Published in the upcoming early online edition of the Proceedings of the National Academy of Sciences (PNAS), the researchers Myrna Weissman, Ph.D and Bradley Peterson, M.D. found that people at high risk of developing depression had a 28 percent thinning of the right cortex, the brain’s outermost surface, compared to people with no known risk.
The drastic reduction surprised researchers, which they say is on par with the loss of brain matter typically observed in persons with Alzheimer’s disease and schizophrenia.
“The difference was so great that at first we almost didn’t believe it. But we checked and re-checked all of our data, and we looked for all possible alternative explanations, and still the difference was there,” said Dr. Peterson.
Dr. Peterson says the thinner cortex may increase the risk of developing depression by disrupting a person’s ability to pay attention to, and interpret, social and emotional cues from other people.
Additional tests measured each person’s level of inattention to and memory for such cues. The less brain material a person had in the right cortex, the worse they performed on the attention and memory tests.
The study compared the thickness of the cortex by imaging the brains of 131 subjects, aged 6 to 54 years old, with and without a family history of depression. Structural differences were observed in the biological offspring of depressed subjects but were not found in the biological offspring of those who were not depressed.
One of the goals of the study was to determine whether structural abnormalities in the brain predispose people to depression or are a cause of the illness. Dr. Peterson said, “Because previous biological studies only focused on a relatively small number of individuals who already suffered from depression, their findings were unable to tease out whether those differences represented the causes of depressive illness, or a consequence.”
The study found that thinning on the right side of brain did not correlate with actual depression, only an increased risk for the illness. It was subjects who exhibited an additional reduction in brain matter on the left side, who went on to develop depression or anxiety.
“Our findings suggest rather strongly that if you have thinning in the right hemisphere of the brain, you may be predisposed to depression and may also have some cognitive and inattention issues. The more thinning you have, the greater the cognitive problems. If you have additional thinning in the same region of the left hemisphere, that seems to tip you over from having a vulnerability to developing symptoms of an overt illness,” said Dr. Peterson.