Plasticity in Response to Music Production

One approach to investigating how long-term experience in music performance may result in lasting changes in the brain involves measuring the size of particular structures from high-resolution MRI images. Differences between musicians and individuals who do not engage in music performance may be the direct results of musical experience, though one can usually not rule out the alternative explanation that these morphometric differences result from innate differences in ability.

Gottfried Schlaug and associates measured the midsaggital areas of the corpus callosum, the bundle of interhemispheric corticocortical fibers. The anterior half of the corpus callosum was significantly larger in musicians, with most of the effect coming from the subgroup of musicians who began training before age 7. Since the fibers connecting the sensorimotor cortices in the two hemispheres are contained in this portion of the corpus callosum, the increase in interhemispheric connections may be related to sensorimotor cortical development in response to musical practice, especially during the first decade of life.

They also measured the intrasulcal length of the posterior bank of the precentral gyrus as an index of primary motor cortex. They found a significantly longer sulcus in the right hemisphere of musicians in comparison to nonmusicians. Furthermore, both right and left sulcal length were positively correlated with the age at which music training commenced.

Finally, Schlaug and associates used MRI morpho-metry to investigate the specialized ability of absolute pitch perception. The surface area of the planum temporale, or the superior temporal plane posterior to

Heschl's gyrus, showed an exaggeration of the normal leftward asymmetry in musicians with perfect pitch. Zatorre and colleagues measured the cortical volume of the same region and found that it was slightly larger on the left for absolute pitch possessors than for musicians without absolute pitch. Furthermore, pitch naming performance was correlated with size of the left planum temporale, such that larger volume was associated with lower error scores. Although studies suggest a critical period early in life for the development of absolute pitch, it is not clear if these asymmetries are innate or result from musical training. In the monkey the majority of projections from the superior temporal gyrus to the frontal area analogous to that activated in absolute pitch musicians while listening to musical tones (the posterior dorsolateral frontal cortex) originate in the planum temporale.

Pantev and associates used MEG to measure the responses of large ensembles of neurons in auditory cortex to tones. Yoshihiro Hirata, Shinya Kuriki, and Pantev contrasted such responses from a group of musicians with absolute pitch to a group of nonmu-sicians. The best fitting single equivalent current dipoles, representing the sources of these auditory-evoked magnetic responses, were located significantly more posteriorly within the left hemisphere for the absolute pitch possessors. These results are consistent with morphometric data indicating a larger planum temporale.

Pantev and associates also measured the magnetic fields evoked by piano tones and by pure tones matched for hearing level and fundamental frequency. The strength of the primary auditory evoked response (dipole moment of the M100) in the left hemisphere to piano tones was about 25% greater than that to matched pure tones for musicians but not significantly different for nonmusicians. Moreover, the strength of cortical activation correlated linearly with the age at which musicians began playing (Fig. 10) and did not vary with absolute pitch ability. It was maximal for musicians who began to play before the age of 9. Thus, it is plausible that this increase results from cortical reorganization induced by the sensory input coincident with musical production—reorganization that results in the recruitment of a larger ensemble of auditory cortical neurons and/or an increased synchrony of neural firing.

Approximately half of the musicians in the previous study were pianists and half were string or wind instrument players, although almost all reported the piano as at least a secondary instrument. Therefore, it is not clear whether reorganization may have been

Figure 10 Enhanced auditory representation of piano tones in musicians. (Top) Mean values for the strength of the magnetic field evoked in response to piano tones (single equivalent dipole moment of the M100), plotted as arrow length, from musicians (black arrow) and nonmuscician controls (white arrow). (Bottom) •, average dipole strength across all piano tones tested from each musician, plotted as a function of the age at which musical training began. The line indicates the average dipole strength across all piano tones and control subjects. Note that dipole strengths tend to increase with earlier training, but only for musicians who began playing before the age of 9 (reproduced with permission from Pantev et al., 2001a; modified from Pantev et al., 1998).

Figure 10 Enhanced auditory representation of piano tones in musicians. (Top) Mean values for the strength of the magnetic field evoked in response to piano tones (single equivalent dipole moment of the M100), plotted as arrow length, from musicians (black arrow) and nonmuscician controls (white arrow). (Bottom) •, average dipole strength across all piano tones tested from each musician, plotted as a function of the age at which musical training began. The line indicates the average dipole strength across all piano tones and control subjects. Note that dipole strengths tend to increase with earlier training, but only for musicians who began playing before the age of 9 (reproduced with permission from Pantev et al., 2001a; modified from Pantev et al., 1998).

specific to the sounds produced by one's own instrument or to spectrally rich harmonic tones in general. To investigate the instrument specificity of this effect, Pantev and colleagues compared the auditory responses evoked by trumpet and violin notes (of the same fundamental frequency) in groups of trumpeters and violinists (none of whom played both instruments). The amplitude of the M100 was greater for a musician's own instrument and was greater overall for the trumpeters (Fig. 11). Although these results do not rule out generalized enhancement of neuronal responses for spectrally rich harmonic tones among musicians, they do indicate at least an additional enhancement in response to one's own instrument. The instrument-specific response enhancement may result from long-term exposure to feedback from one's own instrumental performance.

Plasticity in somatosensory cortex is also suggested by a study carried out by Thomas Elbert, Pantev, and colleagues, who used MEG to measure and compare the cortical representation of the digits of string players to those of nonmusicians. They found that

Figure 11 Enhanced auditory representation of tones played on one's own instrument (violin or trumpet). (a) Time courses of the strength of the magnetic field (as dipole moment) by hemisphere from one violinist and one trumpeter, evoked in response to trumpet tones (fine lines) and violin tones (heavy lines). Note that peak responses at ~ 100msec (M100) are greater for one's own instrument. (b) Average dipole strengths across both hemispheres for all violinists and all trumpeters in response to violin tones (black bars) and trumpet tones (white bars). The interaction of stimulus type with instrument group was significant, and response strength was reliably greater for the timbre of the instrument of training within both groups (reproduced with permission from Pantev et al., 2001b).

Figure 11 Enhanced auditory representation of tones played on one's own instrument (violin or trumpet). (a) Time courses of the strength of the magnetic field (as dipole moment) by hemisphere from one violinist and one trumpeter, evoked in response to trumpet tones (fine lines) and violin tones (heavy lines). Note that peak responses at ~ 100msec (M100) are greater for one's own instrument. (b) Average dipole strengths across both hemispheres for all violinists and all trumpeters in response to violin tones (black bars) and trumpet tones (white bars). The interaction of stimulus type with instrument group was significant, and response strength was reliably greater for the timbre of the instrument of training within both groups (reproduced with permission from Pantev et al., 2001b).

Figure 12 Enhanced somatosensory representation of left-hand digits in string players. (Top) Equivalent current dipoles (ECDs) elicited by stimulation of the little finger (D5) superimposed onto an MRI reconstruction of the cortical surface from one control subject. The arrows indicate the location, orientation, and moment (length) of the modeled dipoles averaged across sting players (black arrow) and nonmusician controls (white arrow) (Bottom) •, magnitude of the dipole moment of D5 somatosensory-evoked responses from each string player, plotted as a function of the age at which musical training began. The line is the mean of the same response across all nonmusician subjects. Note that dipole strengths are particularly enhanced for those who began training at < 12 years (reproduced with permission from Pantev et al., 2001a; modified from Elbert etal., 1995).

Figure 12 Enhanced somatosensory representation of left-hand digits in string players. (Top) Equivalent current dipoles (ECDs) elicited by stimulation of the little finger (D5) superimposed onto an MRI reconstruction of the cortical surface from one control subject. The arrows indicate the location, orientation, and moment (length) of the modeled dipoles averaged across sting players (black arrow) and nonmusician controls (white arrow) (Bottom) •, magnitude of the dipole moment of D5 somatosensory-evoked responses from each string player, plotted as a function of the age at which musical training began. The line is the mean of the same response across all nonmusician subjects. Note that dipole strengths are particularly enhanced for those who began training at < 12 years (reproduced with permission from Pantev et al., 2001a; modified from Elbert etal., 1995).

the cortical representation of the digits of the left hand was greater for string players (smallest for the thumb) but equivalent for the right hand. The magnitude of the effect correlated negatively with the age at which training began (Fig. 12). These results suggest cortical reorganization in response to somatosensory feedback from one's own instrumental performance since violinists typically use the four digits of the left hand to manipulate the strings. The results are consistent with direct mapping of somatosensory cortex in monkeys before and after tactile discrimination training. Greg Recanzone, Michael Merzenich, and colleagues found that the area of somatosensory cortex representing the portion of the skin of a trained digit increased up to threefold.

Focal hand dystonia, the loss of motor control of one or more digits, is a serious occupational hazard for those engaged in rapid, finely coordinated hand movements such as pianists. Robert Schumann may have suffered from focal hand dystonia beginning in his twenties. Its cause is controversial, but some theories suggest it is caused by maladaptive cortical plasticity, particularly an overlap of receptive fields within somatosensory cortex. Using MEG, Elbert, Pantev, and colleagues found that the single equivalent current dipoles representing somatosensory-evoked responses to individual digits were closer together for the affected hand of dystonic musicians than for nonmusician controls. These results are consistent with the hypothesis of maladaptive, overlapping somatosensory receptive fields.

Jesus Pujol, Pascual-Leone, and colleagues used functional MRI to measure brain activity while guitarists played a simple finger exercise (arpeggios) on a computer-interfaced guitar. When not experiencing dystonia, the pattern of motor cortical activation within contralateral sensorimotor cortex and SMA was the same for dystonic and nondystonic musicians. When dystonic symptoms were provoked by guitar playing, activity in sensorimotor cortex became elevated and motor cortex activity declined.

Further brain imaging studies during the active expression of dystonic symptoms will help to reconcile findings of disordered and hyperactive somatosensory functioning associated with focal hand dystonia, and it is hoped that they will lead to successful treatments for this all-too-common affliction.

Finally, we consider the most generalized claims of changes in the brain as a result of exposure to music production. M. Gardiner studied the effects of music training during childhood on development of other cognitive abilities. He found that children who participated in an intensive, active music training program (Kodaly method) also showed large improvements in math and reading. However, a causal link has not been established, nor have the mechanisms by which musical training might result in changes that could enhance other cognitive functions been determined. Continuing developmental research may help to provide new insights into very old controversies concerning the place of music within general education.

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