Longitudinal changes in critical flicker fusion frequency: an indicator of human workload.

The aim of the study was to verify a hypothesis that critical flicker fusion frequency (CFFF) is season-related because of changes in the intensity and duration of sunlight. CFFF has a wide application in different fields, generally as an indicator of human workload. Therefore, confirmation of previous observations that CFFF is subject to a seasonal rhythm could make theoretical as well as practical sense. The analysis was based on data obtained from 23 male participants, aged 28-60 years. The study lasted 12 months. CFFF was measured with the flicker test. The results of statistical analysis indicate that CFFF changes over time. There are two forms of those changes: a linear trend and oscillations around the trend. Moreover, individual differences in the longitudinal course of CFFF (in trends as well as in oscillations) are observed. The conclusion is that CFFF is subject to longitudinal changes, whose pattern depends on individual differences. Additionally, the individual character of the CFFF changes over time allow the assumption that sunlight is probably not as significant a factor as expected, one that can modify the hypothetical pattern of season-related CFFF fluctuation. The obtained results are important for the methodology of experiments with CFFF and for the interpretation of the results of the flicker test.