In findings published in the journal Perception, Kevin Johnston and Brian Timney from the Schulich School of Medicine & Dentistry and Western’s Faculty of Social Science have shown that alcohol greatly affects the ability to adjust vision for brightness and contrast, which may be increasingly problematic when driving at twilight, as the suns dips below the horizon.
“We obviously know that alcohol impairs our decision making and motor skills but until now, we did not know how alcohol affects our vision,” says Johnston, a research scientist at the Department of Physiology and Pharmacology’s Laboratory for Neural Circuits and Behaviour. “What we have done now, using century-old methods, is find out exactly how much vision is impaired after drinking alcohol.”
Johnston and Timney, who serves as the Dean of Western’s Faculty of Social Science, used the Hermann Grid (http://communications.uwo.ca/media/img/grid.jpg), an optical illusion described by Ludimar Hermann in 1870, to understand how alcohol affects the perception of contrast.
“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,” explains Johnston. “It’s the way our visual system processes contrast or brightness differences that creates this illusion.”
The researchers were able to show that the apparent contrast of the illusory spots in the grid is reduced by 30 per cent at a blood alcohol level around the legal driving limit. This means that making distinctions between different objects based on lightness and darkness becomes increasingly difficult.
“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,” says Timney. “It’s at those times when you are going to be most affected, and impaired.”
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