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The Journal of Neuroscience, April 1, 2002, 22(7):2816-2825

Long-Term Consequences of Switching Handedness: A Positron Emission Tomography Study on Handwriting in "Converted" Left-Handers

Hartwig R. Siebner^{1, 2}, Claus Limmer², Alexander Peinemann², Alexander Drzezga³, Bastiaan R. Bloem¹, Markus Schwaiger³, and Bastian Conrad²

¹ Sobell Department of Neurophysiology, Institute of Neurology, London WC1N 3BG, United Kingdom, and Departments of ² Neurology and ³ Nuclear Medicine, Technical University Munich, D-81675 Munich, Germany

Until some decades ago, left-handed children who attended German schools were forced to learn to write with their right hand. To explore the long-term consequences of switching handedness, we studied the functional neuroanatomy of handwriting in 11 adult "converted" left-handers and 11 age-matched right-handers. All participants had used exclusively their right hand for writing since early childhood. Using [¹⁵O]H₂O positron emission tomography, changes in normalized regional cerebral blood flow (rCBF) were assessed while participants repetitively wrote a stereotyped word with their right hand. The kinematics of handwriting did not differ between converted left-handers and right-handers. In innate right-handers, handwriting caused a preponderant left-hemispheric activation of parietal and premotor association areas. In contrast, converted left-handers demonstrated a more bilateral activation pattern with distinct activation foci in the right lateral premotor, parietal, and temporal cortex. Moreover, foci in the right rostral supplementary motor area and the right inferior parietal lobule demonstrated a positive linear relationship between the degree of "left-handedness" and normalized rCBF during right-hand writing. Functional activity in the primary sensorimotor cortex was not affected by handedness. Our findings provide evidence for persisting differences in the functional neuroanatomy of handwriting between right-handers and converted left-handers, despite decades of right-hand writing. Right-hemispheric activation in converted left-handers may reflect suppression of unwanted left-hand movements. Alternatively, this activity may represent persistent left-handedness and, as such, demonstrate a hemispheric asymmetry of hand movement representations in cortical motor association areas in relation to the direction and degree of handedness.

Key words: converted left-hander; functional brain imaging; handwriting; handedness; human; plasticity; positron emission tomography; regional cerebral blood flow

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Copyright © 2002 Society for Neuroscience  0270-6474/02/2272816-10$05.00/0

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