He underlines two main periods of brain development. The first is the Critical Period. During this infancy interval, the brain undergoes dramatic change. Learning does not occur here as of yet. The brain is simply assembling a basic machinery but not by absorbing information as one would suspect, but by the mere exposure to variations of sound. This is when the infant's brain hasn't enlarged to its fullest thus far, hence the soft spot on the top (the posterior fontanelle or occipital fontanelle), giving the brain room to stretch and grow. The second phase is called Adult Plasticity. In this phase the brain refines its apparatus as if to adept to a wide supply of dexterity. There's much complexity happening at this point—heightened awareness and acuity are required. It's when we build on skills on top of skills (paralleling).
There are two noteworthy factors to remember from Merzenich’s account of rewiring the brain. One is that yes our brain is built to modify and adjust itself accordingly based on the challenges we face and the environmental demands we live in. However, and this is crucial to executing the act of change and the second component to digest, it is we who choose what to change about ourselves, which means we decide on which input and behavioral selections we want to keep and which ones to ignore. Our emotional attachments sort and confront the ultimate power on whether or not we want to change. According to Merzenich, “You are in charge of your well being, of your happiness, of your skills. We change what is important to us, and what we put in high regards.” Change comes only when we make it our priority and when we really believe in it enough to make it happen. Our brain is proficient in perpetual adjustments, but with the understanding that there is variation within a population of course. 
