One conclusion that can be drawn from the studies regarding rat learning and brain structure (greenough, et al., 1990; wallace, et al., 1992) is that

Answer;
One conclusion that can be drawn from the studies regarding rat learning and brain structure (greenough, et al., 1990; Wallace, et al., 1992) is that early psychological experience affects the development of the nervous system and influences vulnerability to psychological disorders later in life. 
Learning changes the physical structure of the brain, structural changes alter the functional organization of the brain; learning organizes and reorganizes the brain, and different parts of the brain may be ready to learn at different times.