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Abstract
Functional near infrared spectroscopy (fNIRS) measurements are confounded by signal components originating from multiple physiological causes, whose activities may vary temporally and spatially (across tissue layers, and regions of the cortex). Furthermore, the stimuli can induce evoked effects, which may lead to over or underestimation of the actual effect of interest. Here, we conducted a temporal, spectral, and spatial analysis of fNIRS signals collected during cognitive and hypercapnic stimuli to characterize effects of functional versus systemic responses. We utilized wavelet analysis to discriminate physiological causes and employed long and short source-detector separation (SDS) channels to differentiate tissue layers. Multi-channel measures were analyzed further to distinguish hemispheric differences. The results highlight cardiac, respiratory, myogenic, and very low frequency (VLF) activities within fNIRS signals. Regardless of stimuli, activity within the VLF band had the largest contribution to the overall signal. The systemic activities dominated the measurements from the short SDS channels during cognitive stimulus, but not hypercapnic stimulus. Importantly, results indicate that characteristics of fNIRS signals vary with type of the stimuli administered as cognitive stimulus elicited variable responses between hemispheres in VLF band and task-evoked temporal effect in VLF, myogenic and respiratory bands, while hypercapnic stimulus induced a global response across both hemispheres.
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1 Drexel University, School of Biomedical Engineering, Science and Health Systems, Philadelphia, USA (GRID:grid.166341.7) (ISNI:0000 0001 2181 3113)
2 Villanova University, Department of Electrical and Computer Engineering, Villanova, USA (GRID:grid.267871.d) (ISNI:0000 0001 0381 6134)
3 Drexel University, School of Biomedical Engineering, Science and Health Systems, Philadelphia, USA (GRID:grid.166341.7) (ISNI:0000 0001 2181 3113); Drexel University, Nutrition Sciences Department of College of Nursing and Health Professions, Philadelphia, USA (GRID:grid.166341.7) (ISNI:0000 0001 2181 3113); Drexel University, Department of Teaching, Learning and Curriculum, School of Education, Philadelphia, USA (GRID:grid.166341.7) (ISNI:0000 0001 2181 3113)
4 University of Pennsylvania Perelman School of Medicine, Clinical TBI Research Center and Department of Neurology, Philadelphia, USA (GRID:grid.25879.31) (ISNI:0000 0004 1936 8972)
5 Drexel University, School of Biomedical Engineering, Science and Health Systems, Philadelphia, USA (GRID:grid.166341.7) (ISNI:0000 0001 2181 3113); Drexel University, Department of Teaching, Learning and Curriculum, School of Education, Philadelphia, USA (GRID:grid.166341.7) (ISNI:0000 0001 2181 3113)