Depression. Coloured Positron Emission Tomography (PET) scan of the brain of a patient suffering depression. The left-side of the brain is seen, in external view. Colour-coded regions highlighted in green/red depict areas of low activity associated with depression. These regions are the prefrontal cortex (at left) and parieto-temporal areas (at right). There has been a decrease in blood flow to these regions. In patients treated for depression, metabolic activity and blood flow resumes to a healthy state in these affected areas. Depression is a disorder of varying severity resulting in sadness, hopelessness, and loss of concentration.