The National Institute of Health has contracted with Battelle Memorial Institute to study measuring marijuana impairment with a virtual reality device. Such a device could determine whether a driver who has used marijuana is currently under the psychoactive influence of the drug (a.k.a. impairment) much like a breathalyzer does with alcohol.

Currently, determining impairment through biological testing isn’t reliable, as chemicals identifying marijuana use remain in the blood and urine long after the subject has regained sobriety, and blood/saliva levels of THC (marijuana’s impairing substance) do not track reliably with impairment. Such a device would overcome these problems, proving especially useful in marijuana-legal states, where, as with alcohol, law enforcement must establish a driver’s impairment to arrest for DUI-drug.

The study aims to measure impairment through metrics:

Rebound dilation (this means the pupil responds to continuous light stimulation initially by constricting and then dilating) Lack of convergence (this describes an inability of one’s eyes to converge when following a stimulus from in front of the face to the bridge of the nose) Horizontal gaze nystagmus (this describes an involuntary jerking of the eye that occurs after tracking a stimulus to the extreme outer corner of the eye and holding the eye there)

The study also describes the device as “measuring smooth eye pursuit,” which would provide a fourth metric. “Smooth eye pursuit” means that, when the eyes are tracking a stimulus side to side, they do so smoothly, not in a jerky manner like sticky windshield wipers. Would such a device reliably measure marijuana impairment? Let’s take each metric in turn.

Rebound dilation is a reliable and unique sign of psychoactivity of marijuana in someone’s system, but this device would measure it via a “light flash.” It’s unclear how this might work because assessing rebound dilation requires a continuous light stimulus. It is also unclear that rebound dilation can be measured and correlated to intoxication levels. Law enforcement currently regards this as a “presence or absence” metric—not a measurement of presence. Lack of convergence is also a sign of psychoactivity of marijuana, but occurs frequently in the sober population, as not everyone can cross their eyes. It is therefore not a tell-tale sign of marijuana impairment alone, but in conjunction with other signs. Horizontal gaze nystagmus is not associated with marijuana use at all, but with alcohol, PCP and/or depressant drugs. Lack of smooth pursuit (sticky windshield wiper eye tracking) is also not associated with marijuana use, but alcohol, PCP and depressant drug use. When police officers ask a driver to hold their head still and follow a stimulus side to side with their eyes, they’re assessing lack of smooth pursuit and horizontal gaze nystagmus to determine alcohol and/or certain drug use (depressants and PCP), not marijuana.

It appears that the latter two metrics are designed to detect alcohol impairment with the same device. While a marijuana impairment measuring device would help police remove dangerously impaired drivers from the public roadways, the metrics have to be right to begin with. Other devices purporting to measure marijuana intoxication have hit the market and largely failed to be adopted—as the alleged evidence provided doesn’t stand up to prosecution. The good news is, with a great need out there for a device that addresses true markers of intoxication, rather than blood plasma, technology has bent its expertise in that direction.