Differential changes in motor cortex excitability during speaking and singing

Objectives: Several components of the network underlying speech motor control exhibit a predominantly left-sided hemodynamic response in neuroimaging. Alongside spoken language, singing represents a second mode of auditory-vocal communication which, conceivably, evolved as a transitional state between subhuman primate vocalizations and speech. Transcranial magnetic stimulation (TMS) provides an useful tool to assess the hemispheric lateralization of language and singing processes and their functional connections to motor areas. Using TMS, the excitability of the motor hand area has been found to be increased in the dominant hemisphere during reading aloud. This finding supports the theory of cortical networks which are linked to language processing as well as planning / execution of hand movements. The present study investigated the influence of singing and melody generation on hand motor cortex excitability. Methods: In 12 right-handed subjects hand motor cortex excitability was assessed using TMS during 5 different conditions [singing aloud, humming, speaking aloud, non-verbal oral movement and baseline]. Results: Mean MEP amplitudes were calculated for each condition and statistically compared with analysis of variance (ANOVA). We found that humming and singing increased MEP sizes significantly more after right than left motor cortex stimulation. In contrast, speaking aloud increased only left motor cortex excitability which is a well-described finding of previous studies. Non-verbal oral movement did not change MEPs significantly compared to baseline condition. Discussion: Our findings provide new evidence for a functional-anatomical dissociation, at least partially characterized by opposite lateralization effects, between the production of language (words) and the representation of tunes (melodies). Our results and the clinical observation that patients suffering from aphasia subsequent to left-hemisphere lesions often exhibit strikingly preserved vocal musical capabilities may provide a neurophysiological basis for further implementation of music therapy in rehabilitative processes of various neurological diseases. Acknowledgements: Supported by the Interdisciplinary Centre for Clinical Research „BIOMAT“ at the RWTH Aachen University.