By Cole Hill ( | First Posted: Mar 16, 2013 02:29 PM EDT

Arias was found guilty May 8 in the gruesome premeditated first-degree murder of her ex-boyfriend, 30-year-old Travis Alexander, in June 2008. (Photo : MySpace: Jodi Arias)

As Jodi Arias' defense team attempts to restore her credibility through "expert witness" testimony, a bizarre, shocking video has emerged of Arias laughing, singing and joking to herself in the interrogation room just minutes after she reacted with disbelief and horror to news of her ex-boyfriend's murder. 

(See video below)

A photographer from California, Arias is charged with the the grisly first-degree murder of her ex-boyfriend Travis Alexander in June 2008, when she allegedly stabbed the 30-year-old man 27 times, shot him in the face twice, slit his throat and left his bloodied corpse crumpled over in the bathroom shower of his apartment. Arias' fate depends on whether the jury believes she killed Alexander in self-defense, as she contends, or was actually a jilted lover exacting jealous revenge, as the prosecution argues.

With Arias finally done testifying, after an almost unprecedented 18 days on the stand, the defense is using "expert witnesses" like psychologist Richard Samuels to argue that Post Traumatic Stress Disorder explains the nature of Arias' purported "memory condition" that's caused her to blackout key elements of the case, such as most of the act of killing Alexander, her odd behavior, and why she was acting so suspiciously afterwards if she knew she'd acted in self-defense. PTSD will reportedly play an integral role in the defense's next phase of the trial.

"We call it denial. It's a very intense form of denial where a person cannot deal with the fact that something they did was so opposite of everything else they stood for in life. I determined that by reading her diaries. And in her diaries, she is essentially a pacifist, not a violent word, not a negative word," Samuels claimed in court Thursday, HLNTV reported.

While the court reportedly won't be shown the tape, Fox News obtained an unsettling video of Arias from the day she was charged with Alexander's murder in 2008. Arias has already admitted to lying about Alexander's death to just about everyone; the footage comes from when she was still claiming she had no knowledge of Alexander's killing and was telling police she was never at Alexander's home the day he was killed.

The video shows an initially incredulous Arias breaking down into tears when confronted with news of Alexander's death. "It's not fair. How many times was Travis stabbed?" asks Arias in the video while crying. Shortly after the officer questioning her leaves the room, her demeanor changes as abruptly as flipping a light switch. A playful, sunny mood washes over Arias as she does a headstand against the wall and sings the Dido song "Here With Me" to herself - which includes the ironic lyric, "It might change my memory." Arias is suddenly, inexplicably playful just minutes after sobbing in seeming grief. She even laughs and jokes to herself that she should have at least put on makeup for the interrogation.

While Arias has admitted to the lies shown in the video, the defense is now tasked with convincing the jury she is finally telling the truth. Arias has claimed she lied so often because she was "ashamed" she killed Alexander in self-defense and because she was afraid of revealing the details of their sexual relationship.

A psychologist and self-advertised "expert witness" on PTSD, Samuels testified Thursday that his review of the case and Arias showed she was "depressed" and "in denial" after she killed Alexander, claiming that's just what caused her "mental condition" and strange behavior.

Arias has tried to explain away many of the inconsistencies in her stories by claiming her memory was foggy the day Alexander died. Arias has wavered back-and-forth between providing surprisingly acute details surrounding the murder to claiming she has little to no memory of certain pieces of the case, such as the actual act of killing Alexander, saying her memory of the fateful day has "huge gaps," according to The Tri-City Herald.

Samuels testified that Arias suffered acute stress disorder following the harrowing incident. He said that disorder eventually evolved into PTSD.

"When a persons engages in an activity that is opposite of what they stand for ... can create in ones mind a psychological wall," Samuels said, The Huffington Post reported.

Samuels claimed that people who are afflicted with dissociative amnesia may only be able to recall small scenes of the actual trauma, and may also feel a "sense of detachment."

Arias has claimed throughout the trial that she only remembers small pieces from the day she killed Alexander, including stabbing him almost 30 times and slitting his throat from ear to ear.

"Perpetrators of horrible crimes can also develop post-traumatic stress disorder for having acted as the source of the crime," Samuels said.

After Samuels is done testifying, the defense is expected to call another "expert witness" to the stand, Alyce LaViolette, an expert on domestic abuse who has experience as in criminal trials. LaViolette has authored books on domestic violence, and delivered a speech about post-traumatic stress disorder at a 2011 conference for military families called, "Why Do People Hurt the Ones They Love?",Fox News reported.

Arias faces the death penalty if convicted. Judge Sherry Stephens recessed court proceedings until Monday at 1:30 p.m. EST.

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