Auditory critical periods: a review from system's perspective.

The article reviews evidence for sensitive periods in the sensory systems and considers their neuronal mechanisms from the viewpoint of the system's neuroscience. It reviews the essential cortical developmental steps and shows its dependence on experience. It differentiates feature representation and object representation and their neuronal mechanisms. The most important developmental effect of experience is considered to be the transformation of a naive cortical neuronal network into a network capable of categorization, by that establishing auditory objects. The control mechanisms of juvenile and adult plasticity are further discussed. Total absence of hearing experience prevents the patterning of the naive auditory system with subsequent extensive consequences on the auditory function. Additional to developmental changes in synaptic plasticity, other brain functions like corticocortical interareal couplings are also influenced by deprivation. Experiments with deaf auditory systems reveal several integrative effects of deafness and their reversibility with experience. Additional to developmental molecular effects on synaptic plasticity, a combination of several integrative effects of deprivation on brain functions, including feature representation (affecting the starting point for learning), categorization function, top-down interactions and cross-modal reorganization close the sensitive periods and may contribute to their critical nature. Further, non-auditory effects of auditory deprivation are discussed. To reopen critical periods, removal of molecular breaks in synaptic plasticity and focused training therapy on the integrative effects are required.