A new study published in the journal Current Biology Thursday found that a single dose of some of the most widely used antidepressants change the brain’s architecture after only a few hours.

However, patients who take drugs to treat depression usually don’t report any improvement until weeks later, the L.A. Times reports.

Researchers hope their findings will allow doctors to determine whether or not a patient will respond to certain psychiatric drugs by way of a brain scan.

Low levels of the chemical transmitter serotonin in the brain are associated with depression. Antidepressants work by blocking how serotonin is reabsorbed into the brain.

The drugs are known as Selective Serotonin Reuptake Inhibitors, or SSRIs.

To conduct the study, researchers compared connections in the brain’s gray matter of patients who took antidepressants and those who did not, the Times reports.

“We just tell them to let their minds wander and not think of anything particularly dramatic or upsetting,” said neuroscientist Dr. Julia Sacher, a co-author of the study.

After mapping the connections researchers found that when more serotonin was present in the gray matter it meant a decrease in the brain’s functional connectivity.

But some areas of the brain didn’t follow this pattern.

“It was interesting to see two patterns that seemed to go in the opposite direction,” Sacher said. “What was really surprising was that the entire brain would light up after only three hours. We didn’t expect that.”

Sacher said more research was needed, but the findings of the study could help to tailor individual therapy for those suffering from depression.

[L.A. Times]

Write to Helen Regan at helen.regan@timeasia.com.