|Active music and its impact on the developing brain|
Music, both in its passive and active form, is known to be a stimulus for inducing brain plasticity. Whereas passively taking in music focuses mainly on the auditory cortex, actively production of music (ie. musical training) is much more complex and involves the development of higher-order motor and cognitive regions  . Although the molecular mechanism of brain plasticity has been understood as a balance between LTP and LTD events, recent endeavors focus on identifying areas undergoing plasticity as a result of musical training. The wide variety of musical instruments allows for the development of many specific brain regions, despite this, there are overlapping areas which are altered. These areas are considered the main structural changes and the degree to which they change reflects in our behavioral outputs over various domains. The field of music and the brain is rapidly evolving as it is a novel example of experience-dependent plasticity to explore how much of our development is caused by nurture as opposed to nature (genetics) .
Like most skills, music skill acquisition through musical training has a sensitive period. During this period, neuronal connections are more easily formed, leading to longer lasting changes in our behaviors  . It is important to note that this sensitive period is different from the critical period, where learning of a specific skill is only possible during that window of time . Missing the chance to learn during the sensitive period is less taxing and will only mean that one would have less efficient structural and functional plasticity.
As it relates to musical training, it has been found that this sensitive period resides somewhere around 5-11 years of age. Evidence for this comes from studies looking at the development of perfect pitch (see Neuroanatomy of Absolute Pitch) and brain imaging studies where there are greater changes to the brain from musical training . The wide age range along with its uncertainty is due to the fact that musical training involves various domains of the human brain, each with its own sensitive period as seen in the video below. Learning an instrument is complex and requires at least multiple sensory pathways (visual, auditory, tactile), the motor cortex and cortical regions . Since musical training can induce plasticity to so many areas, there is a possibility of near and far transfer effects (see cross-modal section below). It is important to study and identify this sensitive period as the neuronal circuits formed during this time will be important for the interpretation of experiences later on in life . However this has not been as easy task, it seems other factors, such as genetics, environmental and motivational differences may play a role  .
|Adapted from Schlaug et al. (2009) Increase in size of area 3 and increase in motor performance for high intensity group|
Musicians require the activity of both hemisphere of the brain and their interaction during their music performance where integration of cognitive, motor and multisensory inputs take place. It would make sense that the corpus callosum would be an area for plastic changes since it is the structure connecting the left and right hemispheres to facilitate communication . And indeed, differences in the corpus callosum, particularly the anterior sections, were observed in diffusion tensor imaging studies  . It was found that intense music training for 29 months was enough to produce plasticity in areas of the corpus callosum and predicting heightened motor performance . The callosal subareas of interest were that of areas 3-6 due to their role in linking motor cortical and sensory regions . Using groups of varying music training intensity, the biggest changes observed was in the callosal subarea 3 of the highest intensity group. Area 3 connects the prefrontal, premotor and supplemental motor areas of both hemispheres which are used in preparing, planning and executing motor actions . Seeing improved performance of non-dominant hand activities in the subjects is affirmation that the corpus callosum is involved in plastic changes during musical training  .
Plasticity in the motor area is depending on what instrument the individual plays. In keyboard players, the precentral gyrus were found to be larger than non musicians since it is an area denoting elaborate hand and finger movements . When these keyboard players were compared with strings players, differences were seen in the form of a larger right precentral gyrus for the keyboard players and a larger left precentral gyrus for the strings players . In addition to these differences, the intensity and musical expertise of the musicians also determined the degree of structural changes .
The corpus callosum and motor area are only some of the brain areas thought to be involved in music training induced plasticity. Evidence for gray matter increases in other areas such as the primary sensorimotor cortex, the mesial Heschl’s gyrus, the cerebellum, the sensorimotor cortex, the inferior frontal gyrus and the lateral temporal lobe are also being studied at the moment in hopes of explaining potential transfer effects seen in trained musicians  . Of these areas, the inferior frontal gyrus is an interesting area because it is an area that contributes to cognitive processes involving risk aversion which does not seem to pertain to music production . The auditory cortex (primary auditory region) is also another area seen to increase after music training due to exposure and processing of sounds .
Cross-Modal Transfer Plasticity
In addition to improving musical skills, music training is believed to increase aptitude in other areas outside music. Transfer effects come in two forms, near transfers that are close in resemblance with the music domain and the transfer domain, and far transfers with little resemblance or association of musical competence to the transfer domain . Most studies do not focus on far transfer effects because they are less common and are hard to explain due to the likelihood of confounds while near transfer effects are studied intensively due to their occurrence and straightforward explanations  .
|near transfer effects|
|Adapted from Herholz et al (2012) Enhanced sensorimotor and motor processing in musicians|
Longitudinal studies involving children with music training are often compared to a non-musician children control group to examine transfer effects in sensorimotor and motor processes . It was found that these musically trained children were better at tonal and rhythmic auditory discrimination and finding melodic contours (listening and discrimination skills) by testing for melodic phrases differentiation or pitch discrimination . Motor skills wise, trained children were much better at motor sequencing abilities such as having better speed and accuracy pressing keys on a keyboard or reproducing metronome rhythms, and better fine motor skills as assessed using non-dominant hand tests . Most of these studies demonstrated sensorimotor and motor proficiencies in musically trained children as early as 15 months into training, suggesting that these changes are due to near transfers through familiarity and practice effects.
Cognitive transfer effects come in many forms but are considered by many far transfer effects due to inconsistent findings. Spatial learning, including spatial reasoning has been implied as being enhanced through musical training because of musical notation  . Despite that, meta-analyses have revealed that spatial learning is controversial since it is seen enhanced when given an object assembly task but not so when evaluated using non-verbal reasoning tasks . This has lead some to hypothesize that spatial learning transfer effects are the result of having the brain regions associated with music processing and spatial learning so close together . Other cognitive domains thought to be affected by musical training are mathematical skills and general IQ, owing to the mathematical components in musical rhythms . Again, results in proving the relationship between these two areas are mixed because it is hard to account for inherent skills. However, recent studies have shown that musical training induces increased gamma-band activities in the brain . These bands are thought to be responsible for attention, memory, multisensory integration and expectation which can all lead to better cognitive performance . Lastly, it is well known that music training primes the brain for language skill acquisition, refer to the association between language and music processing  .
Persistence of plasticity
In line with research on transfer effects, researchers were interested in the persistence of these effects and the persistence of brain plasticity. One study by Skoe and Kraus examined how much plasticity is maintained or reversed during adulthood . Using 3 groups of adults, 1-5 years of training, 6-11 years of training and no training, they found that training for a period of only 3 years is enough to induce neuroplasticity with lasting effects for around 7 years after music training has stopped . Results were based on measurements of the auditory brainstem for auditory perception, executive function and communication skills . Other studies in this field are also finding similar results by looking at the age music training was began as well as the intensity or amount of training given .