New York - People who have been drinking may miss objects that appear unexpectedly in their field of sight, even when their blood alcohol levels are just half the legal driving limit.
"In light of this result, perhaps lawmakers should reconsider the level of intoxication deemed legal to operate a vehicle," Dr Seema Clifasefi of the University of Washington in Seattle and her colleagues suggest in a report.
This phenomenon, known as inattentional blindness, occurs commonly among people who are sober, Clifasefi and her team note.
Alcohol is known to impair fine motor skills, reaction time and visual attention, they point out, but to date no one has studied what effect it may have on inattentional blindness.
To investigate, the researchers had 47 volunteers watch a video of two teams passing basketballs back and forth and asked them to count how many times the team wearing white T-shirts passed the ball.
During the video, a woman dressed in a gorilla suit appeared among the players, stood in the middle of the screen and beat her chest, and then walked away.
The subjects were given a beverage and instructed to drink it over a 10-minute period five minutes before watching the video. After viewing it, the researchers interviewed them to determine if they'd seen the gorilla.
Half of the study participants were given a placebo drink, while the other half consumed a vodka and tonic formulated to bring their blood alcohol level up to half the legal driving limit for most US states, or 0.04.
Half of people within each of those groups were then told they were given alcohol, while the other half were told their beverages didn't contain alcohol.
People who were told they had received alcohol reported feeling more intoxicated, even if they had been given the placebo beverage. But whether or not people were told they had consumed alcohol had no effect on their likelihood of seeing the gorilla-costumed woman.
Overall, one third of the study participants didn't notice the gorilla. Among those who were sober, 46 percent spotted the gorilla, compared to 18 percent of the intoxicated group.
"Thus, alcohol appears to be adding an additional layer of blindness," Clifasefi and her team conclude in a report published in the journal Applied Cognitive Psychology.
"In our study, alcohol caused a narrowing of the attention to one specific aspect of a scene (counting passes), such that other information in the scene (the gorilla) was more likely to be ignored," they add, concluding: "Our mildly intoxicated subjects demonstrated a substantial perceptual deficit, indicating that even having one stiff drink can make you blind drunk."