The autocorrelation (AC) of the fMRI signal is assumed irrelevant to the brain function and is eliminated by fMRI preprocessing. Recent findings have suggested that the brain function may alter the AC value of the fMRI signal. We used fMRI data acquired during cognitive processes of working memory (WM), mathematical computations, video watching, and resting-state, and showed that cognitively demanding tasks decrease the AC values in functionally related brain regions. Decrease in AC is related to performance on the target tasks. The AC of the fMRI signal is affected by cognitive processes and can provide complementary information about brain function.