Having a designated driver on hand while drinking is an important part of being responsible on the road. Though blood alcohol concentration restrictions are imposed on drivers, how is our vision affected when we are under this limit? Researchers in Canada set out to answer this question and found that our vision is impaired by up to 30% – before we even hit the legal limit.

The research, published in the journal Perception, was conducted by Kevin Johnston and Brian Timney, from the Schulich School of Medicine & Dentistry and Western University’s Social Science department in Ontario.

They say though it is widely known that alcohol affects decision-making and motor skills, until now, there have been few studies to analyze how alcohol affects vision.

However, rather than using modern technology to conduct their research, they employed a 144-year-old optical illusion, called the Hermann Grid.

This illusion was described by Ludimar Hermann, a German physiologist, in 1870.

Johnston explains how the illusion works:

The Hermann Grid is basically a grid of black squares on a white background. You see ghost-like dark spots at the intersections of the grid, but they are not actually there. It’s the way our visual system processes contrast or brightness differences that creates the illusion.”

Though staring at this Grid for any length of time is enough to make the viewer feel like they have had a couple of drinks, the researchers say it has helped them to “find out exactly how much vision is impaired after drinking alcohol.”

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Only had ‘one or two’? New research suggests even under the legal driving limit, alcohol consumption can impair vision by 30%.

The legal blood alcohol concentration (BAC) limit for driving in Canada and most parts of the US is 0.08%.

To conduct their study, the researchers had participants drink either nonalcoholic drinks or enough alcoholic drinks to keep them just under the legal limit.

After consuming their drinks, the participants then estimated the contrast of the blobs – that Johnston described as “ghost-like dark spots,” which are present at grid intersections – by using a matching procedure.

For the participants who drank the alcohol, the contrast of the spots in the grid was reduced by 30% when their BAC approached the legal driving limit.

The researchers say this visual impairment would make distinguishing differences between objects based on lightness and darkness quite difficult.

They also add that their results suggest alcohol reduces lateral inhibitory interactions – the inhibition neighboring neurons in brain pathways have on each other – in human vision.

Timney says:

This is obviously important when you are driving at twilight, when objects are more difficult to see and more difficult to discriminate, even without alcohol. It’s at those times when you are going to be most affected, and impaired.”

Drivers in the US may want to take heed, as the Centers for Disease Control and Prevention (CDC) recently estimated that at least 38 million Americans drink too much.