How prenatal exposure to marijuana affects kids' vision

Children exposed to marijuana in the womb show a significant improvement in their ability to track moving objects at age four, says a new study, warning that the results do not mean marijuana has a benefical effect on foetal development.

"We were surprised with this initial finding," said Ben Thompson, professor at University of Waterloo in Ontario, Canada. 

"It shows that marijuana and alcohol can have quite an impact on a fundamental aspect of the visual processing happening in our brains. But despite the apparently beneficial impact of marijuana on the development of the brain's visual system, other research shows its use can actually impair the brain development of unborn children," Thompson noted.

The researchers tested higher-level visual processing in a group of four-year-old children who were exposed to different combinations of marijuana, alcohol, methyl amphetamines, or nicotine while in the womb, compared with a non-exposed control group. 

Results showed exposure to marijuana improved global motion perception, a measure of processing within the brain's dorsal visual pathway which is responsible for motion processing and visual-motor control. 

In contrast, exposure to alcohol had a negative effect. Nicotine and methamphetamine had no effect on vision compared with the control group.

The researchers modified a global motion perception test for use with young children as a quantitative means to measure dorsal pathway performance.

The test measures the child's ability to track a group of moving dots together across a screen despite the presence of randomly moving dots. The percentage of randomly moving dots is increased until the child can no longer tell which way the dots are moving.

The study appeared in the journal Scientific Reports.