The critical flicker fusion frequency: illumination matters?

Introduction: The critical flicker fusion frequency (cFFF) refers to the frequency at which one perceives a regularly recurring change of light stimuli as steady. In diving and hyperbaric medicine, cFFF was utilized to assess alertness and cognitive functions. However, the role of confounders like inspiratory oxygen partial pressure (PIO2) and different illumination levels is still unclear. PIO2 increases with depth, O2 fraction, and high pressures are common for Nitrox- and rebreather diving. On the other hand, open-water diving involves reduced lighting conditions with possible effects on cFFF over the time course of the dive.
Methods: (1) cFFF was measured in the morning for 85 consecutive days in the same individual in artificial light, sunlight, or cloudy conditions. (2) 19 divers were investigated on the effects of wearing a mask. (3.1) In a group of 25 divers, cFFF measurements were conducted in two settings: at the poolside during daylight and in a darkened setting in 4 m water submersion. During the ‘darkness’ measure¬ments, participants looked into a tube (25 cm; length: 1.5 m) from one side, while on flicker device was placed on the other side.(3.2) All measurements were repeated after 10 min of breathing 100 % oxygen, simulating the established 1.4 bar PIO2 limit for OC and CCR diving.
Heart rate (HR), HR variability (Polar V800 and Kubios HRV, ver. 3.5.0, respectively), and ventilation (V’E) were assessed throughout the dive to explore potential interactions between cFFF and the autonomous nervous system.
Statistics: Results: (1) Depending on the lighting conditions, cFFF values were 43.1±1.5 Hz, 44.0±1.7 Hz, and 43.2±1.9 Hz under artificial light, sunlight, and cloudy conditions, respectively. (2) No significant difference was observed between wearing a mask or not (35.6 Hz vs. 36.4 Hz). Note: (3.1) and (3.2) require further completion.
Discussion: When utilizing the cFFF, the effects of illumination and PIO2 need to be taken into account for proper interpretation. During dark adaptation, the eye adjusts from a bright environment to a dark one: the cones are relieved of their duty, and the rods take over. As a result, the cFFF decreases significantly. However, in darkness, the pupil dilates, allowing more light to enter the eye, which, in turn, raises the cFFF threshold.