|Overview / Development and plasticity|
|Drawings: S. Blatrix|
|Development of the auditory pathways of the brain|
|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.|
|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:
|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.
- 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.
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