Washington, Feb 10 (IANS) Depression does not always lead to failing memory or difficulty in concentrating and paying attention, research says.

A review of nearly 20 years of literature on depression was conducted by researchers from the University of Texas Southwestern Medical Centre (UTSWMC).

'The relationship between cognition - thinking, attention and memory - and depression remains poorly understood from a neuro-scientific standpoint,' said Munro Cullum, head of psychology at UTSWMC and a study co-author.

'Our study challenges some of the clinical myths about the effects of depression on cognitive functioning,' Cullum said.

The effects of depression on cognition may vary in patients, said the assistant professor of psychiatry at UTSMC, Shawn McClintock, who led 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,' he added.

The researchers examined 35 studies, published between 1991 and 2007, investigating links between severe depression in patients and specific impairment 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. Some suggested cognitive difficulties; others said there were none,' McClintock said.

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.

The findings appeared in the January issue of Neuropsychology, a journal of the American Psychological Association.