primary auditory cortex. These studies support the notion that exposure to certain sounds within the critical period can influence the development of tonotopic maps and the response properties of neurons. Critical periods are important for the development of the brain for the function from a pattern of connectivity. In general, the early auditory environment influences the structural development and response specificity of the primary auditory cortex. Musical ability Absolute pitch manifests itself almost always before adolescence and rarely if ever among individuals who are first exposed to music after mid-childhood, suggesting that exposure to music or similar phenomena (e.g., tonal languages) in early to mid-childhood is a necessary condition for its development or refinement. Studies that ask musicians and non-musicians to sing or hum well-known popular songs that have definitive recordings (and hence are sung in standardized keys) show that—on average—participants sing within a semitone of the standardized key but that outside the small subset of participants with absolute pitch there is broad variation (the "bell curve" that reflects the degree of approximation to the standard key is broad and flat). These results suggest that almost all humans have some innate aptitude for absolute pitch recognition—though other factors may enhance or limit the level of that aptitude. Also, the results' conjunction with the aforementioned chronological observations suggests that early to mid-childhood exposure to environments whose interpretation depends on pitch is a developmental "trigger" for whatever aptitude an individual possesses. Vestibular system In our vestibular system, neurons are undeveloped at neuronal birth and mature during the critical period of the first 2-3 postnatal weeks. Hence, disruption of maturation during this period can cause changes in normal balance and movement through space. Animals with abnormal vestibular development tend to have irregular motor skills. Studies have consistently shown that animals with genetic vestibular deficiencies during this critical period have altered vestibular phenotypes, most likely as a result of insufficient input from the semicircular canals and dopaminergic abnormalities. Moreover, exposure to abnormal vestibular stimuli during the critical period is associated with irregular motor development. Children with hypofunctioning vestibular receptors frequently have delayed motor development. The results of the studies done on ferrets and rats reinforced the idea that the vestibular system is very important to motor development during the initial neonatal period. If the vestibular receptors are present during the initial six months to a year when the infant is learning to sit and