New research has found that alcohol intoxication can increase inattentional blindness — meaning the failure to consciously perceive fully visible objects. The study was published in the scientific journal Psychopharmacology.

“Being intoxicated – even to within the legal driving limit – will significantly reduce your likelihood of noticing a novel or unusual event unfold right in front of you, if you happen to be engaged in some simple but on-going task,” explained study author Alistair J. Harvey of the University of Portsmouth.

“My interest in this field was sparked by jury service in 2009 when I was summoned to sit through two physical assault trials,” he said. “The defendant, victim and eyewitness claims about the causes and nature of the violence were crucial to the jury’s judgment, but in both cases most of these individuals were intoxicated with alcohol at the scene of the alleged crime.”

“I later discovered that this is the case for around 30-50% of violent crimes committed in public in the UK yet relatively little is known about the effects of alcohol on eyewitness attention and memory.”

The concept of “inattentional blindness” emerged thanks to a 1999 study known as the invisible gorilla experiment. In the study, participants were asked to watch a video showing a group of people passing a basketball. They were instructed to count how often the ball was passed.

While the participants were focused on counting how many times the basketball moved from person to person, a man in a gorilla suit walked into the scene and beat his chest before walking off. Surprisingly, only a small percent of the participants reported seeing the gorilla.

For their new study, Harvey and his colleagues recruited 104 patrons from a university’s student bar and used a breathalyzer to assess their level of intoxication.

As in the original experiment, the participants watched the basketball clip and were asked how many passes were made. They were then asked if they had noticed anything unusual about the game.

Alcohol intoxication increased inattentional blindness in an easy condition, where the participants only had to keep track of the number of total passes. But intoxication did not increase inattentional blindness in a harder version of the experiment, where the participants had to keep track of the number of aerial and bounce passes separately.

“If we drink sufficient alcohol and are engrossed in some on-going activity we are significantly less likely to notice what’s going on around us than when sober. This is because alcohol reduces our cognitive reserves, forcing the brain to allocate its dwindling mental resources to only the most important tasks at hand,” Harvey told PsyPost.

“However, our study reveals that alcohol does not impair peripheral attention when we are absorbed by a particularly demanding task and we think this is because hard tasks leave little to no mental capacity leftover for alcohol to deplete.”

The study has some limitations.

“Although our study shows significant effects of attentional narrowing at blood alcohol levels lower than this country’s legal driving limit, it is restricted to a simple situation in which the viewer’s attentional focus is engaged by only a single visual tracking task,” Harvey said. “Our findings cannot be generalised to more complex activities, such as driving, where the viewer must attend to an array of important visual stimuli.”

The study, “Alcohol increases inattentional blindness when cognitive resources are not consumed by ongoing task demands“, was authored by Alistair Harvey, Sarah J. Bayless and Georgia Hyams.

