The central idea of "Phineas Gage: A Gruesome but True Story About Brain Science" is the story of Phineas Gage and how his brain injury changed our understanding of the brain.
A supporting detail that helps develop this central idea can be found in paragraphs 7-9 of the text. These paragraphs describe the details of the accident in which Phineas Gage's brain was severely damaged by a metal rod that went through his skull. The text explains how the rod entered Gage's head, passing through the frontal lobes of his brain. It also mentions how Gage survived the accident and continued to function physically but experienced significant changes in his personality and behavior.
This supporting detail directly supports the central idea by highlighting the impact of Phineas Gage's brain injury.