The police sergeant’s voice was quiet but firm. She told the college student exactly what he was going to do, and then he did it.
“You’re going to take a series of nine heel-to-toe steps,” she said. “You’re going to look at your feet, you’re going to count your steps out loud, you’re going to keep your hands by your side, and you’re not going to stop once you start. … Then you’re going to come back.”
He put one foot carefully in front of the other, like a tightrope walker who’d made the mistake of looking down.
That sobriety test might have taken place on a windswept roadside, where Sgt. Deborah Batista had just pulled the student over for swerving across lanes. But they were going through the motions in the relative comfort of a Massachusetts General Hospital office building, where researchers were testing a brain imaging device to see whether it could identify people driving under the influence of pot.
As more and more states legalize marijuana, scientists and entrepreneurs are rushing to devise a Breathalyzer for pot—something more objective than officers’ observations of people as they drive, and then as they attempt the walk-turn and the one-leg stand. At Mass. General, they’re taking a different tack: Instead of looking for the chemical ghosts of pot on your breath, researchers are using a sensor-studded cap to look for impairment in the brain.
They know it’s a long shot. “Whether or not it will be useful, practically, is anyone’s guess,” said Jodi Gilman, an assistant professor at Harvard Medical School and Mass. General’s Center for Addiction Medicine, who is one of the researchers running the study.
Even other approaches that are closer to market aren’t ready for the roadside yet.
“Marijuana is a tough one, we admit that,” said Charles Hayes, who helps run the Drug Evaluation and Classification program for the International Association of Chiefs of Police. “There is no test out there at this time that can give us some sort of numerical reading … that would tell us that person is impaired.”
Drug-testing experts tend to break the country down into two categories: blood states and urine states. Get pulled over looking like you’re driving under the influence of drugs, and that’s the bodily fluid that will be sampled and sent to the lab.
Both fluids can provide evidence that you’ve been using marijuana—so if possession or use is illegal, then the police might have a case. But if you’re in one of the 29 states that, along with Washington D.C., allow some of kind of pot use, then it’s impairment while driving they’re after.
Part of the problem is that the chemical signs of marijuana use last for different amounts of time in different bodily fluids, and they don’t necessarily mean you’re high just then. And unlike with alcohol, for THC, the active ingredient in pot, there is no agreed-upon level that would mean a person is legally considered stoned.
A handful of states have, according to the Governors Highway Safety Association, established a threshold THC level drivers can have in their blood. But many experts worry that frequent pot smokers might be well past these thresholds without showing signs of impairment, while police officers say that occasional users might fall below the limit but still be driving dangerously.
Yet eyelid flutters and body tremors and difficulties following walk-and-turn instructions cannot provide the kind of numerical evidence that an alcohol breath test can.
“Police officers have their own biases, and aren’t perfect … look at all the stuff going on with persecution of minorities,” said Gilman. “If you have an objective test, it’s not left up to any one person’s judgment.”
Meanwhile, Batista, the Middleborough Police Department sergeant, who is also the assistant coordinator for the Massachusetts Drug Recognition Experts program, worries that courts aren’t taking standard sobriety tests seriously, which means police are loath to make arrests.
“I know of cases where officers are saying, ‘I know you’ve been smoking weed, I’m seeing signs of impairment, but we’re just going to give you a ride and tow your car,’” she said. “That’s really not addressing the issue.”
Batista said she dreads the day—now set for July 1—when it becomes legal to sell recreational pot in Massachusetts. And what she’s most worried about are the kinds of stories told by the student whose sobriety she was testing.
John, the student who requested his last name not be used, first saw the study advertised on his university’s quickie job board. Gilman and her team were looking for regular pot users in the Boston area and John knew he fit the bill. He smokes about six days a week, almost always before dinner, because that’s when he gets the most bang from his bong: It elevates his takeout burritos and microwaveable orange chicken from plain old sustenance to existential delight.
By his account, John almost never drives stoned. The few times it’s happened—“definitely … no more than between 12 and 24 times,” he said—have been over his winter break, when he’s steered his parents’ car back to their house after hanging and smoking up and listening to Jay-Z with a friend. He talks about the experience the way someone might describe a drive after hot yoga, his eyes more attuned to the glow of headlights and the brightly colored sign outside of his old middle school.
“It’s entirely different from drinking and driving,” he said. “I feel almost more aware of what’s around me when I’m high. I feel almost safer.”
That’s exactly what worries Batista: People don’t just think they’re able to accomplish the multitasking and quick-thinking of driving when they’re high. They think they’re even better at it.
They’re not. “We’ve done a couple of driving studies where we get people stoned, get them behind the wheel in an obstacle course,” said Dr. Mike Lynn, the CEO and co-founder of Hound Labs, one of the companies now working on a Breathalyzer for marijuana. Part of the circuit involved a simulated cyclist popping out of nowhere, as cyclists sometimes do, and, Lynn reported, “all of them hit that cyclist when they were stoned, and most of the time, they didn’t even know they’d hit him.”
John’s trip into the lab—for which he’d get $100 and a chicken avocado wrap—didn’t involve anything as exciting as driving a closed course while blitzed. But if he were lucky, he’d still get a handful of large red pills that contained a strong dose of free THC.
“This is where the magic happens, this tiny windowless room,” said research coordinator Eve Manghis, leading him in.
He wasn’t getting the drug just yet. First, the researchers needed to look at his brain unbaked. Their device of choice was a cloth cap—a kind of wig wired up with emitters and receivers of infrared light—held in place by a chin strap.
The device uses a technique known as functional near infrared spectroscopy, or fNIRS for short. By scattering certain wavelengths of light through John’s skull and into the tissue underneath, the researchers could detect changes in blood flow to assess the extent of communication between different parts of the brain.
Before starting, Manghis and her colleagues fiddled with the cap to get it to work properly. They stuck a long cotton swab through the holes in the cap to twist John’s hair out of the way. They tried another aviator-like cap over the first, to see whether that helped. It didn’t. Then they smeared a bit of ultrasound gel between the sensors and John’s scalp. “I think we’re cruising,” said Manghis, after looking at the data coming in to the computer.
John sat still, his eyes closed, as the blood flow in his brain was translated into red and blue squiggles on a screen.
By looking at these data before and after participants got any THC—and by giving some of them a placebo—the researchers hoped to see whether being high produced any distinctive patterns of brain activity. They also wanted to know how that correlated with a police officer’s assessment of impairment.
At this early stage, it’s a bit hard to imagine—in these beige-carpeted offices where researchers fiddle with wires and cotton swabs, calibrating and re-calibrating, administering psychological tests and surveys—that an officer would be able to rig up one of these caps and look at blood flow in the brain on the side of the road.
After all, the researchers would need to make sure that the signals they are seeing are in fact pot impairment and not something else. As Gilman explained, she wants to avoid false positives at all costs, and that means making sure that marijuana impairment couldn’t be confused with sleep deprivation or a medication the driver is taking.
The side-of-the-road scenario is also challenging because no one would know what the driver’s blood flow in the brain looked like normally. “You don’t have the baseline, so it can’t be a within-person comparison, and that’s problematic,” said Heather Bortfeld, a University of California, Merced, psychologist who sits on the board of directors of the Society for functional Near Infrared Spectroscopy.
Meanwhile, Hound Labs is saying it will start taking pre-orders for its marijuana breath testing device from law enforcement agencies in the spring. The company said in an email that it has done hundreds of tests on human subjects and that there is an ongoing clinical trial at the University of California, San Francisco, to validate the machine’s performance.
John was disappointed not to get the warm “full-body high” he’d felt the last time he’d swallowed a handful of red capsules from the Mass. General pharmacy. “Last time, I was feeling effects at this point, so I think it’s a placebo,” he said. But when Batista arrived, he got up, ready to stand on one leg, touch his nose, and have his pupils peered into by a police sergeant. After that, he’d slip back into the windowless room to have his brain scanned yet again.