"The relationship between cognition -- thinking, attention and memory -- and depression remains poorly understood from a neuroscientific standpoint," said Dr. Munro Cullum, chief of psychology at UT Southwestern and senior author of the review appearing in the January issue of Neuropsychology, a journal published by the American Psychological Association. "This paper represents an important review of the literature that challenges some of the clinical myths about the effects of depression on cognitive functioning."

Part of what contributes to the clinical lore is that difficulties in concentrating can be a symptom of depression, and this may masquerade as other cognitive problems such as variability in memory performance.

"The presentation of depression can vary between people," said Dr. Shawn McClintock, assistant professor of psychiatry at UT Southwestern and lead author of the study. "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."

Just as a higher fever can indicate more-severe illness, researchers wanted to determine if more-severe depressive episodes led to a greater impairment of cognitive abilities. The reviewers examined 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.

"We found a lot of variability between studies that were conducted," Dr. McClintock said. "Some suggested cognitive difficulties; others said there were none."

In the research, processing speed was found to be the cognitive function most often affected by depression. Processing speed refers to an individual's ability to quickly take in information, process and act upon it. The capability slows when some individuals are depressed, the reviewers found. The link wasn't 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. Some studies diagnose depression using clinical research criteria, while others use depression severity scales.

"The research has not been the most rigorously controlled," Dr. McClintock said.

The review suggests that researchers need to collect more comprehensive neurocognitive assessment data in patients diagnosed with depression to minimize confounding factors such as age and education. The researchers also recommend that more detailed information be collected about each depressive episode, such as its duration and intensity.

"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," Dr. McClintock said.

"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."

Other UT Southwestern researchers involved in the study were Dr. Mustafa Husain, professor of psychiatry and internal medicine, and Dr. Tracy Greer, assistant professor of psychiatry.

Note: If no author is given, the source is cited instead.

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