Not seeing is believing when it comes to a new device for testing cannabis intoxication.

The Impairment Measurement Marijuana and Driving device — or IMMAD — uses a Samsung VR headset specifically designed to test a driver’s peripheral vision. Subjects are required to push a Bluetooth button every time they see a flashing stripe on the edges of their field of vision.

“Marijuana causes temporary paralysis of the cells operating in the retina,” Denise Valenti, an optometrist and lead developer of the product, told International Business Times. “So, when you have certain neurologic deficit in your retina, you just can’t see the stripes. If you can’t see, you can’t drive.”

Valenti, who developed the IMMAD alongside computer science professor Mark Pomplun at the University of Massachusetts, said that adding eye tracking technology to the device should increase effectiveness in determining intoxication from cannabis.

“The final version will be a quick, simple, objective, sensitive, specific test of marijuana driving impairment for law enforcement,” she said. “This test will be threshold related and have a number value compared to a large normative database. That test will take two minutes per eye.”

Marijuana has proven notoriously difficult to accurately test for in the human body. The presence of THC, the psychoactive component of the drug, does not necessarily mean a person is impaired. And higher levels of marijuana use do not necessarily mean higher levels of impairment. And because marijuana-impaired drivers might also be using alcohol, past tests have proven less than effective.

“We’re going to need more research and more help from the medical community, from medical researchers, to help us understand different products with different levels of THC, how different individuals are affected by that, how that relates to impairment, and ultimately how that relates to the ability to drive a vehicle and a potential crash risk,” David Harkey, president of the Insurance Institute for Highway Safety (IIHS) and Highway Loss Data Institute (HLDI), told Consumer Reports.