Depth Perception and Drunk Driving

December 29, 2004

While it’s well-known that drinking and driving don’t mix, new research sheds light on another reason drunk drivers are likely to be dangerous on the road. A particular type of depth perception, called motion parallax, is disrupted by ethanol intoxication.

As revelers prepare to ring in the New Year, a recently published neuroscience study by researchers at North Dakota State University (NDSU), Fargo, provides insight on depth perception and drunk driving. While it’s well-known that drinking and driving don’t mix, new research from the North Dakota State University Center for Visual Neuroscience sheds light on another reason drunk drivers are likely to be dangerous on the road. A study by Mark Nawrot, Ph.D., associate professor of psychology, shows how a particular type of depth perception, called motion parallax, is disrupted by ethanol intoxication. The study is reported in the December issue of Psychological Science, a flagship journal of psychology research.

The goal of the NDSU neuroscience researchers is to understand the role of eye movements in depth perception, according to Dr. Nawrot. While the study revealed some important facts regarding brain function, eye movements and depth perception, results of the study help explain why driving while intoxicated can be so dangerous. “In addition to other 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,” said Nawrot. That inability to judge the depth or distance from the driver’s vehicle to a tree, a light pole or another vehicle, for example, can result in accidents.

Police conducting road-side sobriety tests know that alcohol affects eye movements. It’s one of the reasons suspected drunk drivers are asked by law enforcement to follow the movement of an object when they are initially stopped road-side. Jerky eye movements, also called horizontal gaze nystagmus, are observed during field sobriety tests used by law enforcement.

Though depth perception with two eyes, called binocular stereopsis, is very important, depth perception caused by our own motion—as in a moving vehicle—is much more important. NDSU researchers are studying the brain mechanisms underlying this process called motion parallax.

The motion parallax experiments

Nawrot and researchers Benita Nordenstrom and Amy Olson used alcohol intoxication to disrupt the brain’s control of eye movements and to then study the impact on depth perception. The study demonstrates that because of its influence on what is called the slow eye movement system, ethanol intoxication impairs the perception of depth from motion parallax. Intoxicated drivers may have inaccurate or inadequate information for judging the relative depth of obstacles from motion parallax.

The eye movements of 15 participants who met stringent screening criteria were measured with a head-mounted infrared eyetracker. Subjects were an average age of 24, with an average body mass index of 25. Psychophysical tests included binocular disparity, motion parallax, pursuit eye movements, compensatory eye movements in light and dark, and motion perception. Participants first completed the psychophysical tests while sober. They then returned and in a precisely-controlled laboratory test environment, completed the tests while intoxicated.

Observers were administered ethyl alcohol to achieve a blood alcohol content approaching 0.1% which was then measured with a breath analysis. Observers in the intoxicated condition experienced great difficulty performing the motion parallax task—one that they performed flawlessly when sober. Participants remained in the lab until their blood alcohol content declined sufficiently and were then given a taxi ride home.

Relevance of findings

“Although the current study suggests that intoxicated drivers may have difficulty determining the relative position of obstacles using motion parallax,” said Nawrot, “this may be only one part of a broader, but more poorly understood, set of visual perceptual problems caused by ethanol’s effect on the eye movement system.”

Study of ethanol’s effect on eye movement could lead to a better understanding of the precise blood alcohol levels at which drivers become impaired, said Nawrot. Data from the National Center for Statistics and Analysis-National Highway Traffic Administration (NHTSA) shows that from 1998 to 2003, of the total fatalities on New Year’s Day, 42 percent involved an impaired driver.*

Research into the role of eye movements in motion parallax is relatively new. There are approximately four published studies on the subject, three of them by Nawrot, whose work has also appeared in the Journal of Vision and Vision Research.

The recent study was part of a four-year project funded by the National Institutes of Health-National Eye Institute. NDSU researchers were also recently awarded $8.9 million for a five-year National Institutes of Health grant to establish a Center for Biomedical Research Excellence (COBRE) for visual neuroscience. Through the competitive grant, NDSU researchers will examine various aspects of the neural mechanisms and functional significance of visual perception, visual attention, visual cognition and action.

Research article: “Disruption of Eye Movements by Ethanol Intoxication Affects Perception of Depth from Motion Parallax,” by Mark Nawrot, Benita Nordenstrom, and Amy Olson, Psychological Science, Vol. 15, No. 12, December 2004.

Source: North Dakota State University Center for Visual Neuroscience