Nature and nurture in hearing: critical periods for therapy of deafness

The present manuscript reviews the contribution of cochlear implants to the understanding of the impact of congenital deafness on brain development. The results show that many characteristics of the afferent auditory system, particularly the anatomical features, are genetically determined (are in origin ''nature''), and that experience is used to maintain and improve them to allow discriminating auditory stimuli. Experience (''nurture'') is additionally required to group the auditory features at the level of the auditory cortex into abstract ''auditory objects.'' This requires interaction between bottom-up and top-down streams of information processing, since features define objects and context (i.e. active objects) defines which features may carry relevant information in the given condition. The interaction of feature- and object-level-representation is allowed by columnar microcircuits. The integration of bottom-up and top-down streams of information also controls adult learning. Since congenital deafness interferes with the relevant microcircuitry of the cortical column, congenital deafness, if persisting beyond certain age, also leads to failure of key high-level auditory processes including the switch between juvenile and adult learning and therefore closes the sensitive periods for its therapy.