Depression and lack of concentration do not necessarily go together, researchers at the UT Southwestern Medical Center have found.
The study team reviewed nearly 20 years of literature to come up with their conclusions.
The study has appeared in the January issue of Neuropsychology, a journal published by the American Psychological Association.
Dr. Munro Cullum, senior author of the study and chief of psychology at UT Southwestern, said: "The relationship between cognition - thinking, attention and memory - and depression remains poorly understood from a neuroscientific standpoint.
"This paper represents an important review of the literature that challenges some of the clinical myths about the effects of depression on cognitive functioning."
Lead author and assistant professor of psychiatry at UT Southwestern Dr. Shawn McClintock said: "The presentation of depression can vary between people.
"Many symptoms can be used to diagnose depression, so we tried to dissect and better understand how specific factors in depression might contribute to cognitive difficulties."
The researchers analysed 35 studies published between 1991 and 2007 that investigated links between depression severity in patients and specific impairments in their cognition. The areas of cognition included processing speed, attention, memory, language abilities and executive functioning.
Dr. McClintock said: "We found a lot of variability between studies that were conducted...Some suggested cognitive difficulties; others said there were none."
The research found that processing speed - the ability of an individual to quickly take in information, process and act upon it - was the cognitive function most often affected by depression. The capability slows when some individuals are depressed, it was discovered. The link was not as clear for other types of cognitive abilities, including attention, concentration, memory and executive function.
Researchers found that part of the variability in the literature may be due to inconsistent measurement and diagnosis of depression among studies.
Dr. McClintock said: "The research has not been the most rigorously controlled."
The researchers recommend that more detailed information should be collected about each depressive episode, such as its duration and intensity.
Dr. McClintock said: "If we do this, clinicians can help a depressed patient with processing speed deficits by decreasing the amount of information a patient has to process at one time, while researchers could work out nuances to discover if we can target cognitive deficits and improve them.
"Research for the past few decades has been very beneficial, but it has actually provided more questions than answers. We need to take the heterogeneous, nuanced concept of depression and better characterize it, so we can refine future investigations and guide clinical practice."