Hearing Happens in the Brain: How Nerve Signals Become Voices, Words and Music

We tend to give the ears all the credit for hearing. They do essential work: they collect sound, amplify it and convert it into electrical signals. But a signal on its own is not yet a voice, a word or a melody. Turning a stream of nerve activity into something you recognise, care about and understand is the job of the brain. In a very real sense, the ears are the microphones and the brain is the listener.

What route do signals take from ear to brain?

Signals leave each cochlea along the hearing nerve and enter the brainstem, at the base of the brain. From there they pass through a series of relay stations on their way upward. Each station does more than pass the message on. Some compare the two ears to work out direction, as described in hearing with two ears. Others pick out rhythms, sudden changes or the start of a sound, which helps you react quickly to a slammed door before you know what it was.

The pathway eventually reaches the auditory cortex, an area on each side of the brain roughly above the ears. Signals from each ear travel to both sides, so each half of the brain receives information from both ears. In the cortex and the regions around it, sound is linked to everything else you know: language, memory, emotion and the scene in front of your eyes.

How does the brain fill in what the ears miss?

Real listening is full of gaps. A cough covers a syllable, a bus drowns out a word, a speaker turns away mid-sentence. Most of the time you never notice, because the brain quietly fills the holes using context, grammar and what it expects to hear next.

This prediction is powerful enough to fool you. Many people have sung a song for years with the wrong lyrics, because the brain locked onto a version that made sense to it. In experiments, listeners report hearing sounds that were actually replaced by noise, because the surrounding words made them so likely. Prediction is a strength in everyday life, but it also means that what you hear is partly what you expect.

Why can you pick out one voice in a crowd?

At a busy gathering, the sound reaching your ears is a jumble of voices, music and clatter. Yet you can follow one person, and even notice your own name spoken across the room. This is the work of auditory attention: the brain turns up the stream it cares about and turns down the rest, using cues such as direction, pitch, the speaker's voice and their lip movements.

Attention is a limited resource. When the incoming signal is weaker or less detailed, the brain has to work harder to separate and rebuild speech. That extra effort is one reason noisy places feel so draining for people whose hearing has changed, a theme explored in hearing but not understanding.

Does the brain change when hearing changes?

The hearing parts of the brain are not fixed. They respond to the input they receive. When certain pitches arrive more weakly over a long period, the brain areas that usually handle them may reorganise and respond differently.

This adaptability helps explain some puzzling experiences. Tinnitus, the sound some people hear without any outside source, is thought to involve the brain's response to reduced input from the ear, as discussed in tinnitus basics. It also means that the effort of listening and the way sounds are perceived can shift over time, which is one more reason to talk to a professional about any change you notice rather than wait.

Key takeaways

• The ears code sound; the brain gives it meaning by linking it to language, memory and context.
• Signals pass through several relay stations, and each side of the brain receives input from both ears.
• The brain constantly fills gaps using prediction, which can sometimes lead you to hear what you expect.
• Attention lets you follow one voice in a crowd, but it takes effort, especially when hearing has changed.
• The hearing brain adapts to the input it receives.