How could they test their brain activity when they are
out in the middle of a glade not hooked up to anything?
Movie so dumb
If you read the books, or used your imagination, this would be easily answered. This is Sci fi/fantasy after all. Not to mention it's set the future, and in this world they obviously have a great deal of evolved and advanced technology.
In the books they had devices implanted in their brains that both monitored and "mapped" their brains and also induced variables that let to desirable situations to study. For example, the implants made the boys perceive the wall as higher than it was, so that they would think climbing it was impossible.