But then you have to ask, what caused this defect.
... a defective avenue... (For information I studied Neurobiology...) I don´t want to go into detail, well I couldn´t, because I am not into this topic. The only way you can really show if this "avenue" or better pathway shows a defect, is by doing electrophysical recordings on the brain... on the living brain, from a living person. Not a adequate way to get information about this "illness", by doing a surgery. And even with the not invasive techniques, that are used today, you can only say if a different part of the brain is activated or not... with a bad resolution also...
Maybe this avenue isn´t defective, but was inhibited... and later on the depression was leading to a degeneration of this pathway... hmmm...
Maybe something like a psychosomatic feedback-effect.
I don´t know any of this theories in detail, but I would prefer the "Cross Circuiting Theory". Maybe a problem during development of the neuronal circuits... I don´t know... No one knows... Most probably it is a mixture of nearly all theories, because they get many things in common. They only have different answers to what the reasons are for these phenomena...
Well it maybe is my prejudice, but many of the psychologist don´t know that much about Neurobiology and they don´t have to, because they see the effects changes on the neuronal level have on the behaviour of their patients...
WELL, isn´t this the Anathema-Forum, it feels like being somewhere else at the moment... It is somehow a bit off-topic. But interesting... so.