The relationship between the spontaneous slow cortical potential shifts and the detection of visual stimuli at sensory threshold were investigated. The mean slow cortical potentials preceding the detected stimuli were more negative than those preceding the missed stimuli. Accordingly, the stimulus detection performance was higher during negative compared to positive cortical potential shifts. These findings demonstrate that the cortical negativity reflects increased excitability of neural networks, thereby facilitates the detection of threshold stimuli, in contrast to cortical positivity. Therefore, at near-threshold stimulus intensities one reason for detecting the stimulus in one trial but missing it in another could be the change in the EEG baseline between the trials. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.