HOUSTON (AP) — Andrea Yates was a sweet friend, dedicated nurse and loving mother, but after the birth of her fourth son she turned into a "total zombie" who stared into space and couldn't finish sentences, her best friend testified Tuesday in her murder trial.

Debbie A. Holmes, who met Yates about 20 years ago when both were nurses at the University of Texas M.D. Anderson Cancer Center, also said that a few months after Yates returned from a psychiatric hospital following two suicide attempts, she asked Yates why she had been so depressed.

"She asked me if I thought Satan could read her mind and if I believed in demon possession," Holmes said.

Yates, 42, is being retried in her children's 2001 bathtub drownings because her 2002 murder conviction was overturned by an appeals court. The appeals court ruled erroneous testimony might have influenced the jury.

Yates has again pleaded innocent by reason of insanity. If the jury agrees, she could be committed to a state hospital, with periodic hearings to determine whether she should be released. A guilty verdict would mean life in prison.

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Her attorneys say she suffered from severe postpartum psychosis and did not know it was wrong to kill 7-year-old Noah, 5-year-old John, 3-year-old Paul, 2-year-old Luke and 6-month-old Mary.

Earlier Tuesday, prosecutors cross-examined a neuropsychologist who evaluated Yates about six months after the drownings. Dr. George Ringholz had started testifying Friday but because of a scheduling conflict could not return to the witness stand until Tuesday.

He said Yates recounted a hallucination she had after the birth of her first child.

"What she described was feeling a presence ... Satan ... telling her to take a knife and stab her son Noah," Ringholz said.

Ringholz acknowledged that he did not perform certain tests to see if Yates was trying to make her mental illness appear worse, but he said other tests and safeguards as part of the extensive two-day evaluation indicated she was not faking. Ringholz diagnosed Yates as having schizophrenia.

Ringholz said Yates was suffering from a delusion on the day of the drownings and did not know her actions were wrong, even though she called 911 and knew she would be arrested. Her delusion was that Satan had entered her and that she had to be executed in order to kill Satan, Ringholz said.

"Delusions cannot be willed away," Ringholz said.

After the defense rests its case later this week, prosecutors will begin their rebuttal.