Investigating the biological impact of light on brain function using 7T ultra-high field MRI

Lighting environments can influence our health and wellbeing with several health issues being associated with aberrant lighting. The human retina detects light through two different pathways, the classical visual system, which is required for visual formation, and the non-image forming (NIF) system, which detects environmental irradiance. The NIF system is maximally sensitive to the shorter blue wavelength visible light (~480nm) and mediates light influence on several circadian, neuroendocrine, and neurobehavioral functions. Electrical lighting environments have evolved over the decades with the development of incandescent and fluorescent lighting and now on to the increasingly more energy-efficient light-emitting diode (LED) bulbs. The change in electrical lighting has also come with a change in the emission spectrum of our indoor light environments, which are becoming more blue-light heavy due to common ‘white’ LEDs being a blue-enriched light source with a peak around 440–460 nm.
Knowledge of the NIF system coupled with advancements in LEDs led to the emerging idea of integrative lighting. Integrative lighting would consider the visual acuity and biological effects of light. To ensure integrative lighting benefits human health and wellbeing and doesn’t exacerbate light misuse, fundamental questions about how light influences cognitive and emotional functioning need to be addressed.
The cortical circuitry underlying light’s stimulating impact is not established in humans. To
gain a deeper understanding of how the NIF impact of blue-enriched light may influence the subcortical areas of the brain, we utilised two techniques, ultra-high-field 7 Tesla (7T) functional magnetic resonance imaging (fMRI) and infrared eye tracking equipment. Young healthy adults completed an fMRI protocol with simultaneous eye tracking whilst exposed to the light of various illuminance and engaged in auditory cognitive tasks.
In my thesis, we first aimed to characterise the task-evoked pupil response (TEPR) associated with auditory inputs under different light levels. The analysis showed that even with a smaller sustained pupil size during brighter light blocks, a higher light level triggers a stronger TEPR to auditory stimulation. We presume this is through the recruitment of the locus coeruleus, a brainstem region that may be involved in some of light's NIF functions.
Secondly, we analysed if there were regional differences in light illuminance across the human hypothalamus whilst engaged in cognitive tasks. The findings reveal that there were distinct response dynamics to increasing illuminance across the hypothalamus. Specifically, higher illuminance triggered an increase in activity over the posterior part of the hypothalamus and the opposite was observed in the anterior and ventral parts of the hypothalamus. The nuclei comprising the posterior part of the hypothalamus may be among the key initial sites of the stimulating impact of light on human cognition and alertness, putatively through orexin and histamine signalling.
Thirdly, we analysed if there were regional differences in illuminance across the human
amygdala whilst engaged in an emotional task. The findings reveal that there were distinct
response dynamics to increasing illuminance across the amygdala. We found that the medial nucleus and other medial/superior parts of the amygdala show a marked reduction of activity under higher illuminance when processing emotionally charged stimuli. These findings provide additional insight into the mechanisms that may underlie the benefits of light therapy.
Overall, the thesis highlights part of the brain circuitry underlying the impact of blue-enriched light on (cognitive) brain functions. The work emphasises that a better understanding of how light impacts cognitive and emotional functioning will help to achieve integrative light solutions to benefit health and wellbeing in the future.