ABSTRACT
Until the mid-20th century, most scientists believed that primarily genetic processes governed brain development. Only in the last few decades has it become clear that the brain is genetically underdetermined and experience is essential to brain development and learning. If genes are incapable of furnishing the instructions necessary for the complete development of a fully functional brain, then the brain must rely on cues from its environment. Pioneering neuropsychologist Hebb (1949) provided a theoretical framework that enabled subsequent researchers to make sense out of how experience becomes a resource for synaptic modification and change. His theory stresses that synaptic processes are highly plastic and use-dependent, and that learning and memory involve physical alterations in the firing and wiring of neurons. Indeed there is considerable evidence from animal studies that stimulation produces significant but temporary changes in dendrite branching and synapse and spine growth, among other effects, although scientists remain uncertain how these changes affect the underlying neuropsychological mechanisms involved in learning and memory.
