And what process/es do they go through?

I think we have to agree about the term insanity first before we see which part. The question is too general I think.

Islamic jurisprudence equalizes between insanity and absence of choice, on this basis it can be said that failure of executive functions or its reference to previous experience makes one insane with no respect to the resultant behavior. The prefrontal cortex plays a major role, also the anterior cingulate is particularly involved in choice, but one single aspect i learned from neuroscience; localization is misleading and most of time you have to deal the whole system.

But after all, this may not exactly matches the term because the term as I think is describing a behavior more.


Wafa

In general the mesolimbic system.

Dopamine plays a role- you can get people acutely psychotic on dopamine agonists, you treat psychosis with dopamine antagonists
Glutamate plays a role- you can also make people psychotic by screwing with a part of the major excitatory transmitter system of the brain (NMDA antagonists)
Stress plays a role- you can make people psychotic if you stress them enough

it's general and uncertain question, so we cannot clarify the meaning of this term unless we understand what we're talking about.
Sorry (

also degeneration in the frontal lobes such as in Pick's Disease can make a person disinhibited, sexually inappropriate, labile, euphoric