Question:
I was wondering if anyone (Viscount 11 or perhaps Shukoku) have ever done any research or reading that implicates an association between closed head injury/trauma and the onset mood disorders (sudden or otherwise)?

I find the possibility of closed head injury/trauma having a cause - effect relationship to the onset of mood disorders fascinating and worthy of discussion.

Answer: Here's my version of the answers to your questions about closed head injury.

Concussion means head impact resulting in confusion, amnesia or loss of consciousness. There may or may not be bruising (contusion) of the brain. The mechanism of concussion is unknown. There is a discussion about this now on biosci.neuroscience.

Severe closed head injury is empirically defined as that which causes amnesia, etc., of more than 20 minutes. Mild concussion (less than 5 minutes, I think) gets nearly complete recovery of all neuropsych function by 1 month, and no deficit is detectable at 3 months. Repeated concussion before the deficits from the last have cleared increases likelihood of permanent deficit (boxer's dementia, punch-drunk). Little or no concussion, headache, nausea practically guarantees complete recovery (I'm sure there must be rare exceptions).