Commentary: “Compensatory plasticity: time matters”

The mammalian nervous system can adapt to the challenges of life through neural plasticity. The brain will undergo extensive reorganization following sensory deprivation or damage to afferent pathways (Kaas, 2001). This plastic reorganization develops as a function of time. A recent review on plasticity in the blind (Lazzouni and Lepore, 2014) stressed the importance of critical periods and the influence of the duration of sensory deprivation on the reorganization of sensory cortices. Such considerations are paralleled in the hearing sciences, sharing the authors' opinion that " time matters. " Restoring lost sensory function to a blind or deaf cortex, via surgically implanted devices offers a unique insight into brain reorganization, allowing scientists to follow the transition from deaf to hearing, from blind to sighted. While retinal implants are just becoming available in a clinical setting (Zrenner et al., 2010), cochlear implants (CIs) have been offered since the 1980s (Clark, 2003). Here we argue that, for auditory implants, time matters along two dimensions: pre and post-implantation. On the one hand, plasticity is especially strong when sensory deprivation occurs at early stages of development. Referred to as the sensitive period for brain development, it provides cutoff ages to guide implantation (Sharma et al., 2002; Bedny et al., 2010). On the other hand, the functional maturity of the auditory cortex crucially depends on sensory experience (Kral et al., 2005), emphasizing the importance of rehabilitation. Age at implantation plays a substantial role in performance with a CI. Research has shown the existence of an early critical period for brain development and demonstrated how deprivation-driven functional changes in the cortex are affected by age. Cats that were implanted after the fifth month of age had smaller activation areas of the auditory cortex, compared with cats implanted earlier, even when they had longer experience with implant (Kral et al., 2002, 2005; Kral and Sharma, 2012). In humans, the latency and morphology of the P1 component of auditory-evoked potentials can serve as a biomarker for the development of the central auditory pathways (Sharma et al., 2005b; Dorman et al., 2007; Kral and O'Donoghue, 2010). Using this measure, a cutoff age for optimal auditory cortical plasticity was identified. Children implanted before the age of 3.5 years showed a faster and more robust development of the P1 than children implanted past age seven. Sharma et al. (2002) observed that 55 out of 57 early-implanted children had P1 latencies within …