Critical flicker fusion frequency measurement through a chamber porthole

The critical flicker fusion frequency (cFFF) is a non-invasive measure of central nervous system function and cortical arousal, increasingly used in diving and hyperbaric medicine to assess the effects of breathing gases under pressure. This feasibility study aimed to evaluate whether cFFF can be reliably measured through the porthole of a hyperbaric chamber. Forty-five experienced male divers underwent cFFF testing at various pressures (101.3 kPa outside chamber, then 101.3, 608, 132, 101.3 kPa inside [1.0 bar outside then 1.0, 6.0, 1.3, 1.0 bar inside]) using a manually operated LED flicker-device while standing at a fixed distance from the chamber window. Results showed that cFFF values were higher inside the chamber at 101.3 kPa (1.0 bar) compared to outside (45.6 Hz vs. 40.2 Hz), decreased under hyperbaric conditions (608 kPa [6 bar], 43.5 Hz), and declined further during decompression (132 kPa [1.3 bar], 42.1 Hz; 101.3 kPa [1.0 bar], 43.5 Hz). These findings support previous observations of gas-induced central nervous system effects and highlight the sensitivity of cFFF to pressure-related neural changes. The successful external measurement protocol addresses challenges associated with observer narcosis and movement artifacts in underwater settings. While limited by the homogenous participant group and lack of confirmatory measures, this approach may still be a valuable tool for future research into the temporal dynamics of gas narcosis and cortical excitation.