~) Pergamon Neuropsychologia, Vol. 33, No. 8, pp. 1047-1055, 1995 Copyright © 1995 Elsevier Science Ltd Printed m Great Britain. All rights reserved 0028-3932/95 $9.50 + 0.00 0028-3932(95)00045-3 INCREASED CORPUS CALLOSUM SIZE IN MUSICIANS GOTTFRIED SCHLAUG,*~f LUTZ J,ANCKE,$ YANXIONG HUANG,t JOCHEN F. STAIGERI" and HELMUTH STEINMETZ1" 1"Department of Neurology, Heinrich-Heine University, P.O. Box 101007, D-40001 Diisseldorf, Germany (Received 20 June 1994; accepted 27 February 1995) Al~tract--Using in-vivo magnetic resonance morphometry it was investigated wbetheT the midsagittal area of the corpus callosum (CC) would differ between 30 professional musicians and 30 age-, sex- and handedness-matched controls. Our analyses revealed that the anterior half of the CC was significantly larger in musicians. This difference was due to the larger anterior CC in the subgroup of musicians who had begun musical training before the age of 7. Since anatomic studies have provided evidence for a positive correlation between midsagittal callosal size and the number of fibers crossing through the CC, these data indicate a difference in interhemispheric communication and possibly in hemispheric (a)symmetry of sensorimotor areas. Our results are also compatible with plastic changes of components of the CC during a maturation period within the first decade of human life, similar to those observed in animal studies. Key Words: corpus callosum; laterality; magnetic resonance imaging; motor cortex; motor skills; music; neuronal plasticity. INTRODUCTION The corpus callosum (CC) as the main interhemispheric fiber tract plays an important role in interhemispheric integration and communication. Gender and handedness differences in the anatomy of the CC have been a matter of long-standing dispute. A lesser degree of functional lateralization, i.e. increased "ambilaterality" as found in left-handers and in subjects with fight-hemisphere language dominance, has been associated with a larger midsagittal callosal area [14, 31, 42, 43], although this may still be disputable [26, 40]. Nevertheless, group differences in callosal size or shape observed in morphometric studies were generally regarded as a neuroanatomical substrate of differences in cerebral asymmetry and interhemispheric connectivity [10, 11, 14, 31, 40, 42, 43]. Theories to explain these differences in CC morphology include naturally occurring regressive events, such as death of neurons and elimination of axon collaterals [8, 28, 44]. However, this axonal elimination occurs supposedly prior to most environmental influences. Alternatively, it has been proposed that functional maturation of the CC extends into late childhood and adolescence and coincides with the termination of its myelination cycle. According to Rakic and Yakovlev [34] and Yakovlev and Lecours [41] the CC is one of the latest fiber tracts in the central nervous system to be myelinated. Furthermore, in-vivo imaging has revealed that increases of callosal size can be seen at *Address for correspondence: Dr Gottfried Schlaug, Dept. of Neurology, Beth Israel Hospital, 330 Brookline Avenue, Boston, MA 02215, U.S.A. or Dr Helmuth Steinmetz at the above address. 10471048 G. SCHLAUG et al. least up to the middle of the third decade with a maximum during the first decade of human life [2, 9, 33]. This presumed progression in maturation of the CC may also correspond to a period of cortical plasticity since continuing changes in cortical synaptic density occur throughout childhood [20, 27]. There is also a general consensus that movement control and motor coordination as well as intermanual transfer of sensorimotor information improve gradually from ages 4 to 11 years, an age span coinciding with callosal maturation [12, 17, 25, 29, 30]. Although previous reports already suggested that a larger caUosal area may indicate a higher capacity for interhemispheric transfer [10, 40, 42, 43], a positive correlation between midsagittal CC area and the number of fibers crossing through was only established recently [1]. For studying possible differences in callosal morphology in humans with presumed differences in interhemispheric communication, the midsagittal CC area and its subdivisions were measured in musicians and controls using high-resolution in-vivo magnetic resonance (MR) imaging. Recent functional imaging studies already suggested that certain abilities unique to musicians relied on specialized cortical representations in both hemispheres and a more distributed network than similar nonmusical operations [37]. Furthermore, certain cortical areas such as the premotor cortex and the supplementary motor area play a particular role in the temporal control of sequential motor tasks and the integration of bilateral motor behavior [13, 15]. Both aspects of motor control are especially important for performing musicians. In the current study, we tested the hypothesis that early and intensive training in key and string players may facilitate increased and faster interhemispheric transfer in order to perform complex sequential bimanual motor sequences. Furthermore, the recall of stored motor programs could require more and faster interhemispheric exchange than in those not 'routinely' performing bimanual interactions of similarly complex motor sequences. In a more general sense, we also speculated that due to their intense and early exposure to extraordinary stimuli musicians may form a particularly promising group to disclose relationships between brain structure and behavior. MATERIALS AND METHODS Subjects We examined 30 professional classical musicians (keyboard or string instrument players, or both) who described themselves as right-handers. All of them were students or had just finished training at a music school. As a control group, we included 30 subjects matched for sex, age, and handedness. They were taken from our database of more than 100 healthy young adults studied with high-resolution MR morphometry who have been described elsewhere [23, 39, 40]. Most of them were medical