Donte Johnson is 20 years old, but the way he thinks, communicates, and copes with problems is more in line with the mind of child, according to a forensic neuropsychologist who testified in Johnson's murder trial Monday.

Johnson, accused in the June 2010 rape and slaying of Sabina Rose O'Donnell, has an IQ in the 70s, which places him on the borderline of what is considered "mentally retarded," said Gerald Cooke, who administered a series of tests to Johnson this year.

"He's basically in that 11- to 12-year-old range in terms of his intellectual functioning," Cooke said.

Johnson faces life in prison in the death of O'Donnell, who was a few weeks from turning 21 when she died. When Johnson was arrested two weeks after the murder, he confessed to police. Authorities also matched his DNA to O'Donnell's body, and found surveillance video of Johnson following O'Donnell to her home at Fourth Street and Girard Avenue, on the edge of Northern Liberties.

Johnson is accused of raping, beating, and strangling O'Donnell and leaving her body in a vacant lot behind her home. He told police he targeted her because he wanted her bicycle.

Cooke, expected to be the main witness in Johnson's defense, did not appear in Common Pleas Court. Instead, a prerecorded video of his testimony was played for jurors. He was one of two witnesses called after the prosecution rested Monday morning.

Johnson's attorneys are seeking to use Cooke's findings to show, among other things, that Johnson did not fully understand his rights when he gave a statement to police. They have said that Johnson should not be facing a first-degree murder charge.

Cooke testified that he believes Johnson suffers from some type of brain damage sustained early in life, possibly in utero. Johnson tests well in terms of visual and auditory memory, he said, but poorly in other areas. The disparity suggests damage, he said, specifically to the right frontal lobe of his brain.

Johnson processes things slowly, Cooke said, and has difficulty solving problems and reasoning, two functions controlled by that part of the brain. People with brain damage often use the same solutions to problem-solve, as opposed to trying new ideas, Cooke said. When the same methods fail to solve the problem, it can lead to frustration.

Cooke, who said he would be paid more than $9,000 for the testing and for testifying, described Johnson as "friendly and cooperative" during the testing. Johnson was left back three times in school and dropped out after the ninth grade. He was diagnosed with ADHD and learning disabilities as a child, and also got speech therapy. Johnson described his home life positively, and said he had "a lot of girlfriends."

Johnson said he started smoking pot every day when he was 14 or 15, and binge-drinking occasionally around that time, as well, Cooke said. Johnson was arrested for assaulting a school police officer and later for possession of crack cocaine with the intent to deliver, but told Cooke he did not use the drug.

He also maintained his innocence, Cooke said, claiming that he was home with his family at the time of the slaying and did not remember confessing to police because he was intoxicated. Johnson's relatives also have said he was drunk and high when he turned himself in to police, though officers disputed that.

Defense lawyers also called to the stand Dominique Abel, who said Johnson was her boyfriend at the time of the slaying. Abel testified that before giving his confession, Johnson drank six large cups of whiskey and smoked six or seven marijuana joints.

Under cross-examination, Assistant District Attorney Richard Sax implied that Johnson would have been incapacitated by that amount of alcohol and pot, asking, "And who carried him out?"

Earlier Monday, Johnson's lawyer, Gary Server, sought to highlight potential weaknesses in Johnson's confession. Johnson admitted putting O'Donnell in a chokehold, and said she could not breathe and became unconscious. Philadelphia Police Detective Thorsten Lucke said under cross-examination that Johnson never said specifically that he had killed O'Donnell.

Server also questioned some of the language used in the police statement, asking Lucke if Johnson really used the word "ejaculated" when referring to O'Donnell's rape.

"Yes, but he mispronounced the word," Lucke said.

Defense attorney Lee Mandell questioned Philadelphia Detective Frank Kane, the lead investigator in the case, about a brown human hair found on O'Donnell's body. Kane said that to his knowledge, the hair had never been analyzed for DNA.

Contact Allison Steele at 215-854-2641 or Follow on Twitter @AESteele.