Patients diagnosed with major depressive disorder have a 31 percent greater than average number of neurons in the thalamic regions involved with emotional regulation and the regions are physically larger as well, according to an article in the July issue of the American Journal of Psychiatry. Researchers also discovered that this portion of the thalamus is also physically larger than normal in affected people.

The findings are the first to directly link a psychiatric disorder with an increase in total regional neurons, said Dr. Dwight German, lead author of the study.

“This supports the hypothesis that structural abnormalities in the brain are responsible for depression,” he said. “Often people don’t understand why mentally ill people behave in odd ways. They may think they have a weak will or were brought up in some unusual way.

“But if their brains are different, they’re going to behave differently. Depression is an emotional disorder. So it makes sense that the part of the brain that is involved in emotional regulation is physically different.”

Four groups were represented in the study: people with major depression, bipolar disorder, or schizophrenia, as well as a comparison group with no history of mental illness. Brain specimens were provided by the Stanley Foundation Brain Bank, which collects donated postmortem brains for research on mental illness, and the subjects were matched according to age, gender, brain weight, and other variables.

The American researchers used special computer-imaging systems to meticulously count the number of neurons in the thalamus. There was an increase of 37 percent and 26 percent, respectively, in the number of neurons in the mediodorsal and anteroventral/ anteromedial areas of the thalamus in subjects with major depressive disorder when compared with similar cells in people with no psychiatric problems. The numbers of neurons in people with bipolar disorder and schizophrenia were normal.

Researchers also found that the size of the affected areas of the thalamus in subjects with history of major depressive disorder was 16 percent larger than those in the other groups.

“The thalamus is often referred to as the secretary of the cerebral cortex ? the part of the brain that controls all kinds of important functions such as seeing, talking, moving, thinking and memory,” German said. “Most everything that goes into the cortex has to go through the thalamus first.

“The thalamus also contains cells that are not involved with emotion. Our studies found these portions of the thalamus to be perfectly normal. But the ones that are involved in emotion are the ones that were abnormal.”

Researchers also looked at the effect of antidepressant medications on the number of neurons and found no significant difference among any of the subject groups ? whether they had taken antidepressants or not ? reinforcing the belief that abnormalities in brain development are responsible for depression.