Scientists from the Center for Brain Health at the University of Texas reported in the Proceedings of the National Academy of Sciences that regular marijuana use is associated with decline in the volume of the brain’s frontal lobe gray matter.

Cells in this part of the brain control rewards, addiction behavior, and decision making.

Researchers looked at brain scans of 48 volunteers who used marijuana at least four times per week during the previous six months. Those people’s brains were compared to those of 62 volunteers who did not use the drug.

Although an area above the eyes in the frontal lobe gray matter (known as orbital frontal lobe) was smaller in the pot smokers, their brains also showed increases in white matter, which comprises the long wires that connect the grey matter brain cells.

This combined finding — less grey matter and more white matter — suggests that the brain may be compensating for cell shrinkage by increasing the amount of white matter connecting wires, which will accelerate message transmission between the grey matter brain cells.

An earlier investigation showed that volunteers with smaller orbital frontal brain regions were more likely to start using marijuana years after their scans were performed, so the decreased volume of this addiction-behavior area may predispose people to decide to use marijuana on a regular basis.