Dependence of Cerebral Oxygenation and Task Performance on Coloured Light Exposure and Chronotype: Blue and Red Do...

In our previous studies, we investigated the right-left asymmetry (RLA) of cerebral tissue oxygenation (StO₂) at rest in humans and the influence of the individual chronotype (i.e. individual chronobiological disposition) on StO₂. The aim of the current study was to investigate (i) whether the RLA exists during a cognitive task and coloured light exposure (CLE), and (ii) how changes in StO₂ induced by CLE and cognitive performance during a 2-back task are related to the subject's chronotype.

36 healthy subjects (22 female, 14 male, age 26.3 ± 5.7 years) were studied twice on two different days. They were exposed to a sequence of blue followed by red light or vice versa in a randomised crossover study design. During CLE, subjects were asked to perform a 2-back task. We measured StO₂ of the right and left prefrontal cortex (PFC) as well as the right and left visual cortex with functional near-infrared spectroscopy (fNIRS). At the behavioural level, we recorded the number of correct and incorrect answers given by the subjects. The chronotype was determined with the Horne and Östberg morningness-eveningness questionnaire.

(i) We found that the blue and red light caused a RLA in the PFC. For red light exposure, the 2-back performance was negatively correlated with StO₂ in the right PFC (r = -0.283, p = 0.016), and for blue light, exposure in the left PFC (r = -0.326, p = 0.005). (ii) 83% of subjects who performed the 2-back task at their optimal time of day according to their chronotype showed increased and higher changes in StO₂ (ΔStO₂ > 1%) compared to subjects who did not exercise at their optimal time of day. (iii) No correlation was found between chronotype and 2-back task performance (red: p = 0.38; blue: p = 0.42).

We found for the first time that blue and red light exposure target different regions of the PFC during performance of a 2-back task, which can be explained by the approach and withdrawal model. These results illustrate that studying the subregions (i.e. right, left, and even centre) of the cortex provides a better understanding of the CLE effects in the human brain. Our study also shows that individual chronotype plays an important role in the individual changes in StO₂ induced by CLE.