Can you focus all of your attention on one task until it’s complete? While this may be the most productive way to check things off of your to-do list, there may be an unforeseen safety risk associated with extreme concentration. According to a report from ABC News, a new study warns that focusing intently on a task could leave you temporarily deaf.

This may explain why it’s so darn hard to speak to someone engulfed in their smartphone. They aren’t ignoring you, researchers from the University College London say; they simply might not hear you. The study showed that the brain’s sense of vision and hearing both share similar circuitry, and the mind is often forced to choose between which sense to focus on at a given time.

The study was published in the Journal of Neuroscience. Researchers examined 14 different people as they competed visual tasks of increasing complexity while audio played in the background. By keeping tabs on the participants’ brain activity, the researchers were able to tell when exactly the brain chose vision over hearing.

As visual tasks became more demanding, the researchers found that the participants’ early response to sound was significantly delayed. The scientists hypothesized that the participants weren’t simply ignoring the sound samples; they actually couldn’t hear them.

According to the study’s author, Nilli Lavie, a professor of psychology and brain sciences at the University College London, “In order to hear, we don’t just need our ears to be operating; we need our brain to respond to the sound. If our brain doesn’t respond because our attention is fully taken by another task, then we experience deafness.”

The study confirms that your friend’s aren’t just being jerks when they ignore you; they may be so engulfed in what they’re doing that they simply didn’t hear what you had to say. Brain signals occur rapidly after a sound occurs, usually in under a quarter of a second. The brain already has a short window of time to respond to a stimulus, and it appears to have difficulty switching between senses rapidly. So give your busy buddies a break, and maybe a soft tap on the shoulder if you’re trying to get their attention.

A press release from University College London outlining the details of the study can be found here.