Defense lawyers in a Manhattan murder trial try again to convince jurors that the man accused of killing a little boy in 1979 is erratic enough to not know what he's saying in a videotaped confession. NY1's Jose Martinez has that story.

In a 2012 video, Pedro Hernandez confesses to killing Etan Patz, telling Manhattan prosecutors how he strangled the 6-year-old in a bodega basement, then stuffed him into a box and tossed him out with the trash.

But defense lawyers contend it's a false confession wrung out of a man with very low intelligence and a serious mental disorder.

"There are criteria in false confessions that are met by this case because Pedro Hernandez matches people vulnerable to false confessions: low mentality, psychiatric condition, long interrogation. This is very relevant information for the jury,” said Harvey Fishbein.

On Thursday, jurors heard from an expert defense witness who evaluated Hernandez in jail after his arrest in 2013.

Psychologist Bruce Frumkin testified that low intelligence is a risk factor for false confessions, saying, "He doesn't really know one way or the other what's real or what's not real … He's just very vulnerable to law enforcement telling him over and over, 'We believe you had something to do with the death.’”

The psychologist added that Hernandez spoke of having visions of a woman dressed in white with ballet shoes and no facial features - and that he's been on anti-psychotic medication for years.

But prosecutors say the confession will stand up in court.

Countering the defense's claim that Hernandez has one of the lowest IQs in the country, prosecutor Joan-Illuzzi-Orbon pointed out that he correctly uses words like "alleviate," "diagnose" and "plasma."

One thing jurors won't get to see is up 50 pounds of potatoes the defense wanted to introduce as evidence.

Defense lawyers claim that Etan was too heavy for the slim defendant to carry, a point they hoped to make by having jurors try to lift the spuds themselves.

They even brought the potatoes into the courthouse.

But the judge sided with prosecutors, who said lifting them is vastly different than carrying a body.

The trial resumes Friday.