Dependence of Cerebral Oxygenation and Task Performance on Coloured Light Exposure and Chronotype: Blue and Red Do Not Have the Same Effects on the Prefrontal Cortex.

Abstract

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 lightexposure 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.