Its a kind of neural exercising, called kindling . It happens when we learn. It happens in epilepsy. It even happens in meditation when a person learns to choose the meditative state of consciousness instead of the more normal ones. If the ones on the left are selected, then a person will be more likely to respond to a neutral event with a positive emotion, and to anticipate that it will have a positive result.
Depression invites a negative emotion and expectation in response to almost everything that happens.
T he brain has hundreds of structures, all interconnected with one another. When the frontal lobes and the amygdala increase their connectivity, all the rest of the related structures need to adjust their output to compensate. The baselines of activity that influence depression involve lots of different structures, and all of these need to adjust. In order to accommodate the many other structures involved, the sessions have to be spread out over time. S o far, so good.