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Whilst the human cochlea has completed its development by birth, the brain's auditory pathways and centres develop more slowly and progressively, from the brain stem to the auditory cortex. This development ends only between the 4th to 8th years after birth.
The development of the auditory brain strongly depends on the integrity of the cochlear function.
Comparative development of the cochlea and the auditory brain
Development of neurons in the auditory cortex
These two micrographs, from the same area of the cat auditory cortex, are from a cat at birth (left) and from an adult cat (right). The first stage (left) corresponds in man to 5 months in utero, when the cochlea starts functioning. The final stage (right) corresponds to a 6-8 year-old, fully developed, auditory cortex. Between the two stages, the development of dendrites (i.e. synapses) of pyramidal neurons requires a fully functioning cochlea. Without normal stimulation, for example if there is congenital deafness, the auditory brain will remain at the immature stage.
Role of stimulation
During the first years of life, the auditory brain needs stimulation from the normal cochlea to achieve its development. Any abnormality in cochlear function during this period may result in an abnormal development of the auditory brain. This is summarised in the two schematic drawings below.
The cochlea matures normally and, thanks to its normal functioning, the auditory brain achieves its maximum performance.
The cochlea does not develop completely, and so the auditory brain, lacking normal stimulation, does not reach its full developmental potential either.
Normal stimulation helps the auditory brain to obtain its structure and function. As a consequence:
- any middle or inner ear abnormality or pathology should be treated very seriously to avoid any type of deprivation of auditory stimuli;
- rehabilitation at the periphery should be done as soon as a problem is recognized;
- conversely, begining to learn a foreign language or music during the first 6 years of life is recommended: learning will be facilitated by these early stimulations.
Plasticity in adulthood
Plasticity of the brain during development is remarkable. However, in adulthood brain plasticity is still observed, at least in two conditions (cf. ref. b4): learning and repair after lesion.
Si la plasticité du cerveau en cours de développement est remarquable, elle est encore marquée à l'âge adulte, au moins dans deux conditions : l'apprentissage et la réparation post-lésionnelle.
Learning by the adult auditory brain is much slower than during development.
- Learning by the adult auditory brain is much slower than during development.
Two examples of post-lesion plasticity in the human adult auditory brain:
1) a cochlear implant may give spectacular results in a suddenly deafened adult, as its brain shows a remarkable plasticity to adapt to this artificial ear sending stimulations quite different from those of a normal cochlea;
2) in presbycusis (deafness in older age), when the cochlea does not transmit high frequencies any more, neurons in the auditory cortex switch to decoding lower frequencies.