BROCKTON - Just hours after her 4-year-old daughter died at her bedside, Carolyn Riley appeared at the child’s preschool in Weymouth with a “flat’’ demeanor and asked to pick up the girl’s things, as well as a copy of her class photograph, a nurse testified yesterday in the mother’s first-degree murder trial.

When a defense lawyer asked whether the mother’s lack of overt emotion might reflect her shock that Rebecca had died overnight on Dec. 13, 2006, after seemingly having only a bad cold, the nurse at the preschool, Ellen McCarthy, replied: “Someone in shock wouldn’t be in school looking for pictures.’’

The demeanor of the 35-year-old Hull mother was a focus of the first day of testimony yesterday in a case that has drawn nationwide attention as a highly unusual example of alleged child abuse and financial exploitation.

Though defense lawyers say the girl died of rapid-onset pneumonia, prosecutors say autopsy findings show she died of an overdose of psychotropic drugs.

The state also alleges that Rebecca’s parents exaggerated her behavioral issues during her short life to obtain a psychiatric diagnosis of bipolar and hyperactivity disorders, which would pave the way for disability benefits.

At the time Rebecca died, both parents and her two older siblings qualified for federal disability benefits, largely for psychiatric issues, totalling about $2,700 a month.

If Rebecca had qualified, prosecutors say, the family expected to receive an additional $630 a month.

The prosecutor, Frank J. Middleton Jr., said Rebecca’s mother ignored the girl’s worsening condition in the days before her death.

“She was indifferent to the little girl’s death,’’ he said while pacing before the jury box in Plymouth Superior Court. “She poisoned her own girl to death.’’