Other models of gene–environment (G×E) interactions include experience-expectant and experience-dependent processes, outlined by Greenough and colleagues as mechanisms by which environmental factors affect brain development (Andersen, 2003; Greenough, Black, & Wallace, 1987). Experience-expectant refers to the integration of environmental stimuli into normal patterns of brain development, the classic example being the effects of monocular visual deprivation on the developing visual cortex as described by Hubel and Wiesel (1998). Specific times during the development of a particular neural system when certain types of environmental stimulation must occur for normal development to take place are called critical periods. Experience-dependent processes are defined in contrast as means by which unique environmental factors may affect the developing nervous system in distinctive ways. In this case the brain may have particular sensitivities to environmental factors such as a trauma at specific developmental stages, resulting in differing effects on the long-term trajectory depending on when an event occurred. The prolonged developmental course of the human brain suggests that these processes may continue to play a role in reaching the mature phenotype well after birth.
