91 s) for the M1. each experiment) Similar to the onset time, the difference in nadir time between self- paced and externally triggered finger tapping was significant (SMA: la- The average duration of the tapping sequence measured with tency 1.26 s, p = 0.01, M1: latency 0.63 s, p = 0.03). For the latency electromyography (EMG) amounted to 3.9 s (SD ± 1.2 s) in both between SMA and M1 activity within the same trial condition (self-

Fig. 2. Distribution of HbR changes elicited by functional activation of the motor cortex to the self-paced single movement sequence (experiment 2) in a single subject.

Fig. 4. HbR time course in the SMA and M1 to repetitive right-hand and left-hand Fig. 3. Time courses of HbR following self-paced and externally cued single sequence movements. The movement was performed for 20 s (see “movement” bar in the figure) movements in the SMA and M1. The hemodynamic responses in both the SMA and followed by a resting period of another 20 s (20–30 s and −10–0 s in the figure). To illus- the M1 start significantly earlier following self-paced compared to externally cued trate the HbR decrease, the values for HbR of the last 3 s of the resting period were aver- movements, but a significant difference between the SMA and M1 was not detected. For aged and set as the baseline. A significant decrease in HbR with a plateau phase was both conditions, the HbR decrease is slightly steeper in the M1 than in the SMA. observed in both the SMA and the M1 during both conditions. The nadir amplitudes in the M1 and SMA were significantly smaller during left-hand movements than during right-hand movements. paced or externally triggered), no significant difference in hemodynamic responses was observed. to 18 s after movement onset) was averaged. The plateau amplitudes Taken together, the activation of the primary and secondary motor during right-hand repetitive tapping were − 8.46 × 10−5 mol/l systems started significantly earlier during self-initiated than during (SD ± 3.1 × 10−6 mol/l) in the M1 and −6.88 × 10−5 mol/l (SD ± externally triggered movements. As illustrated in Fig. 3, the onset of 4.5 × 10−6 mol/l) in the SMA. The plateau amplitudes during left-hand the HbR drop seemed to start earlier in the SMA following self- repetitive tapping were −2.53 × 10−5 mol/l (SD ± 4.1 × 10−6 mol/l) initiated movement, whereas it was the other way round following in the M1 and −4.47 × 10−5 mol/l (SD ± 0.2 × 10−6 mol/l) in the externally triggered movements. However, it was somewhat subjective SMA. In the left (contralateral) M1 (p b 0.01) and SMA (p b 0.01), the to determine the exact onset of the HbR drop because of the limited oxygenation response was significantly stronger during right-hand temporal resolution and sampling rate of the applied NIRS system as tapping than during left-hand tapping. The results are summarized in well as strong spontaneous fluctuations and basic noise of the HbR Table 1. signal. During both self-initiated and externally triggered movements, HbR fell more steeply in the M1 than in the SMA. 4. Discussion

3.5. Repetitive trial experiments (experiments 3 and 4; n = 12 for Anatomical, physiological, and functional studies have demonstrat- each experiment) ed that the motor system, while extremely complex in primates and humans, nonetheless, can be roughly classified into a primary motor 3.5.1. Amplitudes area and several secondary motor areas [42–49]. The primary motor For the repetition of the finger tapping sequence, the averaged HbR area (M1) comprises the pyramidal cell of Brodmann area 4, ending at in the M1 and SMA regions reached a plateau phase after ~5–7 s that the second motor neuron on the spinal level, and represents the main lasted for the whole movement period (Fig. 4). Significance was found component of the corticospinal tract. The secondary or higher-order for the M1 and SMA in both conditions (all p-values b 0.01) during the motor system includes the SMA [50,51]. Higher-order motor areas movement period compared to baseline. To determine the amplitude have been related to learning, reaction, planning, preparation, readi- of the HbR response, a period of 10 s during the plateau phase (8 s ness, and initiation of voluntary and goal-directed movements

Table 1 The nadir amplitude of HbR decrease shows signiﬁcance for externally cued (experiment 1) compared to self-paced (experiment 2) movements in the SMA but not in the M1 during the single sequence condition. The nadir amplitude in both SMA and left M1 was signiﬁcantly lower during right-hand (experiment 3) than during left-hand (experiment 4) movements following the repetitive sequence trial.

Peak of Hbdeoxy decrease (single and repetitive trials) SMA −5 Single trial [10 mol/l] Self-paced ±SD Externally cued ±SD Δself-cued −5.98 2.47 −5.03 1.87 0.95 (p = 0.03) −5 Repetitive trial [10 mol/l] Right vs. rest ±SD Left vs. rest ±SD Δself-cued −6.88 4.5 −4.47 0.2 2.41 (p b 0.01)

M1 −5 Single trial [10 mol/l] Self-paced ±SD Externally cued ±SD Δself-cued −6.09 3.42 −5.88 1.27 0.21 (p = 0.82) −5 Repetitive trial [10 mol/l] Right vs. rest ±SD Left vs. rest ±SD Δself-cued −8.46 3.1 −2.53 4.1 5.93 (p b 0.01)

[52–54]. The SMA is involved in planning and harmonizing unilateral spatially separated cortical regions that are accessible to scalp record- [55] and bilateral movements [56] and plays a crucial role in the syn- ings with NIRS. Prior MRI studies had revealed a decrease of the BOLD chronization of bimanual movements [57], performance of movement signal in the SMA preceding the M1 following self-paced voluntary fin- sequences [57–59], and complex motor actions [7,8,60]. Moreover, the ger movements which was interpreted as the vascular correlate of the SMA is involved in motor learning, memory-derived motor actions readiness potential. This delay in HbR response between SMA and M1 [61–65], and mental imagery of movements [66–68]. was not reproduced in our NIRS study. In other words, in principle, We performed a scalp-NIRS study in healthy adult humans to investi- fMRI-BOLD and scalp-NIRS-HbR response showed a good spatial [73] gate the Hb oxygenation changes in the M1 and SMA during finger move- and temporal [74] correlation in previous studies. Nonetheless, our ments. Self-paced single sequence finger tapping resulted in a decrease of results suggest that the ability of scalp-NIRS is markedly lower than HbR in the SMA that did not precede the decrease of HbR in the M1 in a that of fMRI to resolve the spatial variability of cerebral activation in statistically significant fashion. In other words, the observed temporal the temporal dimension, although both methods have a similar origin pattern did not correspond sufficiently with previous EEG and magneto- of their underlying signal [75]. encephalography (MEG) studies of the readiness potential [1,13,69]. Several reasons might account for these incongruent findings: The This lack of a clear temporal delay between the hemodynamic responses fMRI study of Weilke et al. [15] only showed a significant difference in the SMA and M1 is not consistent with fMRI results on the preparation for the rostral SMA and M1 but not for the caudal SMA and M1. Regard- and execution of voluntary movements reported by Cunnington et al. ing the location of the NIRS pad in our study, it is possible that only the [17]. In 4 out of 5 subjects, Cunnington and colleagues demonstrated caudal, but not the rostral, SMA was covered by the optodes. Moreover, that the BOLD response in SMA and cingulate motor areas preceded because of the poor spatial resolution of the NIRS setup, our study did that in the primary motor area by 0.40 to 1.16 s. not differentiate between the SMA of the left and right hemispheres or Also the latency between movement onset and nadir during self- between subareas of the SMA, nor did it allow us to distinguish the paced finger movements seems to vary between different neuroimag- SMA from neighboring cortical areas. Hence, it is conceivable that the ing studies. Our NIRS study showed a peak-activation in the M1 after signals originating from the SMA region are influenced by other second- movement onset at 5.31 s, whereas, in two fMRI studies, the BOLD signal ary motor areas such as the premotor cortex. This could have blurred showed a peak activation at ~3.8 s [14] and 4.8 s [17], respectively. Aside the SMA response. Another critical issue might be the small interoptode from the different methods (scalp-NIRS or fMRI), various aspects, in- distance of 2.5 cm. This could have entailed a high fraction of extra- cluding technical setup and study design, have to be taken into account cerebral HbR signal with attenuation of the hemodynamic response to to explain such discrepancies, e.g., (i) sampling rate and time resolution functional neuronal activation in circumscribed cortical areas. More- of the near-infrared spectrometer or MR scanner, (ii) data processing over, the limited temporal resolution (sampling rate: ~3.45 Hz) and and algorithm of analysis, (iii) determination of movement onset, the asynchronous sampling of source–detector combinations within (iv) definition of the investigated HbR or BOLD response (onset, half this time regime were possibly insufficient to demonstrate a latency in maximum, or maximum of the HbR/BOLD signal), and (v) trial design the HbR response between two distinct cortical areas of less than 1 s. and paradigm (duration, complexity, side (ipsilateral, contralateral, or bilateral movement pattern), and the mode of initiation (self-paced or 4.1. Advantages and disadvantages of NIRS technology in the externally cued) of the movement). clinical application During self-paced movements, we observed a stronger hemodynamic response in the SMA, which might reflect a more pronounced Compared to PET and fMRI, NIRS has a couple of limitations. These neuronal activity compared to externally cued movements. This is in limitations concern the following: the investigated cerebral regions, line with a PET study, in which self-initiated movements activated the since the penetration depth of NIR-light is limited; the rough spatial res- rostral SMA and elicited a significantly greater activation of the caudal olution compared to fMRI due to restricting physical properties of light SMA than did unpredictably cued movements [70]. It also corroborates propagation and scattering; the limited number of NIRS channels; and an fMRI study demonstrating a significantly enhanced activity within the resulting sparseness of coverage. While, in most circumstances, the left SMA during self-initiated movements compared to externally the additionally acquired structural scan provides the anatomical infor- triggered movements [71]. mation for fMRI data interpretation, it is to be stressed that NIRS probe Repetitive movements demonstrated a significantly greater decrease positioning strongly relies on extracerebral landmarks that restrict the of HbR in the SMA during right-hand tapping than during left-hand tap- certainty with respect to the anatomical interpretation. ping. It is assumed that the location of the scalp-NIRS grid allowed us to On the other hand, NIRS has the advantage over fMRI to provide measure the entire part of the SMA located at the convexity of both hemi- direct measures of HbO, HbR, and Hbtot, the latter being assumed to be spheres. Thus, we would not construe the difference in HbR response dur- proportional to CBV. Moreover, NIRS can be performed in parallel with ing right-hand and left-hand repetitive movements as an effect of various other techniques since mutual inferences with other devices lateralization in the SMA. It might rather reflect a higher grade of neuronal can be excluded. In addition, the NIRS technique operates silently (by activity and/or neuronal recruitment in the SMA for planning and prepar- contrast, the fMRI gradient noise is between 90 and 130 dB); NIRS is ing movements generated by the dominant primary motor cortex. The moderate in costs and allows long-term acquisition without any risk difference in HbR response during right-hand and left-hand repetitive of radiation exposure (unlike PET) and heat absorption (which occurs movements was, however, contradictory to a PET study, which did not with fMRI). Finally, NIRS devices are comparably small (portability) in demonstrate a significant difference in rCBF of the SMA during left-hand size and allow the technical integration into the clinical environment and right-hand movements [56]. In this context, an fMRI study is also of and bedside application. interest, although it may not be directly comparable, in which differences in relative activation of the SMA between left-handed and right-handed 4.2. Potential clinical implications subjects were found when unilateral left index finger button presses were performed. By contrast, this effect was not detectable when the Despite enormous progress in NIRS research (for review see [76]), same task was performed with the right index finger [72]. NIRS has not reached standardized protocols or any appreciable applica- Taken together, our study was designed to test whether scalp-NIRS tion in clinical routine. In principle, scalp-NIRS is a promising tool for provides a valid tool to map temporally separated onsets of focal HbR noninvasive monitoring, for instance, of cortical spreading depolariza- changes in different cortical regions. The motor system is very amenable tions (CSDs) in patients with stroke and brain trauma. Cortical spread- to such a validation study since voluntary finger movements are associ- ing depolarizations are pathological events of the cerebral gray matter ated with temporally separated onsets of neural activation in several only recently identified as recurring acutely in patients with stroke

Please cite this article as: Drenckhahn C, et al, A validation study of the use of near-infrared spectroscopy imaging in primary and secondary motor areas of the human brain, Epilepsy Behav (2015), http://dx.doi.org/10.1016/j.yebeh.2015.04.006 C. Drenckhahn et al. / Epilepsy & Behavior xxx (2015) xxx–xxx 7 and traumatic brain injury [77]. Clinical evidence suggests that the oc- [8] Cui RQ, Huter D, Egkher A, Lang W, Lindinger G, Deecke L. High resolution DC-EEG mapping of the Bereitschaftspotential preceding simple or complex bimanual currence of clusters of repeated CSDs predict a new stroke/secondary sequential finger movement. Exp Brain Res 2000;134:49–57. neurological deterioration in neurointensive care patients [78].Cortical [9] Deecke L, Kornhuber HH. An electrical sign of participation of the mesial ‘supple- spreading depolarizations are currently observed invasively using direct mentary’ motor cortex in human voluntary finger movement. 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