Dead in the head

Does anybody know the origin and exact meaning of this phrase "dead in the head"? I am especially interested in its use in reference to drug addicts.
Thanks in advance, Denis.

The term "dead head" refers to fans of the rock group "The Greatful Dead" and it is a common misconception that Dead Heads are heavy drug users. Perhaps there is a connection between these two expressions.

I found several meanings of "dead head," including "someone who rides free on a train." But no "dead in the head," even in my "sin slang" references. Dead in the head to me means "nobody home," "the porchlight is out." His brain is fried.