May 2, 2013:
How the brain processes auditory signals
Many small neurons located in the brain are responsible for the processing of auditory information. By passing through various auditory pathways, the signals are decoded into sounds that we are familiar with and make sense to us.
Once the hair fibres of the cochlea, the snail shell-resembling organ of the inner ear, have sent electrical signals to the auditory nerve, these impulses are transferred to the auditory centre of the brain.
In the auditory brain, several groups of neurons receive the impulses and translate them into a language that our brain understands. This translation occurs in order to cause a conscious perception of the sounds that we receive.
The Auditory Brain
Transformation and processing of sound generally occurs on three levels in the brain: As a reflex, in the auditory cortex and in other brain areas.
Thus, the arrival of the message may first of all trigger a reflex and cause us to jump or turn our head. Thereafter, the processing might also unfold in the auditory cortex, where the sound is consciously perceived.
Other brain areas can allow the perception to become conscious as well and hence recognise the sound by relating it to those that have been memorised in the past. This assessment is followed-up by an appropriate voluntary response.
Primary Auditory Pathway
The processing of decoded sound material starts within the primary auditory pathway. This pathway carries messages from the cochlea to a sensory area of the temporal lobe called the auditory cortex.
First stop on this journey is taken in the brain stem, where a decoding of basic signals such as duration, intensity and frequency takes place. Subsequently, the message passes two additional relays that play an important role in the localisation of the sound.
The next stop occurs in the thalamus, an ovoid mass of grey substance situated at the base of the cerebrum. The thalamus integrates the sensory systems in the body and hence functions as an essential factor in the preparation of a motor response e.g. vocal response.
The last neuron of the primary auditory pathway connects the thalamus with the auditory cortex. At this stage the message has already been largely decoded. However, in the auditory cortex the signal is moreover recognised, memorised and may eventually result in a response.
Non-primary Auditory Pathways
In contrast to the primary auditory pathway, non-primary auditory pathways process all sorts of sensory messages. The core function of these pathways is hence to choose the type of sensory message to be treated first. For example, when reading a newspaper while listening to the radio, this system permits the person to focus on the most vital task.
The processing of sensory data within the non-primary auditory pathways also starts in the brain stem. Hereafter, the auditory information passes through the reticular formation, a region in the brain stem consisting of more than a hundred small neural networks.
In the reticular formation, the information that should be treated as a priority is selected in accordance with the wake and motivation centers and further treated. Finally, the messages continue to the thalamus and end up in the sensory areas located in the cerebral cortex, the outer layer of our cerebrum.