Ever wondered why it is so difficult to ignore the sound of a crying baby when you are trapped aboard a train or aeroplane? Scientists have found that our brains are hard-wired to respond strongly to the sound, making us more attentive and priming our bodies to help whenever we hear it – even if we're not the baby's parents.

"The sound of a baby cry captures your attention in a way that few other sounds in the environment generally do," said Katie Young of the University of Oxford, who led the study looking at how the brain processes a baby's cries.

She scanned the brains of 28 people while they listened to the sound of babies and adults crying and sounds of animal distress including cats meowing and dogs whining.

Using a very fast scanning technique, called magnetoencephalography, Young found an early burst of activity in the brain in response to the sound of a baby cry, followed by an intense reaction after about 100 milliseconds. The reaction to other sounds was not as intense. "This was primarily in two regions of the brain," said Young. "One is the middle temporal gyrus, an area previously implicated in emotional processing and speech; the other area is the orbitofrontal cortex, an area well-known for its role in reward and emotion processing."

Young and her colleague, Christine Parsons, presented their findings this week at the annual meeting of the Society for Neuroscience in New Orleans.

The study suggests there is something special about the way baby sounds are processed by the brain, said Young. The fact that there was activity in the emotional areas of the brain could mean that the sound of a baby's cries are tagged as important even before our brains have had a chance to fully process them.

None of the study participants was a parent or had any particular experience of looking after babies, yet they all responded in the same way, after 100 milliseconds, to the baby cries. "This might be a fundamental response present in all of us, regardless of parental status," said Parsons.

The researchers also investigated how a baby's cry might subconsciously affect a person's behaviour as a result of activation of the sub-cortical areas of the brain. These are among the brain's most primitive parts and are important in controlling behaviours such as the fight-or-flight response and other responses that keep us alive in dangerous situations.

Parsons tested whether a baby cry somehow switched on such survival responses by asking volunteers to play a game of whack-a-mole, before and after listening to various sounds including babies, adults and animals in distress.

"They were faster and more accurate after listening to the infant vocalisations. It's almost like we have this improvement in our effort for motive performance immediately after listening to vocalisations that might facilitate care-giving behaviour," said Parsons.

She added that this subconscious switching of the body into alert mode could explain why a baby crying on a plane can be particularly problematic for anyone who is not the parent. "When you hear a baby on a plane, you're immediately alert, even if you don't want to hear it," said Parsons. "It's a sound that's very difficult to ignore."