New York, July 5 (Reuters): 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, Seema L. 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, the report notes. Alcohol is known to impair fine motor skills, reaction time and visual attention, but to date no one has studied what effect it may have on inattentional blindness.

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, 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, they were asked to determine if they had seen the gorilla.

Half the participants were given a placebo drink, while the other half had vodka and tonic to bring their blood alcohol level up to half the legal driving limit for most US states, or 0.04. Half of the 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 alcohol reported feeling more intoxicated, even if they had been given the placebo.

Overall, one third of the study didn’t notice the gorilla. Among those sober, 46 per cent spotted it, compared to 18 per cent of the intoxicated group.