9.9. Human motor cortex function characterized by navigated transcranial magnetic stimulation

 Public examination of a doctoral dissertation in the field of clinical neurophysiology

Doctoral candidate: M.Med.Sci. Laura Säisänen

Time and venue: 9.9.2011 at 10 AM, Medistudia Auditorium ML3, Kuopio campus

 Transcranial Magnetic Stimulation (TMS) can be used to assess corticospinal, cortico-cortical and cortico-subcortical network physiology and function. In the motor system, TMS-induced cortical activation leads to peripheral muscle responses. Various motor functions can be characterized extensively by applying specific stimulation protocols. However, the clinical applications are limited by large variability and by uncertainty of whether the results reflect true pathophysiological changes or merely non-optimal stimulation and methodological variation. To overcome these issues, TMS has been optimized by neuronavigation. Navigated TMS (nTMS) refines traditional “blind” TMS by continuously visualizing the induced electric field within three-dimensional magnetic resonance images (MRI) of the subject’s brain. Hence, it is possible to provide repeatable stimuli to a certain location with control of coil orientation and tilting to any previous target both in the experimental session and repeated sessions even years later.

The aim of this thesis was to establish the basis for studies of motor cortex functions with nTMS. First, navigated and traditional TMS were compared. Then a large population of healthy volunteers of different ages was studied to define normative values for motor threshold, motor evoked potentials (amplitude and latency) and silent period for both hand and leg muscles. Thereafter, measures of cortical functions were refined: first, intracortical inhibition-excitation balance was characterized by paired stimulus pulses. Intracortical inhibition was then studied in silent period measurements by determining the effect of stimulus intensity and muscle contraction on the TMS induced silent period. Finally, a novel method for preoperative motor mapping was constructed to study patients with focal drug-refractory epilepsy caused by a lesion near the motor areas, and thus at risk of losing motor function in surgery. The clinical usefulness of preoperative nTMS mapping in operative decision making was evaluated.

Overall, nTMS reduced the variability of motor evoked potentials but did not affect the motor threshold values. Reference values were calculated for the most common TMS parameters (amplitude, latency) when stimulation was targeted to the individually optimal cortical area. Finally, nTMS mapping of cortical motor areas was shown to be clinically useful in the preoperative evaluation of epilepsy surgery candidates.

The doctoral dissertation of Master of Medical Science Laura Säisänen, entitled Human motor cortex function characterized by navigated transcranial magnetic stimulation will be examined at the Faculty of Health Sciences. The opponent in the public examination will be Professor Paolo Maria Rossini of the University of Rome, and the custos will be Professor Esa Mervaala of the University of Eastern Finland. The public examination will be held in English.

Photo available for download at http://www.uef.fi/vaitoskuvat  

For further information, please contact: Laura Säisänen, laura.saisanen@kuh.fi, p. 044-7175452

Publishing year: 2011

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