In depth
Psychology researchers study the effects of alcohol on eye movement
The fifteen volunteers knew what going to the eye doctor was like. But this was, well, different. In a small and narrow basement laboratory not even as big as a bedroom, the darkness of four walls wrapped in black felt material makes it vaguely claustrophobic. The hum of computer fans and shafts of light from monitors add little in the way of ambiance. Sitting down in front of a computer screen, each volunteer wears functional, but clearly unfashionable, goggles.
A series of black dots passes from one side to another on the computer monitor, with each tiny movement of the eyes measured, as the goggle-clad volunteers intently follow the floating dots. Streams of data are pumped into computers, resulting in paper strips with print-outs resembling those from an EKG machine. More tests ensue. Some are familiar, like placing one's chin and forehead in a tracking device that moves from side to side, similar to a test in the optometrist's office. Others are a jumble of passing dots. After two hours, it is time to go home - at least for now. The volunteers know that their second visit here will be very unlike their first.
The fifteen participants return to the visual neuroscience research lab at North Dakota State University soon enough. No muscle-bound bouncers are present for the second visit, although IDs are checked and the volunteers are scrupulously screened. There is only one choice of beverage to order here. Each volunteer receives a dose of ethyl alcohol according to body weight. Ten minutes later, another dose. Out comes the large beige steel box exotically named the Intoxilyzer(R) 5000, an infrared evidentiary breath analyzer. Breathe into it, measure and record. In another ten minutes, administer another precisely-measured dose of alcohol. Out comes a Lifeloc FC10, unfortunately familiar to anyone ever stopped by law enforcement and asked to blow into a portable breathalyzer. The volunteers know the drill. Breathe out, measure and record. Another dose is administered. Breathe, measure and record yet again, until a blood alcohol content approaching 0.1 percent is achieved by each participant. Try following the dancing black dots across a computer screen after that. More eye movement measurements, more data, more waiting. Then it's time to sober up. Researchers call cabs to take everyone home.
Mark Nawrot thought a research study involving 100 proof vodka and orange juice might raise a few eyebrows, particularly on a college campus. But he was focused on finding the answers to some questions. Little did he know the answers he found would appear in media around the world.
As a neuroscience researcher and associate professor in the Department of Psychology at NDSU, Nawrot studies how the eyes and the brain work together. He and researchers Benita Nordenstrom and Amy Olson are particularly intrigued by the crucial connection between eye movements and depth perception. What would happen if alcohol disrupted it?
Nawrot patiently explains the science of eye movements, with infectious enthusiasm for the subject. "Eye movements track in two speeds - fast, called saccadic, and slow, known as pursuit," he says. "We use fast eye movements to dart our eyes and direct our attention to an object," he says. "We use slow eye movements to fix our gaze and track moving objects." As we read a book or a magazine article, for example, we use fast eye movements as our eyes jump from point-to-point, following each line of text. A basic understanding of eye movements is one step to understanding the perception of depth, which relies on the slow eye movement system.
Try remembering the first time you parallel parked a car or made a left turn without a green-arrow light in rush-hour traffic. Judging your spatial relationship to other objects can result in successful driving or in a fender bender. It is the practical application of one type of depth perception known as motion parallax-depth perception caused by our own motion.
When going about our daily routines, the visual perception of depth is usually quick and effortless. We judge it almost automatically, or at least our brain interprets it through our eyes. Whether walking or driving, we constantly use motion parallax to judge what's nearer or farther. What's our spatial relationship to the door or the tree or that car?
The messages received from our eyes and processed at lightning-fast speeds by our brain allow us to move about without running into things. What's going on in our brain as we unknowingly calculate this depth? And what is the impact of alcohol on our ability to do it?
Such questions led to the motion parallax experiments by Nawrot and his research team. Methodology for the study was painstakingly developed. The flyer posted to recruit participants noted anyone under 21 need not apply, among other restrictions. But before volunteers could even be found, Nawrot provided details of his proposed research before the university's Institutional Review Board. Made up of faculty, medical and clergy members, it examines proposed studies involving human research subjects. The board concluded that appropriate safety measures were in place, in compliance with federal and school guidelines. Approval to conduct the study also hinged on review by university administration, which granted approval.
Although one might think there would be legions of volunteers on a college campus for a study where they're asked to drink alcohol, the stringent criteria ruled out many people. Participants committed to separate 3-hour and 2-hour time periods and extensive vision testing. The average age of volunteers in the study was 24.
The volume and types of eye movements measured in the experiments were significant. Pursuit eye movements were monitored and recorded. Just visualize a quarterback passing a football or think of watching other vehicles on the road while driving. Both use pursuit eye movements. Then imagine a wide receiver in a football game as he runs while following the ball and trying to catch it. These compensatory eye movements, as they are known, are what we use when we drive to allow for our constant head movements.
The researchers also measured other factors such as binocular disparity, which is the difference between the viewpoints of our two eyes. Crunching all that data provided some interesting research results.
Making headlines
The editor of the flagship journal Psychological Science, in which Nawrot's research was published, noted its importance. He called it "a near perfect blend of an applied and theoretical advance. ... " A peer reviewer commented that "results are of importance on theoretical grounds. ... and are also of practical importance because they provide additional evidence about a specific type of deficit alcohol can induce in a drinking driver."
That connection between depth perception, alcohol and driving made headlines. "Perceptions: Drunk, and Out of Your Depth" appeared in the New York Times. "Alcohol impairs depth perception" said CNN, the Canadian Broadcasting Corporation and the BBC in the United Kingdom. "It's all Greek to me," was the subject line in one email that contained a story from a publication in Greece. It was only the beginning. Research results discovered by the team at NDSU's Center for Visual Neuroscience appeared in media around the world, with information published in German, Russian, Spanish and Italian. "Why drunk ones more frequently against lantern stakes drive," was one headline translated from a German publication's Web site. Publications in Slovenia, India, Turkey and Asia covered it. Nawrot also received media inquiries from Ireland and Switzerland.
He is pleasantly surprised by the attention. The pages of the New York Times reach a different audience than that of scholarly journals. "I still find it funny that the words 'motion parallax' could appear in the general press. Drunk driving must be an issue in many countries," he says. A news release summarizing the study found its target audience shortly before New Year's Eve by mentioning statistics from the U.S. National Highway Traffic Safety Administration which showed that from 1998 to 2003, on New Year's Day, 42 percent of fatalities involved a drunk driver, compared with 31 percent during the entire year.
How it applies to real life
Although experiments were conducted in a lab, they offer another clue as to why drunk drivers are likely to be dangerous. At a blood alcohol content approaching 0.1 percent, motion parallax became at least 4.5 times worse. "Participants were nearly motion parallax blind," says Nawrot, giving additional meaning to the phrase "blind drunk." This aspect of alcohol intoxication was previously unknown. "In addition to well-known problems such as impaired decision-making, poor coordination and balance, the study shows that intoxicated drivers have difficulty judging the relative depth of objects that they are trying to avoid while driving."
It may be only one part of a broader, but less understood, set of visual perceptual problems caused by alcohol on the eye movement system. Study of alcohol's effect on eye movement could lead to a better understanding of the precise blood alcohol levels at which drivers become impaired, says Nawrot.
The road ahead
Research into the role of eye movements in motion parallax is relatively new. Only four studies have been published on the subject, three of them by Nawrot, in the Journal of Vision and Vision Research. It's only relatively recently that researchers have been able to look at eye movements, he says. Earlier equipment to measure them was cumbersome and expensive. The technology progressed and the cost of equipment decreased. "It was the right idea at the right time with the right technology."
An overnight success, if you will, after eight years. Patience is a virtue, as our mothers once told us. Ask any researcher. Nawrot estimates it took methodical, detailed, continual research over four years to develop the new theory of the role eye movements play in depth perception. His recently-completed study funded by the National Institutes of Health's National Eye Institute was a way to test that new theory. Disrupt the eye movement system with alcohol and measure the effect on the perceptual system. Initial design of the motion parallax study and the actual experiments took two years. Add another year to analyze the tremendous amount of eye movement data. Tack on another year for peer review and publishing of results.
Still, Nawrot's enthusiasm doesn't wane. He acknowledges the complexity of that relationship between the brain and the eyes results in more questions than answers. "I'll grab a brain here," he tells a visitor, locating a plastic model in his office for a more detailed 3-D example. "The frontal eye fields are one region that receive and feed information to other brain and brain stem areas for eye movement," he says, cradling the model in his hands. "The movements themselves are driven by nerves coming out of the brain stem. There are just twelve pairs of cranial nerves in the brain and one-third of them are involved with vision and eye movement. We have another twenty or thirty years of study to go on this."
-- Carol Renner
