In the early 1980s, research began into the use of PET in functional neuroimaging in humans. In most studies, increases in activity were observed during task conditions. A meta-analysis revealed a characteristic network of brain areas that showed a decrease in activity during attention-demanding tasks. This discovery was surprising as this network had not previously been recognised as a system. The activity patterns of this network were observed in both PET and fMRI studies. The significance of this network and its function in the brain is the subject of further research - but to summarise, the default mode network is active when the brain is at rest.