Three basic mechanisms have been proposed over the years to explain the formation
of specific neural circuitry: (1) an elaborate predetermined program encoded genetically in each neuron that unfolds according to rigid and unmodifiable rules, (2) a
random process of trial and error in which growing nerve fibers that make the right connections are consolidated and those that fail are reabsorbed and (3) a general program
of circuit formation that is brought to completion by an interplay between genetic and extrinsic factors.