Effect of short-term colored-light exposure on cerebral hemodynamics and oxygenation, and systemic physiological activity. Scholkmann et al, Neurophotonics, 4(4), Nov. 2017, full paper

Abstract
There is not yet a comprehensive view of how the color of light affects the cerebral and systemic physiology in humans. The aim was to address this deficit through basic research. Since cerebral and systemic physiological parameters are likely to interact, it was necessary to establish an approach, which we have termed “systemic-physiology-augmented functional near-infrared spectroscopy (SPA-fNIRS) neuroimaging.” This multimodal approach measures the systemic and cerebral physiological response to exposure to light of different colors. In 14 healthy subjects (9 men, 5 women, age: 33.4±10.5
years, range: 24 to 57 years) exposed to red, green, and blue light (10-min intermittent wide-field visual color stimulation; 15×20s
blocks of visual stimulation), brain hemodynamics and oxygenation were measured by fNIRS on the prefrontal cortex (PFC) and visual cortex (VC) simultaneously, in addition with systemic parameters. This study demonstrated that (i) all colors elicited responses in the VC, whereas only blue evoked a response in the PFC; (ii) there was a color-dependent effect on cardiorespiratory activity; (iii) there was significant change in neurosystemic functional connectivity; (iv) cerebral hemodynamic responses in the PFC and changes in the cardiovascular system were gender and age dependent; and (v) electrodermal activity and psychological state showed no stimulus-evoked changes, and there was no dependence on color of light, age, and gender. We showed that short-term light exposure caused color-dependent responses in cerebral hemodynamics/oxygenation as well as cardiorespiratory dynamics. Additionally, we showed that neurosystemic functional connectivity changes even during apparently stress-free tasks—an important consideration when using any of the hemodynamic neuroimaging methods (e.g. functional magnetic resonance imaging, positron emission tomography, and fNIRS). Our findings are important for future basic research and clinical applications as well as being relevant for everyday life. ....

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Conclusions and Outlook
In conclusion, this SPA-fNIRS study demonstrates a wide variety of effects evoked by colored light: (i) the cerebral response varies with region. While the VC reacted to all colors, the PFC responded significantly only to blue light, which was significantly different to red and green light; (ii) a color-dependent effect on the cardiorespiratory activity was observed for PRQ (blue: decrease), HF (red: increase, blue and green: decrease), LF (green: decrease, red and blue: increase), LF/HF (red and blue: increase, green: decrease); (iii) the magnitude of hemodynamic responses in the PFC and the VC were gender- and age-dependent; (iv) gender and age effects were visible in the cardiorespiratory system: gender and age affected RR and PRQ; age affected LF, LF/HF, and MWA (LPFC); gender affected PETCO2
, PP, Q, and MWA (RPFC); and (v) EDA and psychological state did not show significant changes. Thus, we were able to demonstrate responses in cerebral hemodynamics/oxygenation as well as cardiorespiratory dynamics that were color dependent. Blue light was found to elicit the strongest effect overall.
Our findings contribute to a better understanding of the effects of colored light on human physiology. This is relevant both for medical research and everyday life. Colored light has already been employed as a treatment modality in medicine. This information opens opportunities to specifically treat diseases, using colored light as a potentially powerful tool for future medicine. In addition, we are increasingly exposed to colored light in everyday life (low-energy light bulbs and screens), whose potential for disease-promoting effects should be considered (recent reviews: Refs. 159160.–161). This is a topic for urgent further investigation.”
The significant correlations between cerebral and systemic parameters demonstrate a neurosystemic functional connectivity, which has in most incidences been neglected so far. The presence of this neurosystemic functional connectivity should be taken into consideration in future neuroscientific studies and could constitute a paradigm shift. SPA-fNIRS should become a standard approach for functional neuroimaging experiments using fNIRS because even our apparently nonstrenuous stimulation of colored light significantly altered the human systemic physiology. Mentally or physically strenuous tasks are expected to elicit even more profound systemic effects (whereby the strength of these systemic effects is expected to be related to the exact kind of tasks/stimulations performed and the individual physiology), which are likely to affect cerebral hemodynamics. These effects should be explored further because the concept that cerebral hemodynamic changes are purely associated with brain activity is probably wrong. This is not only true for fNIRS but also for other hemodynamic neuroimaging methods including the most widely used neuroimaging modality fMRI.

learn more/full paper: https://www.spiedigitallibrary.org/journals/Neurophotonics/volume-4/issue-4/045005/Effect-of-short-term-colored-light-exposure-on-cerebral-hemodynamics/10.1117/1.NPh.4.4.045005.full?SSO=1

Neurophotonics covers advances in optical technology applicable to study of the brain and their impact on the basic and clinical neuroscience applications.