The adult brain is not entirely "hard-wired" with fixed neural circuits. There are numerous instances of reorganization in cortical and subcortical neural circuits in response to training and damage. This involves activities in multiple brain regions, changes in cortical thickness, and gray matter density, including somatosensory, motor, insular, and prefrontal cortices, as well as the thalamus, amygdala, basal ganglia, and hippocampus. The concept of neural plasticity is also central to memory and learning theories associated with experience-driven changes in synaptic structure and function.