Functional near-infrared spectroscopy is sensitive to leg activity in the primary motor cortex, but requires rigorous correction for systemic fluctuations induced by movements.

Abstract

Background: functional near-infrared spectroscopy (fNIRS) is an increasingly popular tool to study cortical activity during movement and gait. However, this comes with a number of challenges, like the correction for movement-induced systemic changes (i.e., changes in blood pressure, heart rate, breathing). We investigated gait-related tasks in a controlled and seated environment, to validate whether fNIRS can yield comparable results to functional magnetic resonance imaging. More specifically, we studied the fNIRS sensitivity to leg movements and to movement automaticity, and compared this to finger movements.Methods: Twenty-seven healthy participants performed sequential automatic and non-automatic finger tapping and foot stepping tasks. A multichannel fNIRS device including 12 short channels covered the primary motor cortex (M1), the premotor cortex, the prefrontal cortex, and the posterior parietal cortex. Changes in oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HbR) were assessed by a general linear model (GLM). However, we noticed a correlation between the short channels and the expected functional hemodynamic responses, necessitating performing a separate short channel regression instead of adding them as nuisance regressors to the GLM.Results: Consistent with the M1 somatotopy, we found significant HbO increases of very large effect size in the lateral M1 channels during finger tapping (Cohen’s d = 1.35, p<0.001) and significant HbO increases of moderate effect size in the medial M1 channels during foot stepping (Cohen’s d = 0.8, p<0.05). The cortical activity differences between automatic and non-automatic tasks were in line with expectations, though the effect sizes did not yield significance. Importantly, leg movements produced large systemic fluctuations, which were adequately removed by the use of all available short channels.Discussion: Our results indicate that fNIRS is sensitive to leg activity in M1, though the sensitivity is lower than for finger activity and requires rigorous correction for systemic fluctuations. We furthermore highlight that task-evoked systemic effects may cause destabilization of GLM estimates when short channels are correlated with the task regressor.

Test-retest reliability of functional near-infrared spectroscopy during a finger-tapping and postural task in healthy older adults

“…For finger tapping, reliability of the C3-C1 hand motor channel was good when assessed on the same day (ICC=0.66). This supports the notion that fNIRS is a sensitive neuroimaging tool for capturing HbO2 changes during a specific motor tasks requiring localized cortical activation 29 . It should be noted that there was a trend towards better behavioral tapping accuracy from test 1 to test 2 (p=0.07).…”

“…This supports the notion that fNIRS is a sensitive neuroimaging tool for capturing HbO 2 changes during a specific motor tasks requiring localized cortical activation. 29 It should be noted that there was a trend towards better behavioral tapping accuracy from test 1 to test 2 (p ¼ 0.07). However, when exploring this result, we did not find differences in the HbO 2 levels or that the participants with improved accuracies had better reliability.…”

“…The single channel-based analysis underscored this statement, as no clear activation pattern was found for the PMC and SMA. In addition, fNIRS signals pertaining to the lower limbs are more prone to systematic artifacts 29 and likely more difficult to capture as the motor representation of the lower extremities is located deeply within the interhemispheric fissure, 37 which could result in more inconsistent fNIRS measurements. Note that, similar to the present findings, HHb reliability after cap removal was overall comparable to HbO2 findings in healthy adults (see Table 4).…”

“…First, test-retest reliability of five regions of interest (ROIs) was determined during a weight-shifting task comparing two tests, which were repeated twice on the same day and on two consecutive days following cap removal and repositioning. Next, the task-specificity of test-retest reliability was investigated during a control finger tapping task, focusing on the contralateral finger region of the motor cortex 29 . Based on earlier work during both gross and fine motor tasks in young adults 26 , 27 and the anticipated lower signal to noise ratios in older people, 28 we hypothesized that test–retest reliability would be more compromised than that obtained in young adults.…”

“…Next, the task-specificity of test-retest reliability was investigated during a control finger tapping task, focusing on the contralateral finger region of the motor cortex. 29 Based on earlier work during both gross and fine motor tasks in young adults 26,27 and the anticipated lower signal to noise ratios in older people, 28 we hypothesized that test-retest reliability would be more compromised than that obtained in young adults. Furthermore, we expected lower reliability after cap removal compared to when the cap remained stationary.…”