Depressive Psychosis - Why does taking lamotrigine and amitriptyline help with depression?

I am a patient, no medical training, have been answering questions on Drugs.com for 10 years.

My theory after researching, reading and analyzing your question is the following:

I have read many medical terms, which if one is not a Neurologist/Psychiatrist it becomes quite complicated to explain and comprehend how the human brain works and the many different mental illnesses that exist and treatments used to treat the symptomatology of each one of these mental conditions.
To the point and simple.
You most likely are aware that Depressive Psychosis is a form of severe depression with hallucinations both seeing and hearing things that do not exist , so when you refer to Lamotrigine an anticonvulsant, among many of it´s off label uses it can treat the symptoms of Schizophrenia described by Drs. as a type of "psychosis" and then Amitriptyline a tricyclic "antidepressant" in the treatment of "depressive conditions", endogenous or "psychotic".
So one may be lead to believe that the combination of these 2 medications taken together "may" treat the symptoms of Depressive Psychosis.
There are no clinical trials nor evidence I can use as reference to back up my theory which I tried to keep simple and "logical".

The real deal:

Treating psychotic depression
Treatment for psychotic depression involves:

medicine – a combination of antipsychotics and antidepressants can help relieve the symptoms of psychosis
talking therapies – the 1-to-1 talking therapy cognitive behavioral therapy (CBT) has proved effective in helping some people with severe depression, including those with psychosis
social support – support with social needs, such as education, employment or accommodation
The person may need to stay in hospital for a short period of time while they're receiving treatment.

Electroconvulsive therapy (ECT) may sometimes be recommended if the person has severe depression and other treatments, including antidepressants, have not worked. The mental health charity Mind has more information on ECT.

Treatment is usually effective, but follow-up appointments so that the person can be closely monitored are usually required.

Source: NHS-National Health System, United Kingdom

I tried to keep it logical,

"The brain is neither analog nor digital, but works using a signal processing paradigm that has some properties in common with both. Unlike a digital computer, the brain does not use binary logic or binary addressable memory, and it does not perform binary arithmetic."