A slow brain potential change, the contingent negative variation (CNV), was recorded in a temporal generalization schedule. The task was to judge the duration of a signal (1.250 to 3.125 s) as being equal or not to that of a 2-s target that had been previously memorized. Two signal modalities, visual and tactile, were contrasted in distinct trial blocks, in order to explore possible localization differences. Significant results were found at CPz, irrespective of signal modality. The CNV that developed during signal presentation peaked around 2 s and then declined when the current signal was longer than the 2-s target, instead of peaking at signal extinction as was the case for shorter signals. Thus, for signals longer than the target, the CNV peak and the following slope change provide a memory trace of the encoded target duration, leading to decision making.