Kevin Johnston, PhD
Kevin Johnston, PhD, professor of physiology, Brain and Mind Institute, University of Western Ontario, London, Ontario, Canada. His research was published in Perception.
You know that driving after drinking is dumb and potentially deadly—even if you aren’t legally drunk—because alcohol messes up coordination, impairs judgment and reduces alertness. And you know that people who are drunk may see double.
But you probably don’t know that your vision can be significantly impaired even if you keep your drinking within the legal limit…and the specific way in which alcohol affects vision presents particular dangers while you’re driving.
This new insight comes from a fascinating study that makes use of a particular optical illusion first described in 1870 to explain just how alcohol screws up vision. It all has to do with what you can think of as phantom blobs on roads…
You’ve probably seen the Hermann grid before. It’s a grid made of black squares neatly arranged in parallel rows and columns on a white background. The white lines are like roads running between the black squares and forming multiple intersections. When we look at the Hermann grid, our brains perceive gray areas at these intersections even though the gray areas aren’t actually there. The study researchers call these gray areas “blobs.” Can you see them in the grid at right?
This optical illusion is most likely due to a phenomenon called lateral inhibition that I will explain in a moment. But for now, suffice it to say that these blobs actually allow us to see better in the real world by making it easier to distinguish separate objects.
For the new experiment, researchers used a modified Hermann grid that included two rows and eight columns of black squares on a white background. A white circle was “cut out” of the middle of the grid, so that white replaced the inside corners of the four center squares. In the middle of this white circle was a fuzzy gray spot. Imagine that the white lines are like roads…the circle in the center of the grid is like a traffic roundabout…and the gray spot in the circle is like a fountain in the center of the roundabout.
For this study, participants who were typical, moderate social drinkers viewed this modified Hermann grid and performed some tests on two different days. On one occasion, the participants drank plain fruit juice prior to viewing. On the other occasion, they drank fruit juice spiked with just enough vodka to get their blood alcohol content (BAC) to a maximum 0.08%, which is the legal limit in the US and Canada. The testing began when each participant’s BAC reached 0.06%, a level at which it is still legal to drive.
On both occasions, the drinks were served through straws in lidded cups dabbed with peppermint oil—all to mask any alcohol smell or taste and make it hard for participants to know what they were drinking. Then participants sat in a darkened room and viewed a computer screen showing the modified Hermann grid. They were asked to say whether the real gray spot in the center of the white circle looked lighter or darker than the optical-illusion gray spots (the phantom blobs) they perceived in the intersections of the white lines. The test was repeated numerous times, with the researchers adjusting the relative contrast between the black and white sections of the grid in ways that made the illusory blobs more or less pronounced.
Here’s the really interesting (and scary) part: After the participants consumed the alcohol, they lost a lot of their ability to see the phantom blobs! Specifically, alcohol reduced the perceived darkness of the illusory blobs by 30%, the researchers estimated—based on the contrast level at which participants reported that the actual spot and the illusory spots appeared to be the same.
Now, you might think that it would be a good thing to not see what isn’t really there. But in fact, the phantom blobs are the result of a helpful vision phenomenon known as lateral inhibition. It’s complicated, but the basic premise of lateral inhibition is that the activity of a nerve cell suppresses the activity of nearby nerve cells. Our eyes contain different types of nerve cells, including photoreceptors that respond to varying degrees of light. When one photoreceptor activates in response to light, its neighboring photoreceptors are inhibited. In the Hermann grid, the intersections appear darker than the middle sections of the white roads because four bright areas are coming together, increasing the lateral inhibition. When something interferes with this normal lateral inhibition, the contrast between objects is diminished.
To put this in real-world terms, alcohol messes with lateral inhibition, reducing your ability to clearly distinguish objects from one another, especially at night. This means that, after drinking, you are likely to have more trouble telling where the road stops and the sidewalk starts…or seeing a person who’s crossing the street. And that’s dangerous!
Keep in mind that the study participants were not especially drunk during the experiment. To reach a BAC of 0.06 (the level at which the study participants’ testing began), a 120-pound woman would need to consume only two four-ounce glasses of wine within one hour, and a 175-pound man would need to have just three 12-ounce beers in one hour.
If you want to get a general idea of how drinking affects your BAC, spend some time looking at a blood alcohol calculator. Many factors contribute to alcohol’s effects, including your age, gender and size, how quickly the alcohol is consumed and whether you have anything to eat before drinking.
Bottom line: We already knew that drinking and driving is a bad combination, and now we have a better understanding of why that is. Remember those blobs…if you imbibe, even if you aren’t drunk, leave the driving to someone else.
