Major Depressive Disorder
MDD is characterized by feelings of sadness or emptiness and diminished interest or pleasure in activities. These symptoms are accompanied by numerous concomitant features, including physiological and affective symptoms. In addition to these debilitating clinical symptoms, MDD is associated with deficits in multiple cognitive domains that tend to be mild to moderately severe—about 0.5 to 1.0 standard deviation below population mean. A metaanalysis by Veiel1 estimated prevalence rates for specific neurocognitive deficits in people with MDD. The study classified severe impairment (that is, 2 standard deviation units below the mean), for about 11% of people with MDD on verbal fluency, 18.2% on scanning and visual-motor tracking, 15% on visual-spatial skills, 14.5% on verbal learning, and 50% on executive functions, such as mental flexibility and control. It is important to bear in mind the considerable variability between studies in the severity of symptoms of the patients and in the neurocognitive test batteries used across samples.