Scientists have uncovered the way a controversial treatment for severe depression could work for the first time.
The researchers claimed electroconvulsive therapy could change the way different parts of the brain involved in depression communicate with each other.
Electroconvulsive therapy, or ECT, involves anaesthetising a patient and electrically inducing a seizure.
Despite being used successfully in clinical practice around the world for more than 70 years, the underlying mechanisms of ECT have so far remained unclear.
But a multidisciplinary team of clinicians and scientists at the University of Aberdeen has shown for the first time that ECT could work by "turning down" an overactive connection between the parts of the brain that govern mood, and those responsible for thought and concentration.
Researchers examined the impact of ECT on brain connectivity in six male and three female patients from the Royal Cornhill Hospital in Aberdeen who were successfully treated for severe depressive disorder.
All patients had failed to respond to chemical antidepressants and underwent ECT twice weekly - an average of eight treatments - until their symptoms abated.
MRI scans were used to examine the brains of the patients before and after ECT, and then the experts applied a "new and complex" mathematical analysis to investigate brain connectivity.
In the paper, published in the journal Proceedings of the National Academy of Sciences, the researchers said the decrease in connectivity observed after ECT treatment was accompanied by a significant improvement in the patient's depressive symptoms.
Professor Ian Reid, professor of psychiatry at the University of Aberdeen and consultant psychiatrist at the Royal Cornhill, led the study.
He said: "ECT is a controversial treatment, and one prominent criticism has been that it is not understood how it works and what it does to the brain.
"However, we believe we've solved a 70-year-old therapeutic riddle because our study reveals that ECT affects the way different parts of the brain involved in depression connect with one another.
"For all the debate surrounding ECT, it is one of the most effective treatments not just in psychiatry but in the whole of medicine, because 75% to 85% of patients recover from the symptoms.
"Over the last couple of years there has been an emerging new perspective on how depression affects the brain.
"This theory has suggested a 'hyperconnection' between the areas of the brain involved in emotional processing and mood change and the parts of the brain involved in thinking and concentrating.
"Our key finding is that if you compare the connections in the brain before and after ECT, ECT reduces the connection strength between these same areas - it reduces this hyperconnectivity.
"For the first time we can point to something that ECT does in the brain that makes sense in the context of what we think is wrong in people who are depressed.
"As far as we know no-one has extended that 'connectivity' idea about depression into an arena where you can show a treatment clearly treating depression, changing brain connectivity.
"And the change that we see in the brain connections after ECT reflects the change that we see in the symptom profile of patients who generally see a big improvement."
Researchers now hope to continue monitoring the patients to see if the depression and hyperconnectivity returns. They also want to compare their ECT findings with the effects of other therapies used to treat depression such as psychotherapy and antidepressants.
The study was funded by the Chief Scientist Office.