The researchers applied an electroencephalogram — a cap with metal discs that detect brain waves — to the scalps of each person to study their brain responses. The study found frontal midline theta power, which is associated with focused cognitive control and memory encoding, decreased in the subjects who had inhaled marijuana before cognitive and driving tests.
“It happens instantly, immediately after inhalation, and it continues all the way into five or six hours, even when people report not starting to feel as intoxicated,” she said. “Theta is very important for memory.