Researchers Use Brain Imaging To Gauge Cannabis Impairment

For years, people have searched for a method to measure an individual’s impairment after using cannabis.

While some companies have ventured down the breathalyzer route and been able to measure a person’s THC levels, they have been unsuccessful in determining actual impairment.

Following the results of a new NIDA-funded study focusing on the brain, measuring cannabis-related impairment may soon become more accurate.

A New Method

Researchers at Massachusetts General Hospital (MGH) say they have discovered an objective and reliable noninvasive brain imaging procedure that can identify tetrahydrocannabinol (THC)-related performance impairment in individuals.

“Companies are developing breathalyzer devices that only measure exposure to cannabis but not impairment from cannabis,” says Study Author Jodi Gilman, Ph.D. “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. While it requires further study, we believe brain-based testing could provide an objective, practical, and much-needed solution.”

To measure impairment, the method uses functional near-infrared spectroscopy (fNIRS) technology to measure brain activation patterns.

According to the Neuropsychopharmacology journal, the new technique may have significant implications for highway and workplace safety.

Legalization Has Created a Need

Gilman says that legalization has led to increased cannabis use and created an urgent need for a portable brain imaging procedure that can differentiate between impairment and mild THC intoxication.

“Our research represents a novel direction for impairment testing in the field,” says Gilman. “Our goal was to determine if cannabis impairment could be detected from activity of the brain on an individual level. 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.”

MGH researchers say past studies have shown THC to impair cognitive and psychomotor performance and possibly double the risk of fatal motor vehicle accidents.

Because some people have high levels of THC in their bodies and have elevated tolerances, high concentrations of the cannabinoid do not always directly correlate with impairment.

THC metabolites can also stay in the body for weeks after a person has used cannabis, long after THC intoxication has worn off.

“Identification of acute impairment from THC intoxication through portable brain imaging could be a vital tool in the hands of police officers in the field,” says Principal Investigator A. Eden Evins, MD, MPH. “The accuracy of this method was confirmed by the fact impairment determined by machine learning models using only information from fNIRS matched self-report and clinical assessment of impairment 76 percent of the time.”

The Study

For the MGH study, 169 cannabis users underwent fNIRS brain imaging before and after receiving oral THC or a placebo.

According to MGH, participants who reported intoxication after receiving THC showed an increased oxygenated hemoglobin concentration (HbO) compared to those who reported low or no intoxication.

HbO is a neural activity signature from the brain’s prefrontal cortex section.

MGH says the study did not specifically assess fNIRS in roadside impaired driving assessments, though it did cite considerable advantages for them.

According to MGH, advantages include the feasibility of lightweight, inexpensive, battery-powered fNIRS devices that can store data on wearable recording units or transmit information wirelessly to a laptop.

MGH says fNIRS technology could also be integrated into a headband or cap to minimize set-up time.