The development illustrates the increased brain efficiency through focusing of cortical activity as brain circuits develop. The auditory cortex undergoes a developmental process that involves a progressive sharpening of frequency receptive fields during the maturation of the auditory cortex (Chang and Merzenich, 2003)(Fig. 6). The focal sharpening of cortical activation by sound likely corresponds with improved ability to identify specific tones essential for music and sequences of sounds essential for language. Thus, cortical development leads to increased efficiency and focus that is modified by the environment. Normal development allows the auditory cortex to focus sound and tonal discrimination. However, excessive white noise during the critical period of cortical development disrupts auditory cortex focal sharpening (Fig. 6). Excessive noise during the critical period of auditory cortex development leads to persistent changes in cortical responsiveness, lack of tonal and temporal sharpening of responses and loss of higher order discrimination function, e.g. sounds do activate cortex, but focal specificity is lost (Chang and Merzenich, 2003; Zhou and Merzenich, 2008). These findings indicate that environmental factors during adolescent critical periods of cortical development regulate
