The effects of changes in frequency spectrum of the EEG on the detection of low-intensity visual stimuli near sensory threshold and on resulting visual event-related potentials (ERPs) were investigated on twelve healthy volunteers. The stimulus intensity, at which each subject could perceive 50% of the presented stimuli, was defined as the sensory threshold for that subject. Single ERP epochs were divided into two groups: epochs during detected and undetected stimuli. The ERPs and power spectra of the 1 s pre-stimulus periods were computed for both conditions, and the P300 amplitudes of the ERPs, and total power and relative band powers of the delta (0.5-4 Hz), theta (4-7.5 Hz), alpha (7.5-13 Hz), beta (13-30 Hz), and gamma (30-70 Hz) frequency bands of the pre-stimulus power spectra were measured. Between the two conditions, a specific difference was observed in the relative power of the alpha band, which was significantly lower in trials with detected stimuli (p<0.01) in line with significantly higher amplitudes of the ERPs (p<0.001). These results suggest that small changes in the brain’s excitability state are reflected specifically in the relative alpha power of the EEG, which may explain significant variability in perceptual processes and ERP generation especially around boundary conditions such as sensory threshold.