Abstract
.Functional near-infrared spectroscopy (fNIRS) is a noninvasive functional imaging technique measuring hemodynamic changes including oxygenated () and deoxygenated (HHb) hemoglobin. Low frequency (LF; 0.01 to 0.15 Hz) band is commonly analyzed in fNIRS to represent neuronal activation. However, systemic physiological artifacts (i.e., nonneuronal) likely occur also in overlapping frequency bands. We measured peripheral photoplethysmogram (PPG) signal concurrently with fNIRS (at prefrontal region) to extract the low-frequency oscillations (LFOs) as systemic noise regressors. We investigated three main points in this study: (1) the relationship between prefrontal fNIRS and peripheral PPG signals; (2) the denoising potential using these peripheral LFOs, and (3) the innovative ways to avoid the false-positive result in fNIRS studies. We employed spatial working memory (WM) and control tasks (e.g., resting state) to illustrate these points. Our results showed: (1) correlation between signals from prefrontal fNIRS and peripheral PPG is region-dependent. The high correlation with peripheral ear signal (i.e., ) occurred mainly in frontopolar regions in both spatial WM and control tasks. This may indicate the finding of task-dependent effect even in peripheral signals. We also found that the PPG recording at the ear has a high correlation with prefrontal fNIRS signal than the finger signals. (2) The systemic noise was reduced by 25% to 34% on average across regions, with a maximum of 39% to 58% in the highly correlated frontopolar region, by using these peripheral LFOs as noise regressors. (3) By performing the control tasks, we confirmed that the statistically significant activation was observed in the spatial WM task, not in the controls. This suggested that systemic (and any other) noises unlikely violated the major statistical inference. (4) Lastly, by denoising using the task-related signals, the significant activation of region-of-interest was still observed suggesting the manifest task-evoked response in the spatial WM task.
Highlights
We found the significantly increased O2Hb activations (p < 0.05; Holm–Bonferroni corrected) only in the spatial working memory (WM) task but not in any control tasks [Figs. 5(a)–5(d)]
Despite the various hemodynamic responses in the spatial WM task, neither significant O2Hb increase nor HHb decrease was found during control tasks
O2Hb activation was only detected in the spatial WM task
Summary
Functional near-infrared spectroscopy (fNIRS) is an imaging technique that noninvasively measures the product of cerebral hemodynamics (concentration changes of oxygenated and deoxygenated hemoglobin; ΔCO2Hb and ΔCHHb) and optical path length (L), using light in the near-infrared spectrum (650 to 900 nm).[1,2,3] Because hemodynamic changes are related to local neuronal activity through neurovascular coupling,[4,5] fNIRS is commonly used to interpret brain activity and function. fNIRS has been widely used in research, clinical, and educational purposes[6] due to its cost-effectiveness, safety, flexibility, higher spatial resolution than electroencephalography, and better temporal resolution than functional magnetic resonance fMRI, which enables practical and continuous bedside monitoring even in infants and young children.[8,9,10] fNIRS provides better motion tolerance; fNIRS is suitable for examining the challenging patients with restless symptoms[11,12] and subjects actively engaging in movement such as walking and running.[13,14] cochlear implant patients can safely undergo fNIRS measurement because there is no magnetic field that may endanger patients.[15,16]Despite those advantages, there are three confounding factors in fNIRS studies. FNIRS has been widely used in research, clinical, and educational purposes[6] due to its cost-effectiveness, safety, flexibility, higher spatial resolution than electroencephalography, and better temporal resolution than functional magnetic resonance fMRI, which enables practical and continuous bedside monitoring even in infants and young children.[8,9,10] fNIRS provides better motion tolerance; fNIRS is suitable for examining the challenging patients with restless symptoms[11,12] and subjects actively engaging in movement such as walking and running.[13,14] cochlear implant patients can safely undergo fNIRS measurement because there is no magnetic field that may endanger patients.[15,16]. These factors are: (1) the mixture of neuronal and systemic physiological (nonneuronal) signals which is found in the low frequency (LF) range[17,18] especially when the activation period is relatively short.[19,20] This nonneuronal signal is compounded by several sources such as Mayer waves and vasomotion.[21,22,23,24,25] In addition, because fNIRS data are measured through the intact skull, so every fNIRS channel
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