WATERLOO - Babies exposed to marijuana in the womb showed a significant improvement in their ability as preschoolers to track moving objects, according to new research by a professor at the University of Waterloo's school of optometry.

"The effect of the marijuana was a surprise," said Ben Thompson.

But he warned that the results don't mean people should smoke marijuana while pregnant in the hopes of producing babies with improved vision.

Other studies show drug use during pregnancy can lead to a higher incidence of other types of eye problems.

"The data provide us with really interesting insights," Thompson said.

"But they certainly shouldn't be taken as evidence for taking any kind of recreational drug during pregnancy."

The researchers tested higher-level visual processing in 145 four-year-old children who had been exposed to different combinations of methamphetamine, nicotine, marijuana or alcohol in the womb.

They were compared with a control group of 25 children who weren't exposed to the substances.

The researchers were from the University of Waterloo, the University of Auckland in New Zealand and Brown University in Rhode Island.

Researchers tested the children's ability to track a group of moving dots across a screen despite the presence of other dots randomly moving around. The research was published this month in the journal Scientific Reports.

The research showed exposure to marijuana improved global motion perception. The children exposed to marijuana in the womb were almost 50 per cent better at the task than children with no drug exposure.

Exposure to alcohol worsened motion perception, while nicotine and methamphetamine had no effect on vision.

The research also showed the effect got bigger with larger doses, he said. Babies exposed to more alcohol had worse motion perception, while babies exposed to more marijuana had better results as preschoolers. Interestingly, the research also showed the effects of the two drugs seemed to cancel each other out: exposure to both alcohol and marijuana had no effect on motion perception.

That finding suggests there may be neurochemical ways for doctors to counter the effects of prenatal exposure to drugs, he said.

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The research underlines that exposing a fetus to these recreational drugs is not neutral or benign, Thompson said. "It tells us quite clearly that exposure to these drugs is having an impact on brain development."

The children were part of the international IDEAL (Infant Development, Environment and Lifestyle) study, which looked at the effects of prenatal drug and alcohol use on a variety of motor and cognitive skills.

The researchers hope to use brain imaging to see if differences in prenatal exposure to various drugs actually leads to differences in activity in the visual parts of the brain, Thompson said.