Are critical periods reversible in the adult brain? Insights on cortical specializations based on sensory deprivation studies.

We review here studies with visual and auditory deprived/recovery populations to argue for the need of a redefinition of the crucial role of unisensory-specific experiences during critical periods (CPs) on the emergence of sensory specializations. Specifically, we highlight that these studies, with emphasis on results with congenitally blind adults using visual sensory-substitution devices, consistently document that typical specializations (e.g., in visual cortex) could arise also in adulthood via other sensory modalities (e.g., audition), even after relatively short (tailored) trainings. Altogether, these studies suggest that 1) brain specializations are driven by sensory-independent computations rather than by unisensory-specific inputs and that 2) specific computation-oriented trainings, even if executed during adulthood, can guide the sensory brain to display/recover, core properties of brain specializations. We thus introduce here the concept of a reversible plasticity gradient, namely that brain plasticity spontaneously decreases with age in line with CPs theory, but it nonetheless can be reignited across the lifespan, even without any exposure to unisensory (e.g., visual) experiences during childhood, thus diverging dramatically from CPs assumptions.