But they said more evidence is needed to confirm that.
The results should be interpreted cautiously because of the study's small size, but the data seem robust and bolster the idea that media exposure is a public health issue, said Dr Dimitri Christakis.
He is a child development specialist at Seattle Children's Hospital who wrote an editorial accompanying the study published online on Monday in the journal Paediatrics.
Christakis said parents need to realise that fast-paced programming may not be appropriate for very young children.
"What kids watch matters, it's not just how much they watch," he said.
University of Virginia psychology professor Angeline Lillard, the lead author, said Nickelodeon's SpongeBob shouldn't be singled out.
She found similar problems in kids who watched other fast-paced cartoon programming.
She said parents should realize that young children are compromised in their ability to learn and use self-control immediately after watching such shows.
"I wouldn't advise watching such shows on the way to school or any time they're expected to pay attention and learn," she said.
Nickelodeon spokesman David Bittler disputed the findings and said SpongeBob SquarePants is aimed at kids aged 6-11, not four-year-olds.
"Having 60 non-diverse kids, who are not part of the show's targeted (audience), watch nine minutes of programming is questionable methodology and could not possibly provide the basis for any valid findings that parents could trust," he said.
Lillard said four-year-olds were chosen because that age "is the heart of the period during which you see the most development" in certain self-control abilities.
Whether children of other ages would be similarly affected can't be determined from this study.
Most kids were white and from middle-class or wealthy families.
They were given common mental function tests after watching cartoons or drawing. The SpongeBob kids scored on average 12 points lower than the other two groups, whose scores were nearly identical.
In another test, measuring self-control and impulsiveness, kids were rated on how long they could wait before eating snacks presented when the researcher left the room.
SpongeBob kids waited about 2 1/2 minutes on average, versus at least four minutes for the other two groups.
The study has several limitations. For one thing, the kids weren't tested before they watched TV. But Lillard said none of the children had diagnosed attention problems and all got similar scores on parent evaluations of their behaviour.
- AAP
