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Depressed brains are less 'plastic'

Plastic brains The brains of people with depression show a reduced ability to adapt to their environment, a unique study shows.

The research on 'neuroplasticity' and depression, is a collaboration between the Black Dog Institute and Neuroscience Research Australia and is reported online in a recent issue of Neuropsychopharmacology.

"We have demonstrated plasticity is reduced when you're depressed," says one of the lead researchers, Professor Colleen Loo, from the School of Psychiatry at the University of New South Wales, and colleagues.

While it was once thought the mature brain was fixed and unable to repair itself, scientists now know the brain is plastic and changes throughout our life.

"As you learn something, like a sequence of notes on the piano, the part of your brain that codes for that behaviour actually changes over minutes as you are learning," says Loo.

"Then as you forget that, that part of the brain shrinks again. It's part of the normal function of the brain that it is constantly adapting to its environment."

This "neuroplasticity" involves the growth of new neurones and the branching out of existing ones to make new connections. This is a key component of learning and memory.

Downside of plasticity

The downside of having a plastic brain, however, is that it can also shrink as well as grow, says Loo.

Some studies show people who are depressed don't do as well at learning and recalling tasks as healthy people, and other studies point to this being linked to shrinkage in parts of the brain.

Because learning requires neuroplasticity, this has suggested that depressed people have less plastic brains, says Loo.

But, she adds, apart from neuroplasticity, learning also requires effort, so how can researchers be sure that depressed people are just less motivated to learn?

In order to separate out the effects of effort and neuroplasticity, Loo and colleagues carried out a unique experiment.

They used magnetic stimulation to induce short-term increases in responsiveness in the brain, and to test the magnitude of these changes by measuring tiny electrical potentials in a hand muscle of study participants.

How much the brain changed in responsiveness was a measure of the brains' neuroplasticity.

Magnetic stimulation

The study involved 23 people who had had clinical depression for some months, and 23 controls matched for age and gender.

After 13 minutes of magnetic brain stimulation, the researchers observed the expected neuroplasticity in healthy participants, but not in those with depression.

Loo says this is one of the first objective tests to show that depression is linked to decreased neuroplasticity, which is not confounded by how much effort the person made.

She and colleagues also tested the study participants' ability to learn a new motor skill, which was to use a computer mouse to keep an arrow inside a circle on a screen.

"We found the learning slope was not as good in people who were depressed compared with people who were healthy," says Loo.

New depression treatment

Loo says while the findings are a "bit pessimistic", other more hopeful findings suggest treatment for depression can improve neuroplasticity.

She and colleagues treated some study participants with a new technique called transcranial direct current stimulation (tDCS), which passes a tiny electrical current through the brain.

Preliminary findings suggest not only does this help their depression, but a repeat of the same neuroplasticity tests at the end of the treatment showed they behaved like the healthy controls.

"After four weeks of this treatment the depressed people got better in their mood and their plasticity returned to normal levels," says Loo.

The results, which have been submitted for publication, build on previous reports suggesting tDCS helps improve learning, concentration and memory.

Loo says it would be interesting to test whether psychological therapy and anti-depressants also have the same effect on neuroplasticity.