FORT LAUDERDALE, Fla. -

Wayne Treacy, a teen charged with attempted murder in the beating of a 15-year-old girl at Deerfield Beach Middle School, did not take the stand in his own defense before his attorneys rested their case Thursday afternoon.

A psychiatrist testified Thursday in Treacy's trial, as his attorneys work to prove he was insane at the time of the beating that left Josie Lou Ratley with permanent brain damage.

On Thursday morning, Treacy's defense called its key witness, Dr. Alexander Neumeister, considered one of the most respected men in the study of mental health, especially post-traumatic stress disorder.

Neumeister examined Treacy several times and testified Thursday that Treacy is a textbook case of PTSD. The defense contends Treacy had the disorder after his brother hanged himself.

"Wayne must have been in a state of full-blown PTSD on that day, and he must have been in the midst of a major depressive episode," Neumeister said.

He said Treacy's behavior before, during and immediately after the March 2010 attack of Ratley was consistent with the dissociative stage, which he describes as a prolonged state of detachment during which Treacy lost the ability to consciously control his impulses and understand what he was doing or the consequences of his actions.

"Here's a relatively normal or perfectly normal high-achieving student, who's doing something, who apparently was exposed to a very severe trauma and is doing something unimaginable, in fact, which does not add up at all," Neumeister said. "There's a phenomenon which I cannot explain to you because I don't know, but I always say patients with PTSD are kind of magnets for trouble. What does that mean? They walk from one disaster into the next. And not that they're looking for it; it just happens to them."

That's exactly what Neumeister said happened after the text message exchange between Ratley and Treacy.

"He was angry. He was upset. This is not one more text message that basically put him over the edge. This text message was a game-changer," Neumeister said. "There's no way he knew what he was doing."

Prosecutors said the attack occurred after Ratley sent Treacy a text message that appeared to make light of his brother's suicide, which set him off.

Neumeister said that if Treacy had directed his rage at himself, rather than at Ratley, this could have been a different case, one about alleged cyberbullying that led to a suicide.

After the defense rested its case Thursday afternoon, the state called some rebuttal witnesses to the stand.