On the surface, it sounds positive: Prenatal exposure to marijuana results in an improved ability for young children to perceive activity and movement around them. But the ability to detect motion all around you or an enhanced peripheral awareness may be at the expense of central sensitivity.

In other words, these children may be less able to concentrate on central or near tasks due to excess sensitivity to the movement and activity around them. Research has already recognized that children exposed prenatally to marijuana have attention and memory deficits.

This new study identifying the increase in global motion perception — reported by Arijit Chakraborty and colleagues of the School of Optometry and Vision Science at the University of Auckland in New Zealand — found that global motion perception in 145 children 4.5 years old was deficit in those prenatally exposed to alcohol but improved in those exposed to marijuana. When the prenatal exposure involved use of both, there was no effect.

Evidence of potential dysfunction in central vision processing with prenatal exposure to marijuana was also demonstrated by a team led by Karen Grewen, Ph.D., from the Department of Psychiatry, Neurobiology at the University of North Carolina Chapel Hill.

Her group found decreases in structural connections using magnetic resonance imaging on full-term sleeping infants between 2 to 3 weeks old. There was a significant decrease in the connectivity related to the vision processing region of the brain; the right occipital fusiform. This region processes fine visual signals with the emphasis on the recognition of faces, words and emotional content of human facial expression.

Marijuana is known to suppress functions related to vision both at the retinal level and brain level. At the early processing level for vision in the brain, at the lateral geniculate nucleus, marijuana suppresses up to 75 percent of function while potentially enhancing 25 percent. If we relate this the findings of increased peripheral awareness with enhanced global motion perception, one can state that while a function was improved, 75 percent of other functions may be altered for the worse by marijuana exposure.

Brain imaging studies on older children, ages 6 to 8, have demonstrated changes in the brain. This study compared prenatal exposure to tobacco, prenatal exposure with tobacco in combination with marijuana, marijuana only and no exposure. The research group was led by Hanan El Marroune, Ph.D. from the Department of Child and Adolescent Psychiatry at the Erasmus Medical College in Rotterdam, The Netherlands.

They reported that while prenatal marijuana exposure did not have any impact on the overall brain volume, it did significantly shift volumes with those being exposed to marijuana having thicker frontal cortices.

Clearly, prenatal use of marijuana has a major impact on the brain and brain development. This is not a good thing. It is recognized that the marijuana receptors naturally occurring in the brain play an important role in neurodevelopment and emotional development.

It is critical to limit any disruptions to the normal development of the fetal brain, and marijuana consumption during pregnancy is disruptive to development. The federal government recognizes the complexity of pregnancy and drug use and encourages further research. There are, however, other factors that confound the understanding of prenatal drug use, such as maternal nutrition, exposure to social nurturing, aspects of neglect and multiple substance abuse.

The developing brain is delicate. Gains in one region or function can be at the expense of other regions or functions.