The plastic changes in the brain that are the outcome of enhanced musical experience can be found at two levels: the gross-anatomical differences between professional musicians with absolute pitch and amateurs or laymen, and the subtle functional differences after musical training, which have to be sought in ever finer modifications of synaptic strength in distributed cortical networks (Rauschecker, 2003, p. 357). As such, it is possible to distinguish between macrostructural and microstructural adaptations.
It makes a difference, further, whether musical experience is meant to be 'active performance' or only 'listening' to music. The plastic changes that are triggered by playing a musical instrument are numerous and well-documented. They include changes such as the rapid unmasking of existing connections and the establishment of new ones. As such, both functional and structural changes take place in the brain in an attempt to cope with the demands of the activity of skilful playing (see Pascual-Leone, 2003, p. 396). Some of these changes, however, are triggered by listening as well.
As far as the macrostructural changes are concerned, there are changes, which are the outcome of instrumental playing, such as a difference in a measure of primary motor cortex size (Amunts et al., 1997, p. 210). Other findings are related to the planum temporale, the corpus callosum, and some representation areas.
The planum temporale, first, shows an increased leftward symmetry in musicians with absolute pitch recognition. There is, however, no agreement as to the development of this skill. Two critical notions relate to early exposure to music: almost all musicians with absolute pitch started musical training before the age of 7 and is it unlikely that an individual will develop absolute pitch if he/she commences musical training after the age of 11 (Keenan et al., 2001, p. 1406).
The corpus callosum, secondly, is the main inter hemispheric fibre tract that plays an important role in interhemispheric integration and communication. Its midsagittal size correlates with the number of fibres passing through this structure. In spite of its late development - it is one of the last main fibre tracts to mature in humans - increases in corpus callosum size have been observed until at least the third decade of human life. The maximal growth, however, is in the first decade, which is the period of presumed callosal maturation, which coincides with childhood increases in synaptic density and fine-tuning of the neural organisation. It has been proposed, further, that environmental stimuli, especially early in life, might affect callosal development (Lee et al., 2003, p. 205).
The corpus callosum, further, has been found to be larger in performing musicians who started their musical training before the age of 7. This adaptation can be interpreted as a morphological substrate of increased inter-hemispheric communication between frontal cortices (such as the pre-motor and supplementary motor cortex) subserving complex bimanual motor sequences (Schlaug, 1995, p. 1050). Somewhat generalising, it could be stated that environmental factors, such as intense bimanual motor training of musicians, could play an important role in the determination of callosal fibre composition and size, which, in turn, can be considered as an adaptive structural-functional process. It remains to be determined, however, whether the large corpus callosum of musicians with early commencement of training contains a greater total number of fibres, thicker axons, more axon collaterals, stronger myelinated axons, or a higher percentage of myelinated axons (Schlaug, 1995, pp. 1050-1051).
The microstructural adaptations, finally, must be located at the level of individual neurones and synapses. They reflect the functional plasticity of the brain which can lead to microstructural changes which have been found both as the result of brain lesion (de-afferentiation) and motor skill learning, and which aim at changing the efficacy of the neural connectivity.
As to the first, it has been shown that there is some cortical remodelling which is induced by de-afferentiation. This includes microstructural changes such as the strengthening of existing synapses, the formation of new synapses (synaptogene-sis), axonal sprouting and dendrite growth (Pantev, et al., 2003, p. 385). As to the latter, there are similar significant microstructural changes, which are induced in motor-related brain regions as a consequence of intense and prolonged motor activity. The adaptations include an increased number of synapses per neurone as well as changes in the number of microglia, and capillaries, which can lead to volumetric changes detectable at a macrostructural level (Hutchinson et al., 2003, p. 943; Schlaug, 2001, p. 283).
It is clear that the brain may show some form of adaptation to extraordinary challenges. This is the case with the requirements for musical performance, which may cause some brain regions to adapt. The adaptation, however, does not always involve plastic structural changes. Musical expertise, in fact, influences auditory brain activation patterns, and changes in these activation patterns depend on the teaching strategies applied. To quote Altenmuller:
brain substrates of music processing reflect the auditory learning 'biography', the personal experiences accumulated over time. Listening to music, learning to play an instrument, formal instruction, and professional training result in multiple, in many instances multisensory, representations of music, which seem to be partly interchangeable and rapidly adaptive. (Altenmuller, 2003, p. 349)
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