Critical flicker fusion frequency assessment: robustness across photopic illumination in experienced divers.

INTRODUCTION: Critical flicker fusion frequency (cFFF) is a rapid, noninvasive index of cortical arousal and visual-cognitive performance that has gained renewed interest in diving and other extreme environments. However, the extent to which ambient illumination and water submersion affect cFFF remains insufficiently characterized. METHODS: Nineteen experienced divers (10 females; 24 ± 3 years) completed cFFF testing on land and at 5 m water depth under bright (land: 138 ± 2.4 lx; water: 75 ± 14.8 lx) and dark (0 lx) conditions using a manually operated flicker device. Heart rate (HR) and heart rate variability (HRV) were recorded continuously. RESULTS: For cFFF, mixed-effects ANOVAs revealed a significant main effect of submersion (p=0.0485) but not illumination (p=0.0754), with higher values in submersion compared to land (+1.4 Hz). Heart rate variability showed a main effect for submersion (p=0.004, +30 ms in submersion) and posthoc differences in dark illumination (p=0.004, +41 ms in submersion). No effects were observed for heart rate or respiration rate. Age showed a low negative correlation with cFFF (r = -0.22 to -0.40) but not with heart rate or heart rate variability. Sex effects were negligible (|r|<0.32). DISCUSSION: Based on the current 10-minute exposure time, cFFF is robust between the applied illuminations, but small absolute effects were observed from submersion. For human performance and diving research, these findings highlight the importance of baseline measures under comparable environmental conditions to prevent obscuring genuine changes in alertness or cognitive performance. Exposure time and ambient illumination should be standardized or reported to improve comparability across studies and operational environments.