Don’t worry, your friend looking at their phone isn’t ignoring you when you talk — they didn’t even process your voice in the first place.

A new study by the University College London showed that concentrating one’s attention on a visual task can render them momentarily “deaf” to normal sounds like speech. Published in the Journal of Neuroscience, the research highlights how the brain’s visual and auditory processing centers share limited neural resources. When presented with both, the brain has to choose which to devote its attention to.

The researchers had 13 volunteers complete visual tasks of increasing difficulty while listening to sounds. Brain scans from the volunteers revealed brain responses to sound were dampened while they engaged in the more difficult tasks.

“This was an experimental lab study, which is one of the ways we establish cause and effect,’ explained study co-author Dr. Maria Chait, from the UCL Ear Institute, in a statement. “We found that when volunteers were performing the demanding visual task, they were unable to hear sounds that they would normally hear. The brain scans showed that people were not only ignoring or filtering out the sounds, they were not actually hearing them in the first place."

The idea of “inattentional deafness,” or failing to notice sounds while engaged in other things, has been studied by scientists before. This time, though, was the first occasion in which researchers were able to determine that the effects are driven by brain mechanisms in a very early stage of audio processing.

“Inattentional deafness is a common experience in everyday life, and now we know why,” said co-author Professor Nilli Lavie, from the UCL Institute of Cognitive Neuroscience. “For example, if you try to talk to someone focusing on a book, game, or television program and don’t receive a response, they aren’t necessarily ignoring you, they simply might not hear you! This could also explain why you might not hear your bus or train stop being announced if you’re concentrating on your phone, book, or newspaper.”

We might not be as good at multitasking as we think, and while most of the time the problem is harmless, there are occasions in which this attention deficit could be dangerous.

“This has more serious implications in situations such as the operating theater, where a surgeon concentrating on their work might not hear the equipment beeping,” Lavie said. “It also applies to drivers concentrating on complex directions as well as cyclists and motorists who are focusing intently on something such as an advert or even simply an interesting-looking passerby.”

Lavie explained that loud sounds like sirens and horns would be able to grab their attention, but quieter sounds — like a bicycle bell or car engine — may be in danger of going unheard.

Source: Molloy K, Griffiths T, Chait M, Lavie N. Inattentional Deafness: Visual Load Leads to Time-Specific Suppression of Auditory Evoked Responses. The Journal of Neuroscience. 2015.