How Marijuana Affects Sustained Attention

There is a belief held by many marijuana users that smoking weed heightens their awareness and therefore increases their ability and skills in doing tasks, such as driving a motor vehicle.

Many people who self-report marijuana use claim that they are actually better drivers when they are high than when they are not. But, is that actually the case?

Researchers at the University of Georgia have found that it could be true that smoking marijuana may indeed increase the user's motor skills for a while. But, they conclude that over time any increase in skill level they may acquire is negated by serious attention problems.

The Georgia researchers, led by Jonathon Crystal, found that sustained attention to timing-tasks was substantially altered in laboratory rats when they were given a synthetic cannabinoid. Under the influence, the lab rats displayed difficulty distinguishing between long and short periods of time during tasks for which they were trained.

"In the real world, this suggests that someone smoking marijuana might well be able to do a task briefly, but over time there could be serious attention problems," said Crystal. The implication is that users of marijuana could be lulled into thinking they are capable of using the motor skills for such actions as driving when in fact there could be serious long-term attention-span problems.

For the study, the scientists set up a task which the rats usually perform with a great deal of accuracy. They learned to press one lever to receive a pellet of food after hearing a short sound (four seconds) or another lever if the sound was long (16 seconds).

"Under these circumstances, animals will typically learn to press the correct lever with high accuracy," the authors said.

The research team then played sounds of intermediate length to find a midpoint at which rats were equally likely to respond as if the sound were "short" or "long."

After the rats learned the right levers to press, they were injected with a synthetic cannabinoid, and their sensitivity to time was measured. Being under the influence produced a substantial decline in sensitivity to time.

The researchers said they used a synthetic compound rather than tetrahydrocannabinol (THC), the "active" ingredient in marijuana because the synthetic cannabinoid is more powerful and easier to use in laboratory settings. "However, it is so close chemically to THC," they said, "that the findings can be equated with the effects of THC."

The investigators concluded that the general ability to maintain attention was altered by exposure to the cannabinoid. The cannabinoid produced an attention disorder and disrupted the performance of the task.