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Denise Valenti, OD, advises on why driving under the influence of marijuana is not without visual consequences.
Reviewed by Denise Valenti, OD.
Tunnel vision is an often-mentioned effect of marijuana both by users of the substance, as well as the National Highway Traffic Safety Administration. Despite the fact that educational campaigns also reported tunnel vision, only 1 study has been conducted on humans and peripheral vision.1
In the US, 16 states do not require visual field testing for driver’s licenses, the other 34 states have a binocular horizontal visual field requirement. Fifteen states stipulate a visual field of 140 degrees, and 19 states from 105 to 130 degrees; Maine requires 150 degrees.
Kentucky is the only state that requires vertical visual field testing, i.e., 25 degrees above and below the fixation point. All provinces in Canada require intact vertical fields.
In contrast to that study, 2 studies reported that marijuana may improve night vision.2,3
In the literature: no safety in numbers
An animal study found that tadpoles could see better in extremely low light, when cannabinoid receptors were stimulated.4 In the study, all tadpoles were observed swimming toward the light.
Humans have numerous cannabinoid receptors in the retina, CB1 and CB2. The latter is in the cone and rod photoreceptors, horizontal cells, some amacrine cells, and bipolar and ganglion cells.
When tested in mice5 in the dark, a-waves slower in mice genetically engineered not to have CB2, but in those without CB1, no change was seen. Under photopic conditions, the b-wave in mice without CB2 required more light adaptation.
The authors concluded that CB1 and CB2 receptors could have different roles in visual processing.
Investigators found that when children aged 4 to 5 years were exposed to alcohol and marijuana prenatally, the global motion perception improved with exposure to marijuana, and decreased with alcohol. However, the increase may negatively affect other functions.
The alarming, inaccurate take-home message here as a result of these studies: it is considered safe to drive after marijuana use by many, even while driving.
As a result, marijuana has been detected in markedly higher percentages of drivers, and often with deadly consequences. The rate of marijuana impaired driving fatalities doubled in those states with legal adult use marijuana, according to the American Automobile Association.
Effects on eyes
The following snippets highlight the effects of marijuana use on the eyes:
“If a driver cannot see accurately, they cannot drive safely," Valenti said.
"More research is needed so that we can fully understand how marijuana impacts driving with acute recent use.
Further given that studies show permanent reductions in those brain regions that process vision when initiation of marijuana occurs with early age, adolescent use, that area of research is urgent.
On the other end of the age spectrum, cannabis use in moderate doses may actually be beneficial to the aging brain. "There is so little we do not know. But we do know that recent use of marijuana impairs functions critical for safe driving. Enjoy legal adult use marijuana responsibly. Do not drive,” Valenti concluded.