Years ago, when trepanned skulls began to turn up in the course of field research by archaeologists around the world -- especially in the case of the ancient Peruvians -- astonished scientists said, "L-l-l-l-look!!! They u-u-u-u-used to engage in B-B-B-RAIN S-S-S-SURGERY!!!" And photos with captions talking about ancient brain surgery began to appear in textbooks.
In all overwhelming likelihood, the practice had nothing whatsoever to do with brain SURGERY.
In all overwhelming likelihood, it was all about GETTING HIGH.
Yup. The sensation of being "on drugs" without the drugs, apparently resulting from the slight expansion of the brain at the point where the underlying brain tissue has been freed of its constraining case.
Paul Halvorson, convicted in Utah of Practicing Medicine without a License for jis trepanation work, explains, "It gets you high, but it is a beneficial, non-distorted reality; not like the kind of high you get with poisonous drugs."
It is thus not surprising that we see practice employed by ancient royalty -- ancient Andean kings in Peru...
...and a monarch of the Merovinges of Dark Ages France...
Just as warfare and preparation for warfare frequently leads to scientific advances, warfare probably led to knowledge of the psychological effects of trepanation. Enemies would clock each other on the head, breaking-out a piece of skull in the process. Survivors of the blow would experience the resulting high after the pain subsided, and report this to their people.