Defense attorneys in the trial of Ibrahim Muhammed and Nalik Scott - accused of killing three people in a bodega robbery in 2011 - called a clinical neuropsychologist to the stand Friday, who testified that Muhammed has an IQ of 82 and "very serious brain dysfunction."

Muhammed was arrested in the deaths in 2012, after giving a confession to homicide detectives. Police said Muhammed told them that he and Scott planned to rob Lorena's Grocery in West Philadelphia in September 2011, and killed owner Porfirio Nunez, 50; his wife, Juana, 44; and his sister Lina Sánchez, 48, in the process.

Muhammed's defense team has countered that the confession was false and was taken while he was suffering from unmedicated schizophrenia.

Carol Armstrong, a University of Pennsylvania professor who directed the neuropsychology lab at Children's Hospital of Philadelphia, said she examined Muhammed and concluded that he had impairments in visual memory - "important for learning from social experiences, and remembering where you were."

Muhammed also has issues with verbal comprehension and reasoning, Armstrong testified. She said his father also suffered from schizophrenia and that his family members had a host of psychological issues.

Defense attorney Larry Krasner asked whether Muhammed's condition would have allowed him to negotiate, make deals, or "do business in an adversarial context."

"I don't think he could and represent himself well," Armstrong said.

Later, when prosecutor Kirk Handrich asked if someone with Muhammed's condition could confess to a crime, Armstrong said she had answered Krasner's question hypothetically.

In cross-examination, Handrich argued that court-ordered psychologists found Muhammed competent for legal proceedings a month after his arrest and had never declared him mentally incompetent to stand trial.

Handrich asked whether what they described as Muhammed's extensive drug use - of Xanax, Percocet, and marijuana - and withdrawal from those drugs could explain his symptoms of psychosis.

"Going off [those drugs] does not cause psychosis," Armstrong replied.

Defense attorneys also called a witness to a similar bodega robbery in August 2011 at 62nd and Reedland Streets - which police said was connected to the Lorena's Grocery killings. Video from the Reedland Street robbery led a detective to conclude Muhammed was the man in the video - and subsequently sent him to the homicide unit, where he confessed to the killings and the Reedland robbery.

Amny Rodriguez, who was working in the Reedland bodega when it was robbed, said on the stand that she could not identify the man who had held a gun to her head, though she had described him to police in a statement taken after the robbery.

She said she had taken her description of the men from security video she had viewed of the robbery. And though she had originally told police that she had seen one of the robbers before, she said on the stand that she had not.

Muhammed's sister also testified briefly, identifying family photos that she said showed Muhammed in October 2011 with a full beard. For the last two weeks in testimony, the defense has emphasized an eyewitness account of the murder that described the assailant alleged to be Muhammed as clean-shaven.