Cerebral and systemic physiological effects of wearing face masks in young adults.

The COVID-19 pandemic led to widespread mandates requiring the wearing of face masks, which led to debates on their benefits and possible adverse effects. To that end, the physiological effects at the systemic and at the brain level are of interest. We have investigated the effect of commonly available face masks (FFP2 and surgical) on cerebral hemodynamics and oxygenation, particularly microvascular cerebral blood flow (CBF) and blood/tissue oxygen saturation (StO2), measured by transcranial hybrid near-infrared spectroscopies and on systemic physiology in 13 healthy adults (ages: 23 to 33 y). The results indicate small but significant changes in cerebral hemodynamics while wearing a mask. However, these changes are comparable to those of daily life activities. This platform and the protocol provides the basis for large or targeted studies of the effects of mask wearing in different populations and while performing critical tasks.

The Significance of Lipids for the Absorption and Release of Oxygen in Biological Organisms.

A critically important step for the uptake and transport of oxygen (O2) in living organisms is the crossing of the phase boundary between gas (or water) and lipid/proteins in the cell. Classically, this transport across the phase boundary is explained as a transport by proteins or protein-based structures. In our contribution here, we want to show the significance of passive transport of O2 also (and in some cases probably predominantly) through lipids in many if not all aerobic organisms. In plants, the significance of lipids for gas exchange (absorption of CO2 and release of O2) is well recognized. The leaves of plants have a cuticle layer as the last film on both sides formed by polyesters and lipids. In animals, the skin has sebum as its last layer consisting of a mixture of neutral fatty esters, cholesterol and waxes which are also at the border between the cells of the body and the air. The last cellular layers of skin are not vascularized therefore their metabolism totally depends on this extravasal O2 absorption, which cannot be replenished by the bloodstream. The human body absorbs about 0.5% of O2 through the skin. In the brain, myelin, surrounding nerve cell axons and being formed by oligodendrocytes, is most probably also responsible for enabling O2 transport from the extracellular space to the cells (neurons). Myelin, being not vascularized and consisting of water, lipids and proteins, seems to absorb O2 in order to transport it to the nerve cell axon as well as to perform extramitochondrial oxidative phosphorylation inside the myelin structure around the axons (i.e., myelin synthesizes ATP) - similarly to the metabolic process occurring in concentric multilamellar structures of cyanobacteria. Another example is the gas transport in the lung where lipids play a crucial role in the surfactant ensuring incorporation of O2 in the alveoli where there are lamellar body and tubular myelin which form multilayered surface films at the air-membrane border of the alveolus. According to our view, the role played by lipids in the physical absorption of gases appears to be crucial to the existence of many, if not all, of the living aerobic species.

Infradian Rhythms in Cerebrovascular Oxygenation and Blood Volume in Humans at Rest: A 5-Year Study.

BACKGROUND: All parameters of human physiology show chronobiological variability. While circadian (cycle length ~ 24 h) rhythms of the neuronal, hemodynamic and metabolic aspects of human brain activity are increasingly being explored, infradian (cycle length > 24 h) rhythms are largely unexplored. AIM: We investigated if cerebrovascular oxygen saturation (StO2) and blood volume ([tHb]) values measured over many years in many subjects during resting show infradian rhythmicity. SUBJECTS AND METHODS:  Absolute StO2 and [tHb] values (median over a 5 min resting-phase while sitting) were measured in 220 healthy subjects (age: 24.7 ± 3.6 years, 87 males, 133 females) 2-4 times on different days over the right and left frontal lobe (FL) and occipital lobe (OL) by employing frequency-domain NIRS as part of different systemic physiology augmented functional near-infrared spectroscopy, SPA-fNIRS, studies. The data set consisted of 708 single measurements performed over a timespan of 5 years (2017-2021). General additive models (GAM) and cosinor modelling were used to analyze the data. RESULTS:  The GAM analysis revealed (i) a non-linear trend in the StO2 and [tHb] values over the 5-year span, (ii) a circannual (cycle length ~ 12 months) rhythm in StO2 at the FL (amplitude (A): 3.4%, acrophase (φ): June) and OL (A: 1.5%, φ: May) as well as in [tHb] at the OL (A: 1.2 µM, bathyphase (θ): June), and (iii) a circasemiannual (cycle length ~ 6 months) rhythm in [tHb] at the FL (A: 2.7 µM, φ: March and September, respectively). Furthermore, the circannual oscillations of StO2 (at the FL) and [tHb] (at the OL) were statistically significantly correlated with the day length, outdoor temperature, humidity and air pressure. DISCUSSION AND CONCLUSION:  We conclude that absolute values of StO2 and [tHb] show chronobiological variability on the group-level with a long-term nonlinear trend as well as circannual/circasemiannual rhythmicity. These rhythms need to be taken into account when defining reference values for StO2 and [tHb] and may correlate with the variability of cerebrovascular disease incidents over the year.

Differences Between Good, Moderate and Poor Performers of a Verbal Fluency Task under Blue Light Exposure: An SPA-fNIRS Study.

Individuals have different performance levels for cognitive tasks. Are these performance levels reflected in physiological parameters? The aim of this study was to address this question by systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS). We aimed to investigate whether different verbal fluency task (VFT) performances under blue light exposure were associated with different changes in cerebrovascular oxygenation and systemic physiological activity. The VFT performance of 32 healthy subjects (17 female, 15 male, age: 25.5 ± 4.3 years) was investigated under blue light exposure (120 lux). The VFT, which contained letter and category fluency tasks, lasted 9 min. There were rest periods without light exposure before and after the VFT for 8 min and 15 min, respectively. Based on their number of correct responses, subjects were classified into three groups, i.e., good, moderate, and poor performers. During the entire experiment, we simultaneously measured changes in cerebral and systemic physiological parameters using the SPA-fNIRS approach. We found that the better the subject's performance was, the smaller the task-evoked changes in cerebrovascular hemodynamics and oxygenation in the prefrontal cortex. Performance-dependent changes were also evident for skin conductance, arterial oxygen saturation and mean arterial pressure. This is the first VFT study that applies the comprehensive SPA-fNIRS approach to determine the relationship between task performance and changes in cerebral oxygenation and systemic physiology. Our study shows that these parameters are indeed related and the performance is reflected in the task-evoked cerebrovascular and systemic physiological changes.

Autopsy-Based Pulmonary and Vascular Pathology: Pulmonary Endotheliitis and Multi-Organ Involvement in COVID-19 Associated Deaths.

BACKGROUND: Findings from autopsies have provided evidence on systemic microvascular damage as one of the underlying mechanisms of Coronavirus disease 2019 (CO-VID-19). The aim of this study was to correlate autopsy-based cause of death in SARS-CoV-2, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) positive patients with chest imaging and severity grade of pulmonary and systemic morphological vascular pathology. METHODS: Fifteen SARS-CoV-2 positive autopsies with clinically distinct presentations (age 22-89 years) were retrospectively analyzed with focus on vascular, thromboembolic, and ischemic changes in pulmonary and in extrapulmonary sites. Eight patients died due to COVID-19 associated respiratory failure with diffuse alveolar damage in various stages and/or multi-organ failure, whereas other reasons such as cardiac decompensation, complication of malignant tumors, or septic shock were the cause of death in 7 further patients. The severity of gross and histopathological changes was semi-quantitatively scored as 0 (absent), 1 (mild), and 3 (severe). Severity scores between the 2 groups were correlated with selected clinical parameters, initial chest imaging, autopsy-based cause of death, and compared using Pearson χ2 and Mann-Whitney U tests. RESULTS: Severe pulmonary endotheliitis (p = 0.031, p = 0.029) and multi-organ involvement (p = 0.026, p = 0.006) correlated significantly with COVID-19 associated death. Pulmonary microthrombi showed limited statistical correlation, while tissue necrosis, gross pulmonary embolism, and bacterial superinfection did not differentiate the 2 study groups. Chest imaging at hospital admission did not differ either. CONCLUSIONS: Extensive pulmonary endotheliitis and multi-organ involvement are characteristic autopsy features in fatal CO-VID-19 associated deaths. Thromboembolic and ischemic events and bacterial superinfections occur frequently in SARS-CoV-2 infection independently of outcome.

Changes in Cerebral Oxygenation and Systemic Physiology During a Verbal Fluency Task: Differences Between Men and Women.

BACKGROUND: Sex differences in behavioural performance of a cognitive task have been repeatedly reported in the literature. Whether such differences are also relevant for evoked cerebral haemodynamic and oxygenation responses as well as systemic physiological changes is a topic of ongoing investigations. AIM: We investigated whether changes in cerebral oxygenation and systemic physiological activity are sex-dependent during a verbal fluency task (VFT). SUBJECTS AND METHODS: VFT performance and changes in cerebral oxygenation and systemic physiology were investigated in 32 healthy right-handed adults (17 females, 15 males, age: 25.5 ± 4.3 years). The total duration of the VFT was 9 min. During this phase, subjects were also exposed to blue light (peak wavelength: 450 nm, illuminance: 120 lux). Before and after the VFT, subjects were in darkness. In this study, we used the systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) approach. Absolute concentration changes of oxyhaemoglobin ([O2Hb]), deoxyhaemoglobin ([HHb]), total haemoglobin ([tHb]), as well as tissue oxygen saturation (StO2) were measured bilaterally over the prefrontal cortex (PFC) and visual cortex (VC). Systemic physiological parameters were also recorded in parallel (e.g., end-tidal CO2, heart rate, respiration rate, skin conductance). RESULTS: We found that: (i) Females were better VFT performers in comparison to males. (ii) Changes in [O2Hb] and [HHb] in the VC were higher for males compared to females during the VFT under blue light exposure. (iii) Lower and higher StO2 changes were detected for males compared to females in the PFC and VC, respectively. (iv) Sex-dependent changes were also evident for end-tidal CO2, heart rate, respiration rate, and pulse-respiration quotient. CONCLUSIONS: Our study showed that sex differences are not only observed in task performance but also in the magnitudes of changes in cerebral and physiological parameters. The findings of the current study emphasise that sex differences in brain activation and systemic physiological activity need to be considered when interpreting functional neuroimaging data. These findings offer a broad range of implications for educational purposes and facilitate a better understanding of the effects of sex on behavioural performance as well as human physiology.

Frontal Cerebral Oxygenation in Humans at Rest: A Mirror Symmetry in the Correlation with Cardiorespiratory Activity.

BACKGROUND: Although several studies published reference values for frontal cerebral tissue oxygen saturation (StO2) measured with near-infrared spectroscopy (NIRS) based cerebral oximetry, a detailed investigation, whether and which factors from systemic physiology are related to the individual StO2 values, is missing. AIM: We investigated how the state of the cardiorespiratory system is linked to StO2 values at rest. SUBJECTS AND METHODS: Absolute StO2 values (median over a 5 min resting-phase while sitting) were obtained from 126 healthy subjects (age: 24.0 ± 0.2 years, 45 males, 81 females) over the left and right prefrontal cortex (PFC) by employing frequency-domain NIRS as part of a systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) study. In addition, heart rate (HR) and respiration rate (RR) were measured, and the pulse respiration quotient (PRQ) was determined (PRQ = HR/RR). General additive models (GAM) were used to analyse the data. RESULTS: The GAM analysis revealed a specific relationship between the overall PFC StO2 values (mean over right and left PFC) and the variables HR and RR: HR was positively correlated with mean StO2, while RR showed no correlation. In the mirror case, RR was negatively linearly correlated with the frontal cerebral oxygenation asymmetry (FCOA), which was not correlated with HR. The right PFC StO2 was not linked to the RR, whereas the left PFC StO2 was. Positive correlations of the PRQ with the mean PFC StO2 as well as the FCOA were also found. GAM modelling revealed that the individual FCOA values are explained to a large extent (deviance explained: 88.8%) by the individual mean PFC StO2 and PRQ. We conclude that (i) the state of the cardiorespiratory system is significantly correlated with StO2 values and (ii) there is a mirror symmetry with regard to the impact of cardiorespiratory parameters on the mean PFC StO2 and FCOA.

Cross-Frequency Coupling Between Brain and Body Biosignals: A Systemic Physiology Augmented Functional Near-Infrared Spectroscopy Hyperscanning Study.

BACKGROUND: Understanding the brain and body processes during interaction or cooperation between two or more subjects is an important topic in current neuroscientific research. In a previous study, we introduced a novel approach that enables investigation of the coupling of biosignals (brain and systemic physiology, SP) from two subjects: systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) hyperscanning. AIM: The aim was to extend our signal analysis approach by the cross-frequency time-dependent wavelet transform coherence (WTC) of the fNIRS and SP biosignals to gain new insights into the nature and cause of functional hyperconnectivity. SUBJECTS AND METHODS: 24 pairs of adults took part in a closed-eye versus prolonged eye-contact task of 10 min each. Brain and body activity was measured continuously by SPA-fNIRS hyperscanning. We calculated the time-dependent WTC of the biosignals for four different frequency bands: very low-frequency band (VLF, 0.002-0.08 Hz), low-frequency band 1 (LF1, 0.015-0.15 Hz), low-frequency band 2 (LF2, 0.08-0.15 Hz) and heart rate band (HR, 1-2 Hz). We then performed the cross-frequency correlated-coherence coupling analysis. RESULTS: A stronger cross-frequency coupling during the eye-contact condition (between 99 pairs of biosignals) was found than during the eye-closed condition (between 50 pairs of biosignals). Prolonged eye contact led to entrainment of the brain and body between different frequency bands and two subjects. The strongest hyperconnectivity was between the LF1-VLF frequency band. DISCUSSION AND CONCLUSION: With this exploratory study, we reveal further benefits of the SPA-fNIRS approach for future hyperscanning studies.

Machine Learning Distinguishes Familiar from Unfamiliar Pairs of Subjects Performing an Eye Contact Task: A Systemic Physiology Augmented Functional Near-Infrared Spectroscopy Hyperscanning Study.

BACKGROUND: Eye contact is an important aspect of human communication and social interactions. Changes in brain and systemic physiological activity associated with interactions between humans can be measured with systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) hyperscanning, enabling inter-brain and inter-body synchronisation to be determined. In a previous study, we found that pairs of subjects that are socially connected show higher brain and body synchrony. AIM: To enable a deeper understanding, our aim was to build and automatically detect the best set of features to distinguish between two different groups (familiar and unfamiliar pairs). MATERIAL AND METHODS: We defined several features based on the Spearman correlation and wavelet transform coherence (WTC) of biosignals measured on 23 pairs of subjects (13 familiar and 10 unfamiliar pairs) during eye contact for 10 min. Additional custom features that identify the maximum brain-to-body coupling instants between pairs were generated. RESULTS: After testing on combinations of different feature extraction methods, four subsets of features with the strongest discrimination power were taken into account to train a decision tree (DT) machine learning (ML) algorithm. We have obtained 95.65% classification accuracy using a leave-one-out cross-validation. The coupling features which represent the two maximum mean values resulting from the sum of 7 time-dependent WTC signals (oxyhaemoglobin concentration of the right prefrontal region, total haemoglobin concentration of the left and right prefrontal region, heart rate, electrodermal activity on the left and right wrist, and skin temperature on the right wrist) played an essential role in the classification accuracy. CONCLUSION: Training the DT-ML algorithm with combined brain and systemic physiology data provided higher accuracy than training it only with brain or systemic data alone. The results demonstrate the power of the SPA-fNIRS hyperscanning approach and the potential in applying ML to investigate the strength of social bonds in a wide range of social interaction contexts.

Changes in Water Properties in Human Tissue after Double Filtration Plasmapheresis-A Case Study.

Double-filtration plasmapheresis (DFPP) is a blood cleaning technique that enables the removal of unwanted substances from the blood. In our case study, we performed near-infrared (NIR) spectroscopy measurements on the human hand tissue before and after a specific DFPP treatment (INUSpheresis with a TKM58 filter), along with NIR measurements of the substances extracted via DFPP (eluate). The spectral data were analyzed using the aquaphotomics approach. The analysis showed that the water properties in the tissue change after DFPP treatment, i.e., an increase in small water clusters, free water molecules and a decrease in hydroxylated water as well as superoxide in hydration shells was noted. The opposite effect was observed in the eluates of both DFPP treatments. Our study is the first that documents changes in water spectral properties after DFPP treatments in human tissue. The changes in tissue water demonstrated by our case study suggest that the positive physiological effects of DFPP in general, and of INUSpheresis with the TKM58 filter in particular, may be associated with improvements in water quality in blood and tissues.

Long-Term Blue Light Exposure Changes Frontal and Occipital Cerebral Hemodynamics: Not All Subjects React the Same.

BACKGROUND: In modern society, we are increasingly exposed to numerous sources of blue light, including screens (e.g., TVs, computers, laptops, smartphones, tablets) and light from fluorescent and LED lamps. Due to this wide range of applications, the effects of blue light exposure (BLE) on the human physiology need to be thoroughly studied. AIM: To investigate the impact of long-term BLE on frontal and occipital human cerebral hemodynamics and oxygenation using functional near-infrared spectroscopy (fNIRS) neuroimaging. MATERIALS AND METHODS: 32 healthy right-handed subjects (20 females, 12 males; age: 23.8 ± 2.2 years) were exposed to blue LED light for 15 minutes. Before (baseline, 8 min) and after (recovery, 10 min) the BLE, subjects were in darkness. We measured the concentration changes of oxyhemoglobin ([O2Hb]) and deoxyhemoglobin ([HHb]) at the prefrontal cortex (PFC) and visual cortex (VC) by fNIRS during the experiment. Subjects were then classified into different groups based on their hemodynamic response pattern of [O2Hb] at the PFC and VC during BLE. RESULTS: On the group level (32 subjects), we found an increase in [O2Hb] and a decrease in [HHb] at both cortices during BLE. Evoked changes of [O2Hb] were higher at the VC compared to the PFC. Eight different hemodynamic response patterns were detected in the subgroup analysis, while an increase of [O2Hb] in both cortices was the most common pattern (8 out of 32 cases, 25%) during BLE. DISCUSSION AND CONCLUSION: Our study showed that the hemodynamic and oxygenation changes at the PFC and VC during BLE (i) were generally higher in the VC compared to the PFC, (ii) showed an intersubject variability with respect to their magnitudes and shapes, and (iii) can be classified into eight groups. We conclude that blue light affects humans differently. It is essential to consider this when assessing the impact of the BLE on society.

Comparison of Two NIRS Tissue Oximeters (Moxy and Nimo) for Non-Invasive Assessment of Muscle Oxygenation and Perfusion.

BACKGROUND: Near-infrared spectroscopy (NIRS) tissue oximeters enable non-invasive measurement of muscle oxygenation and perfusion. Several NIRS oximeters are currently available, particularly for muscle measurements. AIM: To evaluate the agreement of oxygenation and perfusion measurements obtained by two devices (Moxy, Fortiori Designs LLC, USA, and Nimo, Nirox, Italy) during an arterial occlusion test on the arm. SUBJECT AND METHODS: Arterial occlusions were conducted at the arm of one individual for 10 min with 200 mmHg. Measurements were made twice a day on five different days. Both NIRS devices were fixed at the arm (covering the muscles extensor carpi ulnaris, extensor digitorum, and flexor carpi ulnaris). RESULTS: The experiment revealed that i) both devices could detect changes in muscle oxygenation and perfusion during the occlusion, but ii) the magnitudes and dynamic changes differed between the two devices. DISCUSSION AND CONCLUSION: Both devices had different performances with regard to the measurement of tissue oxygenation and perfusion. This study shows that it might be worthwhile to compare all NIRS tissue oximeters currently available for muscle measurement in a large systematic study to increase the comparability of measurements obtained with different devices.

Right-Left Asymmetry of Prefrontal Cerebral Oxygenation: Does it Depend on Systemic Physiological Activity, Absolute Tissue Oxygenation or Hemoglobin Concentration?

BACKGROUND: We have repeatedly observed a right-left asymmetry (RLA) of prefrontal cerebral oxygenation of subjects during the resting state. AIM: To clarify if the RLA is a reliably observable phenomenon at the group level and whether it is associated with systemic physiology, absolute tissue oxygen saturation (StO2) or total hemoglobin concentration ([tHb]). MATERIAL AND METHODS: StO2 and [tHb] values at the right and left prefrontal cortex (PFC) were calculated for two 5- min resting phases based on data from 76 single measurements (24 healthy adults, aged 22.0 ± 6.4 years). StO2 and [tHb] were measured with an ISS OxiplexTS frequency domain near-infrared spectroscopy device. In addition, end-tidal CO2 (PETCO2), heart rate (HR), respiration rate (RR) and the pulse-respiration quotient (PRQ = HR/RR) were measured and analyzed for the two phases. RESULTS: On the group level it was found that i) StO2 was higher at the right compared to the left PFC (for both phases), ii) RLA of StO2 (∆StO2 = StO2 (right)-StO2 (left) was independent of PETCO2, HR and PRQ, and iii) ∆StO2 was associated with absolute StO2 and [tHb] values (positively and negatively, respectively). DISCUSSION AND CONCLUSION: This study shows that i) RLA of StO2 at the PFC is a real phenomenon, and that ii) ∆StO2 at the group level does not depend on PETCO2, HR, RR or PRQ, but on absolute StO2 and [tHb]. We conclude that the RLA is a real effect, independent of systemic physiology, and most likely reflects genuine properties of the brain, i.e. different activity states of the two hemispheres.

A Multi-Layered Study on Harmonic Oscillations in Mammalian Genomics and Proteomics.

Cellular, organ, and whole animal physiology show temporal variation predominantly featuring 24-h (circadian) periodicity. Time-course mRNA gene expression profiling in mouse liver showed two subsets of genes oscillating at the second (12-h) and third (8-h) harmonic of the prime (24-h) frequency. The aim of our study was to identify specific genomic, proteomic, and functional properties of ultradian and circadian subsets. We found hallmarks of the three oscillating gene subsets, including different (i) functional annotation, (ii) proteomic and electrochemical features, and (iii) transcription factor binding motifs in upstream regions of 8-h and 12-h oscillating genes that seemingly allow the link of the ultradian gene sets to a known circadian network. Our multifaceted bioinformatics analysis of circadian and ultradian genes suggests that the different rhythmicity of gene expression impacts physiological outcomes and may be related to transcriptional, translational and post-translational dynamics, as well as to phylogenetic and evolutionary components.

Changes in Spinal Muscle Oxygenation and Perfusion During the Biering-Sørensen Test: Preliminary Results of a Study Employing NIRS-Based Muscle Oximetry.

Low back pain (LBP) is suggested to be related to deconditioning of back muscles by a decreased capacity for hyperemia in exercising muscles. However, only inconsistent evidence exists regarding back muscle perfusion and oxygen saturation in healthy subjects and patients suffering from (chronic) LBP. AIM: We measured muscle perfusion in healthy subjects during the Biering-Sørensen (BS) test (the gold standard for evaluating back muscle endurance) using a commercial near-infrared spectroscopy (NIRS) device. We analysed data sets of five female healthy subjects (age: 34 ± 15 years) who reached the maximum of 4 min during the BS test. Muscle oxygenation (SmO2) and perfusion ([tHb]) were measured using the Moxy NIRS device (Fortiori Design LLC, Hutchinson, USA). Probes were set unilaterally on M. longissimus, M. iliocostalis and M. multifidus. Additionally, mean arterial blood pressure (MAP), pulse pressure (PP), heart rate (HR), arterial oxygen saturation (SpO2) and lactate (pre, task, post) were measured. We observed (i) a large inter-subject variability in the SmO2 and [tHb] responses in the three muscles (i.e., SmO2 desaturations in the in M. longissimus across subjects during the task ranging from 1.1% to -56.6%), and (ii) a consistent response of the systemic signals in all subjects (i.e., increase in MAP, PP and HR). Lactate changes (post task minus task period) correlated with changes in PP and SmO2 of the multifidus muscle. Our preliminary results showed that during the BS test the response in the peripheral muscles was more variable than the central systemic response. A goal for future investigations is to explain this variability in the periphery by considering, for example, subject-specific changes in systemic cardiovascular activity, lactate and in the microvascular perfusion of muscle tissue.

Impact of Changes in Systemic Physiology on fNIRS/NIRS Signals: Analysis Based on Oblique Subspace Projections Decomposition.

Measurements of cerebral and muscle oxygenation (StO2) and perfusion ([tHb]) with functional near-infrared spectroscopy (fNIRS) and near infrared spectroscopy (NIRS), respectively, can be influenced by changes in systemic physiology. The aim of our study was to apply the oblique subspace projections signal decomposition (OSPSD) to find the contribution from systemic physiology, i.e. heart rate (HR), electrocardiography (ECG)-derived respiration (EDR) and partial pressure of carbon dioxide (pCO2) to StO2 and [tHb] signals measured on the prefrontal cortex (PFC) and calf muscle. OSPSD was applied to two datasets (n1 = 42, n2 = 79 measurements) from two fNIRS/NIRS speech studies. We found that (i) all StO2 and [tHb] signals contained components related to changes in systemic physiology, (ii) the contribution from systemic physiology varied strongly between subjects, and (iii) changes in systemic physiology generally influenced fNIRS signals on the left and right PFC to a similar degree.

Absolute Values of Optical Properties (µa, µÎ„s, µeff and DPF) of Human Head Tissue: Dependence on Head Region and Individual.

BACKGROUND: Absolute optical properties (i.e., the absorption coefficient, µa, and the reduced scattering coefficient, [Formula: see text]) of head tissue can be measured with frequency-domain near-infrared spectroscopy (FD-NIRS). AIM: We investigated how the absolute optical properties depend on the individual subject and the head region. MATERIALS AND METHODS: The data set used for the analysis comprised 31 single FD-NIRS measurements of 14 healthy subjects (9 men, 5 women, aged 33.4 ± 10.5 years). From an 8-min measurement (resting-state; FD-NIRS device: Imagent, ISS Inc.; bilateral over the prefrontal cortex, PFC, and visual cortex, VC) median values were calculated for µa and [Formula: see text] as well as the effective attenuation coefficient (µeff) and the differential pathlength factor (DPF). The measurement was done for each subject one to three times with at least 24 h between the measurements. RESULTS: (i) A Bayesian ANOVA analysis revealed that head region and subject were the most significant main effects on µa, [Formula: see text] and µeff, as well as DPF, respectively. (ii) At the VC, µa, [Formula: see text] and µeff had higher values compared to the PFC. (iii) The differences in the optical properties between PFC and VC were age-dependent. (iv) All optical properties also were age-dependent. This was strongest for the properties of the PFC compared to the VC. DISCUSSION AND CONCLUSION: Our analysis demonstrates that all optical head tissue properties (µa, [Formula: see text], µeff and DPF) were dependent on the head region, individual subject and age. The optical properties of the head are like a 'fingerprint' for the individual subject. Assuming constant optical properties for the whole head should be carefully reconsidered.

Long-Term Changes in Optical Properties (µa, µ's, µeff and DPF) of Human Head Tissue During Functional Neuroimaging Experiments.

Frequency-domain near-infrared spectroscopy (FD-NIRS) enables to measure absolute optical properties (i.e. the absorption coefficient, µa, and the reduced scattering coefficient, [Formula: see text]) of the brain tissue. The aim of this study was to investigate how the optical properties changed during the course of a functional NIRS experiment. The analyzed dataset comprised of FD-NIRS measurements of 14 healthy subjects (9 males, 5 females, aged: 33.4 ± 10.5 years, range: 24-57 years old). Each measurement lasted 33 min, i.e. 8 min baseline in darkness, 10 min intermittent light stimulation, and 15 min recovery in darkness. Optical tissue properties were obtained bilaterally over the prefrontal cortex (PFC) and visual cortex (VC) with FD-NIRS (Imagent, ISS Inc., USA). Changes in µa and [Formula: see text] were directly measured and two parameters were calculated, i.e. the differential pathlength factor (DPF) and the effective attenuation coefficient (µeff). Differences in the behavior of the optical changes were observed when comparing group-averaged data versus single datasets: no clear overall trend was presented in the group data, whereas a clear long-term trend was visible in almost all of the single measurements. Interestingly, the changes in [Formula: see text] statistically significantly correlated with µa, positively in the PFC and negatively in the VC. Our analysis demonstrates that all optical brain tissue properties (µa, [Formula: see text], µeff and DPF) change during these functional neuroimaging experiments. The change in [Formula: see text] is not random but follows a trend, which depends on the single experiment and measurement location. The change in the scattering properties of the brain tissue during a functional experiment is not negligible. The assumption [Formula: see text] ≈ const during an experiment is valid for group-averaged data but not for data from single experiments.

Liquid Blood Phantoms to Validate NIRS Oximeters: Yeast Versus Nitrogen for Deoxygenation.

Liquid blood phantoms are a tool to calibrate, test and compare near-infrared spectroscopy (NIRS) oximeters. They comprise a mixture of saline, blood and Intralipid, which is subsequently oxygenated and deoxygenated to assess the entire range of tissue oxygen saturation (StO2) from 0% to 100%. The aim was to investigate two different deoxygenation methods: yeast versus nitrogen (N2) bubbling. The phantom was oxygenated with pure O2 in both experiments, but deoxygenated by bubbling N2 in the first and by addition of yeast and glucose in the second experiment. A frequency domain NIRS instrument (OxiplexTS) was used as reference and to monitor changes in the reduced scattering coefficient (µs') of the phantom. Both deoxygenation methods yielded comparable StO2 values. The deoxygenation was slower by a factor 2.8 and µs' decreased faster when bubbling N2. The constant bubbling of N2 mechanically stresses the Intralipid emulsion and causes a decrease in µs', probably due to aggregation of lipid droplets. Deoxygenation by N2 requires a more complex, air tight phantom. The gas flow cools the liquid and temperature needs to be monitored more closely. Consequently, we recommend using yeast for phantom deoxygenation.