Why we understand what we are hearing: 'Context' matters when the brain interprets different sounds

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If you were walking alone during the middle of the night and heard a loud bang, you might be scared.

But if you were standing in a group on 5 November or 4 July, when the sky is full of fireworks, you might fully expect to hear a loud bang, or a series of them.

In a new study, researchers have worked out how we process the same kind of noises in different situations - and found the brain's interpretation of sound is influenced by cues from other senses.

If you were walking alone during the middle of the night and heard a loud bang, you might be scared. But if you were standing in a group on 5 November or 4 July, you might fully expect to hear a loud bang, or a series of them

The scientists looked into the way the same sense can be mapped in the brain in different situations. 

The new results explain how we interpret what we hear at a particular moment, according to a new study.

The report, published today in Nature Neuroscience, shows that nerve cells dedicated to hearing also rely on surrounding context to properly interpret and react to familiar sounds.

Researchers at NYU Langone Medical Centre used mice to map how the same sense can be perceived differently in the brain.  

'What the brain "hears" depends on what is "seen" in addition to specific sounds, as the brain calculates how to respond,' says study senior investigator and neuroscientist Dr Robert Froemke, PhD, an assistant professor at NYU Langone. 

Dr Froemke said the findings show mammals recognise sounds in the auditory cortex of their brains.

But, at the same time, the signalling levels of nerve cells in this brain region are being strengthened or weakened in response to surrounding context. 

If further experiments find similar activity in human brains, the researchers say their work may lead to precise explanations of why we act the way we do in certain situations.

These could be anxiety brought on during exams, sudden post-traumatic stress among combat veterans hearing loud noises, or the ability of people with dementia to better remember certain events when they hear a familiar voice or see a friend's face.

'Our study shows how the same sound can mean different things inside the brain depending on the situation,' said Dr Froemke.

'We know, for instance, that people learn to respond without alarm to the honk of a car horn if heard from the safety of their homes, but are startled to hear the same honk while crossing a busy street.'

The researchers monitored nerve circuit activity in mice when the animals expected, and did not expect, to get a water reward through a straw-like tube after the ringing of a familiar musical note.

When mice heard the sounds, researchers observed patterns based on a basic divide in the nature of nerve cells.

Dr Froemke said the findings show mammals recognise sounds in the auditory cortex of their brains. But, at the same time, the signalling levels of nerve cells in this brain region are being strengthened or weakened in response to surrounding context. Human auditory cortex shown

This work may lead to precise explanations of situation-specific behaviours. These could include anxiety brought on during examsor sudden post-traumatic stress among combat veterans hearing loud noises (stock image)

Each nerve cell 'decides' whether a message travels onward in a nerve pathway. 

Nerve cells that emit chemicals to tell the next cell in line to amplify a message are excitatory, while those that stop messages are inhibitory.

Combinations of the two strike a balance critical to the function of the nervous system.

Researchers found most of the nerve cells in auditory cortex neurons that stimulate brain activity had signalled less when the mice expected and got a reward.

Meanwhile, a second set of remaining 'excitatory' neurons signalled more when mice expected a reward, based on exposure to the two sensory cues, and got one.

Dr Froemke said the team next plans to assess how the hormones noradrenaline and dopamine affect auditory cortex neurons under different situations.

'If we can sort out the many interactions between these chemicals and brain activity based on sensory perception and context, then we can possibly target specific excitatory and inhibitory neurological pathways to rebalance and influence behaviors,' says Froemke.