What is Critical periods and the diseases of plasticity?

Plasticity is greatest in windows of development. Hubel and Wiesel (1960s) covered one eye of a kitten for a few weeks after birth: the visual cortex, normally driven by both eyes in alternating columns, became driven almost entirely by the open eye, and the deprived eye stayed functionally blind for life — while the same deprivation in an adult cat changed nothing. Such critical periods, for vision, for language, for the songs of birds, close when inhibitory circuits mature and the extracellular matrix stiffens, and they can be reopened a little by drugs and training. Plasticity’s pathologies are of dose and target: addiction is learning driven by a counterfeit prediction error, which builds habits that outlast the pleasure; post-traumatic stress is a fear memory over-consolidated by stress hormones; and Alzheimer’s disease begins where declarative memory begins, in the entorhinal cortex and hippocampus, with the loss of synapses — which tracks the dementia better than any plaque count — before the loss of neurons.