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

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.

The Role of Systemic Physiology in Individual Hemodynamic Responses Measured on the Head Due to Long-Term Stimulation Involving Colored Light Exposure and a Cognitive Task: An SPA-fNIRS Study

In our previous investigations using systemic physiology augmented functional near-infrared spectroscopy (SPA-fNIRS) neuroimaging, we found larger variability between subjects in changes of cerebral hemodynamics and oxygenation induced by an intricate experimental paradigm involving colored light exposure and a cognitive task. We aimed to investigate the role the activity of the systemic physiology has on individual variations in the fNIRS data. Thirty-two healthy subjects (17 female, 15 male and age: 25.5 ± 4.3 years) were exposed to blue and red light for 9 min (colored light exposure, CLE) while performing a verbal fluency task (VFT). We found that (i), at the group level, the visual cortex showed a stronger deoxyhemoglobin concentration response during blue light exposure than during red light exposure, and (ii) this relationship was influenced by individually different baseline blood pressure values. Furthermore, we found other correlations between changes in fNIRS signals and changes in systemic physiology. Our study demonstrates the usefulness and necessity of the SPA-fNIRS approach to gain insights into the individual variability of hemodynamic responses measured with fNIRS, especially in the case of an intricate experimental paradigm (i.e., CLE-VFT) as used in our study.

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.