There is general agreement that music is an important value "in itself", providing joy, feeling for aesthetic values and a unique means to explore and to express emotions. During the last decade, however, music educators have become increasingly interested in understanding "secondary" effects of music education, especially on brain activation patterns and brain networks. Evidence from neurobiological research, demonstrating that music education causes remarkable central nervous adaptations has fueled this interest. In short, music making turns out to be the behavior, which probably most effectively induces short- and long-term brain plasticity.

Neural plasticity permits the adaptation of the brain to environmental factors that cannot be anticipated by genetic programming. The neural and behavioral changes attributed to plasticity have been observed on different time scales, ranging from several minutes to the whole life-time of the individual. Very different processes are likely to support plastic changes at the extremes of this time-line. Accordingly, experience-driven neuroplasticity has been explained by both the improvement and de novo growth of new dendrites, synapses, and neurons and the disinhibition or inhibition of pre-existing lateral connections between neurons by sensory input. The former mechanism entails structural changes at the microscopic and macroscopic level, whereas the latter can be achieved by strengthening or inhibiting pre-existing synaptic connections in the spirit of Hebbian learning. Sometimes even more rapid changes of brain responses occurring in the order of milliseconds have been discussed under the heading of neural plasticity. These are likely due to attentional modulation of neural circuits, however, and should be distinguished from true plastic changes.

Research into brain plasticity due to music education is still in its infancy, but already many of the animal findings have found their parallels in studies on musicians. At one extreme, years of musical experience, especially in those musicians who begin training early on, might lead to an increase in gray and white matter volume in several brain regions. In professional pianists and violinists for example having started with their training before age 7, the anterior portion of the corpus callosum - the most important interhemispheric connection - is larger compared to non-musicians or to musicians with later onset of practice. Since both violin and piano require subtle bimanual coordination, this phenomenon seems to reflect a specific training-induced structural adaptation, due to either more pronounced myelination of the axons or to preservation of axons which otherwise are subject to the normal developmental loss of nerve fibers, the so called apoptotic process. A similar enlargement of brain areas has been demonstrated for sensory-motor areas, for the posterior portion of temporal lobe and for the cerebellar hemispheres.

These anatomical alterations appear to be confined to a critical period. The fact that in several of the studies a correlation was found between the extent of the anatomical differences and the age at which the musical training commenced strongly argues against the possibility that these differences are pre-existing and the cause for rather than the result of practicing music. Further research employing advanced imaging techniques such as MR-spectroscopy and diffusion tensor imaging, and the extension of studies beyond the conventional cross-sectional design, are needed to investigate the underlying neurophysiological changes. At the other extreme, several minutes of training can induce changes in the recruitment of motor cortex areas or establish auditory-sensorimotor coupling. Some of the other findings discussed here probably require training on the order of months to several years, and it is currently unclear what neural processes support this behavioral plasticity.

The investigations convincingly demonstrate the utility of the musician's brain as a model for neural plasticity and have thus set the stage for further research. A list of some of the questions that need to be tackled includes: What are the training parameters that lead to successful learning and plasticity? Can these parameters be exploited in musical education or to enhance learning in other domains? What is the role of genes in determining auditory neural plasticity in musicians? What is the range of structural regularities that can be extracted from the auditory input in an automatic, pre-attentive fashion? As making music undoubtedly requires intense self monitoring and error detection and correction, are there any plastic changes in the executive brain systems that are responsible for performance monitoring?

Finally, one has to bear in mind that music can elicit powerful emotional reactions. Strong emotional responses to music leading to shivers down the spine and changes in heart rate are accompanied by the activation of a brain network that includes the ventral striatum, midbrain, amygdala, orbitofrontal cortex and ventral medial prefrontal cortex - areas that are thought to be involved in reward, emotion and motivation. Further research will show whether activity in these areas is also directly involved in mediating neural plasticity.

It seems plausible that an increase in cortical neuronal connectivity or in gray matter density might improve general cognitive abilities. Many complex mental processes rely crucially on the rapidity of cognitive operations and on the amount of processing resources involved. Surprisingly, "hard data" proving transfer-effects of musical abilities on other cognitive domains are rare. Although there are several reports demonstrating a positive correlation between musical aptitude and intelligence in school children, it is still unclear whether this is a mere coincidence (for instance due to socio-economical backgrounds, allowing families with better financial resources to educate children more sophistically, to afford expensive musical instruments and to enable the children to take music lessons) or whether there is a causal relationship. In my opinion, the most convincing transfer effects can be found in the domain what might be called "emotional intelligence". Music education for example improves the ability to decode affective states in spoken language. In summary, although there is ample evidence that music education modifies our "mind-machine", I suspect that we have not yet found the right tests or not done the necessary studies for demonstrating the (probably enormous) long term impact of music education for daily life in reasoning and feeling.