The number of inner hair cells in the cochlea is about 3000, compared with as few as 20 for cochlear implant (CI) electrodes. However, CI users can understand speech rather well. This finding suggests that our brain may play a substantial role in understanding spoken language coded by CI. We used functional brain imaging to examine how the brain manages to perceive speech in CI users. We subtracted brain activities of normal individuals from those of CI users during listening to speech, and found increased activities in various cortical areas of CI users, including the areas that are not usually activated during speech perception. These findings show that more data processing has to be done in the brain to decode the fewer and artificial signals delivered by CI.
We know that the plasticity of the brain has a strict time limitation. For acquisition of spoken language, the most sensitive period of the brain appears to be from 0 to 3 years of age. This developmental plasticity decreases gradually with age, and is lost almost completely after puberty. The human brain develops dramatically after birth. The weight of the brain of a newborn baby is about 400 grams but at the age of 13 years the brain weight has almost reached adult values of around 1300 grams. The rapid growth of the brain is not due to an increase in the number of neurons, but an increase in the synapses that connect them together with development of the myelin sheath that envelopes the nerve fibers and blood vessels that supply the neural structures with glucose and oxygen. The neural networks for verbal activity are established during this period.
The brain is considered to respond flexibly to changes in the input. Sign language significantly activates the temporal cortex in congenitally deaf native signers. The temporal cortex, expected to be activated by speech, is also activated by visual input. It has also been revealed that in congenitally blind subjects, brail reading with the fingers significantly activated the visual cortex.
We know that CI users do not enjoy music due to technical limitation of the CI processor in coding pitch. However they enjoy singing a song a cappella utilizing the pitch sensation located laryngeal muscles.
It may be important to develop a viewpoint from which we can see not only the peripheral organs that code speech, but also the brain that decodes the auditory input signals and processes their meaning.
