Will Brain Scans Replace Breathalyzer Tests for Determining Cannabis DUI?

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It’s no secret that consuming marijuana affects driving skills.

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However, accurately proving that a driver is impaired as a result of cannabis consumption has been a challenge for toxicologists and researchers for years. The main issue has been that, unlike alcohol, impairment from cannabis cannot be determined by a single measure.

More specifically, the results of a recent study conducted by researchers at the University of Sydney’s Lambert Initiative indicated that blood and oral fluid THC concentrations are relatively poor or inconsistent indicators of cannabis-induced impairment.

RELATED: Can Measuring Eye Characteristics Determine Cannabis DUI?

Can noninvasive brain imaging detect cannabis impairment?

Perhaps. A new study published in the journal Neuropsychopharmacology has found that cannabis impairment can be detected from according to the brain activity on an individual level, the Harvard Gazette writes.

Known as functional near-infrared spectroscopy (fNIRS), a non-invasive brain imaging procedure, proved to be an objective and reliable way to identify those with performance impaired by THC, the psychoactive ingredient in the plant. The study was led by researchers at Massachusetts General Hospital.

The technique was used to measure brain activation patterns that correlate to impairment from THC intoxication.

“Our goal was to determine if cannabis impairment could be detected from the activity of the brain on an individual level,” Jodi Gilman, an investigator in the Center for Addiction Medicine, MGH and associate professor of psychiatry at Harvard Medical School. “This is a critical issue because a ‘breathalyzer’ type of approach will not work for detecting cannabis impairment, which makes it very difficult to objectively assess impairment from THC during a traffic stop.”

The study also addresses the issue of determining impairment from cannabis intoxication, considering that people who use cannabis regularly can have high levels of THC in their system but are not impaired. That is because the concentration of THC in the body does not correspond well to functional impairment.

“We need a method that won’t penalize medical marijuana users or others with insufficient amounts of cannabis in their system to impair their performance,” Gilman said.


The study included a total of 169 cannabis users who underwent fNIRS brain imaging before and after receiving a dose of oral THC or a placebo.

As a result, those who experienced intoxication after using oral THC showed an increased oxygenated hemoglobin concentration (HbO) in the brain’s prefrontal cortex.

All subjects received the same dose of THC, however, a variety of impairment measures were recorded, which further affirmed the discordancy between THC concentrations and impairment, the researchers pointed out.

“As we showed that there was no difference in THC dose between those who became impaired from those who did not follow THC, it is likely that a brain- or behavior-based metric (e.g., eye tracking or cognitive testing), rather than a per se blood or oral fluid limit of THC, is required to distinguish THC impairment from simple exposure,” the researchers conclude in the study. “Future work is warranted to determine if observed brain signatures are specific to THC intoxication-related impairment or are a more general signature of impairment.”

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