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The auditory brain receives the signals sent by the cochlea, interprets them and starts either reflexes or conscious perception, depending upon motivation and alertness. It is also responsible for the creation of memories, which is vital for future percepts!
Auditory brain centres
Auditory nerve fibres transmit the signals sent from the cochlea to the brain. In the brain, numerous relay stations (groups of neurones) receive the signals and decode them (soft or loud sound, high or low, its location etc.) in order to cause a sensation or conscious perception.
In exchange, the brain can alter how the cochlea functions. For example, in the general noise of a cocktail party we are able to focus on a friendly conversation, even though our ears are getting stimulated by many different sources, which are often louder. Our brain has "asked" to prioritise the information coming from the interesting conversation!
Auditory perception depends upon our alertness
Sound, which is transformed in the ear into a neural signal, is processed in the brain at a number of different levels:
- A reflex where the arrival of the message causes us to jump or turn our head
- The auditory cortex where the sound is perceived
- Other brain areas, which allow the perception to become conscious, recognise the sound by comparing it to those that have previously been memorised and determine an appropriate voluntary response.
These animations show the different stages of sound processing in the brain.
When awake, all three levels above are activated.
Example: when we hear the sound of a voice, we start to listen (reflex), recognise a friend’s voice (memory)asking an important question (motivation, emotion) and then we answer.
When asleep, our ears are still working; sound enters the auditory pathway (and reflexes can therefore still occur) up to the auditory brain, but the other brain regions (involved in emotions, motivations, memory, etc…) are inactive: There are therefore no voluntary responses or conscious perception.
Example: speaking to someone who is asleep (or a sound from the street) can make them move without waking them, and without them remembering it when they wake up.
- When asleep, a loud and unexpected noise can wake us up, it then becomes ‘conscious’ and will cause a voluntary response (a prime example is an alarm clock!)
- We can ‘regulate’ our wakefulness so that an important sound, even if it is soft, will not be ignored (example: a baby’s muttering).
- The areas used when awake can also be completely turned off after a trauma or stroke. For example, a person in a coma on artificial life support no longer has conscious perception, but can still react to sound by ‘jumping’ if the cochlea and the first central relay stations are still intact !