What's to lose and what's to learn: Development under auditory deprivation, cochlear implants and limits of cortical plasticity

Sensory and environmental manipulations affect the development of sensory systems. Higher-order auditory representations (auditory categories or “objects”) evolve with experience and via top–down influences modify representations in early auditory areas. During development of a functional auditory system, the capacity for bottom–up reorganizations is successively less well expressed due to a molecular change in synaptic properties. It is, however, complemented by top–down influences that direct and modulate the residual (adult) capacity for circuit reorganization. In a deprived condition, this developmental step is substantially affected. As higher-order representations cannot be established in absence of auditory experience, the developmental decrease in capacity for “bottom–up regulated” reorganizations (as repeatedly demonstrated in also in deprived sensory systems) cannot be complemented by an increasing influence of top–down modulations. In consequence, the ability to learn is compromised in sensory deprivation, resulting in a sensitive period for recovery.