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.