Cardon, Campbell, and Sharma (2012) report that the auditory cortex is highly plastic (aka "neuroplastic," i.e., the ability of neuronal groups to adjust function based on input) particularly during the first 3.5 years with respect to visual, auditory, and pre-frontal cortices. By 12 months of age, the cortex has generally developed all six layers and by age 4 years "pruning" occurs. During pruning, extraneous synapses and neurons that do not contribute to the (same) system are eliminated from the specific sensory system (likewise, "neurons that fire together, wire together"). Indeed, age 3.5 years has been described as the terminus of the "sensitive period" for cochlear implantation in congenitally deaf children. Multiple studies have shown significantly improved outcomes for children implanted earlier, rather than later.
Cardon, Campbell, and Sharma report on two major components impact clinical outcomes: (1) the input (to the cortex) and (2) the timing of the input. Of note, the mere existence of (normally) developed intrinsic pathways cannot guarantee normal transmission or function of sensory information—extrinsic stimulation is of significant importance, too. The authors report that if extrinsic input is not delivered to the auditory cortex during periods of optimal plasticity, deficits will occur once auditory stimulation occurs.
Visual processing abilities have often been reported to be enhanced in congenitally deaf people. The authors note cross-modal plasticity may impact outcomes such that adult cochlear implant (CI) users with less cortical activity in their temporal lobes (in response to visual stimuli) tended to have better speech perception than did those with greater cortical activity (in response to visual cues). That is, recruitment of the higher order auditory cortex may occur (benefitting visual and somatosensory systems) if and when auditory stimulation is not delivered in a timely manner to the developing cortex.
These findings are consistent with Giraud, Lazard, and Lee (2011) who stated that temporal lobe hypo-metabolism at rest (i.e., temporal lobe is less active in deaf subjects as compared to hearing subjects) more or less indicates an improved opportunity for successful cochlear implantation as these same brain regions have not been dominated by other cognitive functions (such as vision, etc.), which could render them "no longer susceptible to auditory stimulation."
For More Information, References and Recommendations
Cardon G, Campbell J, Sharma A. (2012) Plasticity in the Developing Auditory Cortex—Evidence from Children with Sensorineural Hearing Loss and Auditory Neuropathy Spectrum Disorder. Journal of the American Academy of Audiology 23:396-411.
Giraud AL, Lazard D, Lee HJ. (2011) Cochlear Implant Outcome and Functional Brain Organization in Deaf Subjects. Seminars in Hearing 32(2):142-146.