A high school student has helped to solve the long-standing mystery of how the brain is damaged by head injury.
The brain is a delicate structure
As one might expect the brain is injured at the place where impact takes place during an accident or fall.
But less predictably it is also injured on the opposite side of the head to where the impact took place - and this injury is usually the more severe.
It has been thought that this is simply because the brain first collides against the skull near the point of impact, and then rebounds, hitting the skull at the back.
However, this theory does not explain why the second injury is often worse.
New Scientist magazine reports that Laura Drew, a student at Oakwood High School in Dayton, Ohio, has helped to solve the problem.
Developing an idea put forward by her father William Drew,
chief of neurology at the Wright Patterson Air Force Base in
Dayton, Laura built a model to simulate what happens inside the head.
Her 'skull' consisted of a tough transparent container
filled with a liquid of the same density as the cerebrospinal fluid (CSF) which cushions the brain inside the skull.
The 'brain' was a balloon filled with a slightly less dense saline solution and tethered to the base with a hook.
When the model was propelled into a wall, video footage showed that the momentum of the denser CSF carries it forward, forcing the brain backwards, and into the back of the head.
Only after this impact takes place does the brain rebound and hit the skull at the point where the original blow took place.
This explains why injury on the opposite side of the head - called the contrecoup injury - is usually more severe.
It also explains what happens when someone is hit on
the head by, say, a baseball bat.
In this case the head is stationary at the point of impact, and so the brain is not forced backwards to the same extent by a change of momentum. Thus the contrecoup injury will not be noticeably worse than that at the point of contact (the coup injury).
Steve Gilbert, a forensic scientist specialising in brain trauma at the State University of New York in Canton, said the work will help investigators distinguish accidental head injuries from intentional ones.
He said: "Kind of reverses some of the theories, doesn't it?"
Professor Lindsay Mclellan, and expert in head injuries at the University of Southampton, told BBC News Online the research was "quite fascinating".
"It shows how effective the system is in
limiting the effects of less severe impacts - it is only when the compensation system saturates that contracoup and coup occur.
"While one has to be cautious given that the model of a balloon in a container is an inexact model of the brain in the skull and the mecahanical factors operating at the moment of impact in real life will vary considerably, there could well be implications from the point of view of vehicle design.
"It certainly supports the policy of reducing driving speeds, since the momentum of the CSF and of the brain will increase exponentially with each increase in the speed at which the head is travelling in space at the moment of impact."
Graham Nickson, spokesperson for the charity Headway, told BBC News Online: "There is an awful lot to find out about brain injury.
"If this model helps move us a little further down the road towards understanding how the brain works and how it responds to injury that would be most welcome."