Full distinctive concussion
A concussion is a status condition caused by having your head critically crippled in combat.
Fallout 2
In Fallout 2, crippling of the eyes resulted in an enemy's blindness.
Fallout 3 and Fallout: New Vegas
Having a concussion will reduce overall accuracy and Perception of the Character by 4 Points. In addition, the screen will intermittently get fuzzy and a ring will be heard through the speakers. It is unknown if enemies really undergo a concussive state when they have their heads Crippled.
As with the crippling any body part, targets will momentarily pause to clutch their head.
Using a stimpak, sleeping, visiting a doctor, using My First Infirmary (if you've purchased it for your residence), or through some other means relieving the crippled head will remove the Concussion effect and restore your Perception.
