Why do some people have trouble distinguishing left and right?

When British neurosurgeon Henry Marsh sat by his patient’s bedside after surgery, he knew the bad news he was about to deliver was his mistake.

The man had a pinched nerve in his arm that required surgery, but after making a midline incision in his neck, Marsh punctured the nerve on the wrong side of his spine.

Preventable medical errors often involve surgery done on the wrong side — an injection in the wrong eye, for example, or a biopsy of the wrong breast.

These “never events” (medical jargon for serious and largely preventable patient safety accidents – i.e., not supposed to happen) highlight that while most of us were taught left and right as children, not all of us do well.

While for some people distinguishing left from right is as easy as distinguishing top from bottom, a significant minority (about one in six people, according to a recent study) struggle with the distinction. Even for those who think they have no difficulty, distractions such as ambient noise or having to answer questions unrelated to what you are doing can get in the way of getting the right answer.

“Nobody has a hard time saying (that something is) back and forth, or up and down,” says Ineke van der Ham, assistant professor of neuropsychology at the University of Leiden in the Netherlands.

But differentiating left from right is different. “It’s because of the symmetry and because when you turn around it’s the other way around, and that makes it very confusing.”

Multiple parties involved

Distinguishing between left and right is actually quite a complex process, requiring memory, language, visual and spatial processing, and mental rotation.

In fact, researchers are just beginning to understand what exactly happens in our brains when we do this, and why it’s so much easier for some people than others.

“Some people can innately tell right from left. They can do it without thinking,” says Gerard Gormley, a physician and professor at Queen’s University Belfast in Northern Ireland. “But others have to go through a process.”

In an effort to understand what happens in medical errors involving the wrong side, Gormley and his colleagues conducted research on the experience of medical students making left-right decisions and examined the process.

“First of all, you must orient yourself from right to left,” he says.

When the answer doesn’t come instantly, participants describe various techniques, from making an L with their thumb and forefinger, to thinking about which hand they use to write or play the guitar. “For some people, the reference might be a tattoo or body piercing,” says Gormley.

So when you figure out which side the other person is on, left or right, the next step is to mentally face the same direction as the other person. “If I’m facing you, my left hand will be facing your right hand,” says Gormley.

“This idea of ​​mentally rotating an object adds an extra degree of complexity.”

Other research shows that people find it easier to judge whether an image shows their left or right hand by imagining their own hand or body turning.

Research published by Van der Ham and colleagues in 2020 revealed that around 15% of people consider themselves insufficient when it comes to identifying left and right.

Nearly half of the study’s 400 participants said they used a hand-related strategy to identify which is which.

The researchers used the so-called Bergen left-right discrimination test to delve deeper into how these strategies work.

Participants looked at images of people drawn as stick figures. Some figures were looking at them and some were not, and they had their arms in various positions. People then had to identify the hand that stood out, whether it was the left or the right.

“It sounds simple, but it’s a little frustrating if you have to do it a hundred times as fast as possible,” says Van der Ham.

In the first experiment, participants sat with their hands on a table in front of them.

“There was a very clear effect in the way this little doll was placed,” says Van der Ham. If you looked at the back of the head, then it was in line with you, so people were much faster and more accurate.”

Likewise, when the puppet was facing the participant but with the hands crossed so that the left hand was on the same side as the participant’s left hand, people tended to do better.

“It tells us that the body is really involved in this,” says Van der Ham.

Strategies

The next question was whether participants were using signals from their body at the time of the test to identify left and right or whether they were using a stored idea about their body as a reference.

To answer this, the researchers repeated the experiment, but this time they tested four different scenarios: Participants sat with their hands crossed or uncrossed on the table in front of them and had their hands visible during the test or covered with a cloth. black.

But the researchers found that none of these changes influenced test performance. In other words, participants didn’t need to see their hands to use their own bodies to tell right from left.

“We haven’t completely solved the problem,” says Van der Ham. “But we were able to identify our bodies as a key element in distinguishing left from right, and we queried our body representation in a more static way.”

In Van der Ham’s experiments, the increase in performance resulting from being aligned with the puppet was most pronounced in people who said they used a hand-related strategy to differentiate left from right in their daily lives, as well as in women in general.

The researchers also found that men tend to respond faster than women, but the data do not corroborate previous studies showing that men perform better overall on tests of left-right distinction.

It’s unclear exactly why people differ in their ability to tell left from right, although research indicates that the more asymmetrical a person’s body is (in terms of hand preference for writing, for example), the easier it is to it differentiate left and right.

“If one side of your brain is slightly larger than the other, you tend to have better left-right differentiation,” says Gormely.

First years

But it could also be something we learn in childhood, like other aspects of spatial cognition, Van der Ham says.

“If kids are in charge of leading the way, if you just let them walk a few feet ahead of you and make the decisions, those are the kids who end up being better navigators,” she says.

Research by Alice Gomez and colleagues at the Lyon Neuroscience Research Center in France suggests that left-right differentiation is something children can learn quickly.

Gomez designed a two-week, teacher-led intervention program to increase body representation and motor skills for children aged 5 to 7 years.

When the ability to locate the correct body part on oneself or on a partner (the right knee, for example) was tested after the program, the number of left-right differentiation errors was almost halved.

“It was very easy for us to increase the children’s skills in locating (the body part) based on the name,” says Gomez.

One reason for this may be that the children learned a strategy: think about which hand they use to write when they couldn’t remember right and left.

The show’s focus on children’s own bodies is another possible explanation, especially since other research shows that an egocentric frame of reference is key when making left-right decisions.

In a regular classroom, kids might label a puppet’s body parts instead of their own bodies, because the latter is more time consuming and harder for a teacher to assess, says Gomez.

the most challenging

While there are many everyday scenarios where it’s important to distinguish between left and right, there are some situations where this is absolutely vital.

Neurosurgeon Marsh managed to correct his nerve decompression surgery, but a scenario of a surgeon removing the wrong kidney or amputating the wrong limb, for example, would have devastating consequences.

Medicine isn’t the only field where left-right errors can mean the difference between life and death: it’s possible that a helmsman turning the boat to the right instead of the left contributed to the sinking of the Titanic.

But while some people have to work harder to tell left from right, everyone is likely to make those decisions wrong, says Gormley.

The expert hopes that greater awareness of the ease of making such mistakes will lead to less stigma against those who need to verify their decision.

“As healthcare professionals, we spend a lot of time labeling spatial orientations — proximal, distal, top, bottom — but we don’t pay attention to left or right,” he says. “And actually, of all the spatial orientations, this one is the most challenging.”

a different brain

About one in ten people are left-handed, and twin studies have shown that genetics play a role.

A study from the University of Oxford in the UK recently revealed four regions in human DNA that appear to play a crucial role in determining whether someone is left-handed or right-handed.

Those who were left-handed had “mutations” in four genes that code for the body’s cytoskeleton, the complex scaffolding found inside cells to help organize them.

Scans of people with these mutations showed that the white matter in their brains had a different structure.

The left and right sides of left-handers’ brains were also better connected than those of right-handers.

*This text was originally published in English on BBC Future