A clear difference between active drug and placebo is not always found when using the ascending and descending thresholds of the critical flicker fusion test (CFF). We selected a test requiring attention and concentration and performed again the CFF after this 6-minute concentration-test (reactive CFF). This reactive CFF was then used in a double-blind study in 30 ambulant patients with cerebro-vascular arteriosclerosis treated for 8 weeks with 80 mg/day cervoxan or placebo. After the treatment period the reaction of the CFF after the 6-minute concentration-test demonstrated a significantly more frequent improvement of the vigilance in the Cervoxan group compared to the placebo group.