The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Imaging Language Function with MEG and fMRI

Mia Liljeström

Doctoral dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Faculty of Information and Natural Sciences for public examination and debate in Auditorium F239a at the Aalto University School of Science and Technology (Espoo, Finland) on the 28th of May 2010 at 12 noon.

Overview in PDF format (ISBN 978-952-60-3176-7)   [2750 KB]
Dissertation is also available in print (ISBN 978-952-60-3175-0)

Abstract

This Thesis considers the cortical mechanisms underlying language function, as measured by magnetoencephalography (MEG) and functional Magnetic Resonance Imaging (fMRI). In MEG, interpretation of the data critically depends on the ability to estimate the underlying neural activity and localize it to a certain part of the brain. In this Thesis, the accuracy of this procedure is explored in localization of cortical rhythms and for different head conductor models. A comparison of different source modeling techniques shows that rhythmic activity can be identified reliably with a variety of tools, such as equivalent current dipoles (ECDs), minimum norm approaches (MCEFD, minimum current estimates in the frequency domain), and beamformers (DICS, dynamic imaging of coherent sources). The results show that DICS is more sensitive to weak sources than the two other methods, both in measured and in simulated data. Computer simulations also demonstrate that for source localization performed under normal noisy conditions, a simple spherically symmetric head conductor model is in most cases a sufficient model for the conductivity geometry of the head.

This Thesis specifically considers the cortical processing of action and object naming. We investigate whether different cortical regions are activated when actions or objects are named from the same images, and how the content of the image affects the brain correlates of naming. The MEG and fMRI results presented in this Thesis indicate that verbs and nouns are processed within the same cortical network, and demonstrate that image category (action/object) has a stronger influence than naming category (verb/noun) on the activation pattern within this network.

In addition, we consider the relationship between MEG evoked responses and fMRI BOLD (blood-oxygen-level-dependent) signals in language tasks. We demonstrate differences between these two measures in both picture naming and reading, and show that such differences do not depend on experimental procedures such as different participants, languages, or task. In particular, we demonstrate an opposite stimulus effect for symbols and letter strings in the left occipito-temporal cortex in MEG vs. fMRI in reading, although the simultaneously measured electroencephalogram (EEG) was similar. We argue that the observed differences within this region reflect different neural generation mechanisms of the MEG evoked response and fMRI BOLD signals.

This thesis consists of an overview and of the following 5 publications:

  1. M. Liljeström, J. Kujala, O. Jensen, and R. Salmelin. 2005. Neuromagnetic localization of rhythmic activity in the human brain: a comparison of three methods. NeuroImage, volume 25, number 3, pages 734-745. © 2005 Elsevier Science. By permission.
  2. A. Tarkiainen, M. Liljeström, M. Seppä, and R. Salmelin. 2003. The 3D topography of MEG source localization accuracy: effects of conductor model and noise. Clinical Neurophysiology, volume 114, number 10, pages 1977-1992. © 2003 International Federation of Clinical Neurophysiology (IFCN). By permission.
  3. M. Liljeström, A. Tarkiainen, T. Parviainen, J. Kujala, J. Numminen, J. Hiltunen, M. Laine, and R. Salmelin. 2008. Perceiving and naming actions and objects. NeuroImage, volume 41, number 3, pages 1132-1141. © 2008 Elsevier Science. By permission.
  4. Mia Liljeström, Annika Hultén, Lauri Parkkonen, and Riitta Salmelin. 2009. Comparing MEG and fMRI views to naming actions and objects. Human Brain Mapping, volume 30, number 6, pages 1845-1856.
  5. Johanna Vartiainen, Mia Liljeström, Miika Koskinen, Hanna Renvall, and Riitta Salmelin. 2010. MEG and fMRI reveal different activation patterns in reading. Espoo, Finland: Aalto University School of Science and Technology. 30 pages. Helsinki University of Technology, Low Temperature Laboratory Publications, Report TKK-KYL-022. ISBN 978-952-60-3051-7.

Keywords: magnetoencephalography, functional magnetic resonance imaging, brain, language

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© 2010 Aalto University School of Science and Technology

Last update 2011-05-26