Research shows that ‘hands-free’ doesn’t mean ‘brain free’ and that talking while driving is a risky proposition.

Many people pride themselves on their multi-tasking abilities, but maybe the human brain just isn’t equipped to handle more than one complex task at a time.

Researchers from St. Michael’s Hospital in Toronto proved this point when they tested how well drivers could make a left turn while talking on a hands-free cell phone. Their results were published in the journal Frontiers in Human Neuroscience.

Researchers monitored the brain activity of young, healthy drivers using a functional magnetic resonance imaging (fMRI) machine while the research subjects used a driving simulator.

As the test subjects drove in increasingly difficult situations, researchers monitored their brain activity, namely which parts of the brain were activated during different driving tasks. They even spiced things up by having the subjects answer true-or-false questions while driving, simulating the experience of making a hands-free phone call.

They found that during a left turn—especially while talking—the brain’s focus shifts from the visual cortex to the prefrontal cortex. Basically, the brain diverts power away from the part that let’s you see where you’re going and toward the part dedicated to decision-making.

“Visually, a left-hand turn is quite demanding,” lead researcher Dr. Tom Schweizer said in a press release. “You have to look at oncoming traffic, pedestrians, and lights, and coordinate all that. Add talking on a cell phone, and your visual area shuts down significantly, which obviously is key to performing the maneuver.”

So, for the safety of everyone involved, don’t confuse your brain and keep your eyes—and the rest of your focus—on the road, especially when taking a left.

The bottom line: Just don’t use your cell phone while driving.

“’Hands free’ does not mean ‘brain free,'” Schweizer said.

Left turns while talking aren’t the only things we’re not equipped to deal with. Research from earlier this year conducted at the University of Wisconsin-Madison found that those who multi-task the most are actually the least able to handle it.

The researchers found that those most likely to multitask do so not to get things done efficiently, but because they show high levels of impulsivity and sensation-seeking.

In fact, the main reason people attempt more than one task at a time is that they are unable to focus on a single item.

“Multi-tasking was shown to be particularly high amongst impulsive individuals who act without thinking and who have difficulty regulating their attention,” the researchers concluded. “These findings clearly suggest that multi-tasking is a matter of who is able to not multi-task as much as it is a matter of who is able to multi-task.”