Just a Thimbleful of Alcohol Could Impair Driving Ability

Alcohol levels as low as 0.015% can impair ocular tracking, a new study reports.

Posted Dec 24, 2020

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On Christmas Eve, many adults celebrate their holiday cheer with beverages that contain alcohol such as egg nog, glögg, champagne punch, hot buttered rum, or mulled wine. If, for any reason, you're planning to drink—even a little bit—and drive, new research suggests that blood alcohol concentration (BAC) levels far below the legal driving limit of 0.08 percent may dramatically impair hand-eye coordination. These findings (Tyson et al., 2020) were published on December 17 in The Journal of Physiology.

During this study, a research team from NASA's Ames Research Center identified how BACs as low as 0.015 percent can cause sensorimotor impairments related to human ocular tracking. Notably, first author Terence Tyson and colleagues at the National Aeronautics and Space Administration's Visuomotor Control Laboratory found that consuming the equivalent of less than half-a-bottle of beer or just a few sips of wine can result in previously unrecognized alcohol-induced impairments.

"Our findings provide a cautionary tale that the subjective experience of drunkenness is often not aligned with objective impairment of sensorimotor coordination. In other words, most people feel they are unimpaired after one drink, yet they are to a significant degree," Tyson said in a news release. "Thus, driving may be affected by drinking just a small amount of alcohol, even though the driver may feel fine and be well within the legal limit."

Over the years, NASA has developed non-invasive techniques for detecting mild cognitive impairments based on pupillary responses to flashes of light and ocular tracking of rapid or jerky eye movements (i.e., saccades). 

Although much of NASA's research is designed to monitor how various factors affect performance in aerospace environments such as living on a space station, their ultra-sensitive tools can also be used to study brain impairments we experience on Earth in ways that have real-world implications.

For this study, Tyson et al. measured 21 different performance metrics and gauged sensorimotor disruptions in response to dose-dependent amounts of ethanol (i.e., ethyl alcohol). During these randomized experiments, study participants had the neurological impact of alcohol consumption on their sensorimotor functions and ocular tracking assessed before and after drinking mixed cocktails containing different alcohol percentages.

This study involved a cohort of men and women in their 20s who typically consumed one or two drinks per week. Prior to being tested, each participant abstained from consuming any alcohol for several nights. They also self-reported getting a good night's sleep the evening before visiting the lab.

After conducting a series of different tests, the researchers discovered that ocular behaviors and pursuit eye movements used to track a target's speed and direction became impaired at very low blood alcohol concentration levels. "Our results show significant impairment of pursuit and visual motion processing at 0.015 percent BAC, reflecting degraded neural processing within extrastriate cortical pathways," the authors wrote.

Additionally, Tyson et al. found that saccade dynamics started to become "sluggish" at BAC levels as low as 0.035 percent. Smooth rapid eye movements were noticeably reduced after consuming relatively small quantities of alcohol.

Based on these findings, the researchers conclude that ultra-low BACs may impair neural activity and oculomotor functions in brain regions specifically associated with visually processing motion.

Not everyone responds to alcohol the same way. Although this study provides general insights into alcohol-induced impairments related to specific BAC percentage levels, countless variables—body weight, empty stomach, sleepiness, etc.—can influence how the same amount of alcohol might affect someone differently.

That said, if your BAC is anywhere above 0.00 percent, this research suggests that any high-risk activity such as driving or operating heavy machinery may be more dangerous than previously believed—and sobering up can take longer than you think. This chart, adapted from the American Education Systems of Eau Claire, shows how many hours it typically takes for men and women of different body weights to metabolize all the alcohol in their system and get back to a zero BAC. For example, it typically takes a woman weighing 120 pounds two-and-a-half hours to return to a 0.00 percent BAC after one drink; if a man weighing 160 pounds has one drink, it usually takes about two hours to get back to zero on a breathalyzer test.

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Terence L. Tyson, Nathan H. Feick, Patrick F. Cravalho, Erin E. Flynn‐Evans, Leland S. Stone. "Dose‐Dependent Sensorimotor Impairment in Human Ocular Tracking After Acute Low‐Dose Alcohol Administration." The Journal of Physiology (First published: December 17, 2020) DOI: 10.1113/JP280395

Accompanying Perspective piece by Philippe Lefèvre: "Want to Win a Bowling Game? Beware of Even One Glass of Alcohol!" The Journal of Physiology (First published: December 17, 2020) DOI: 10.1113/JP280984