Magnetoencephalography

1. Magnetoencephalography Papanicolaou 1998 Fundamentals of Functional Brain Imaging

2. Functional Brain Imaging • The brain is constantly sending electrochemical signals • Coordinated signaling activity of a large set of neurons somewhere in the brain creates a transient deviation in electromagnetic signal intensity beyond the normal range of variation • Temporal variations of the signal can be associated with functions

3. Measuring brain activity • Neurons are the current sources • Synaptic currents • Dendritic currents • Axonal currents • These currents are very small • Neurons must be in a open field configuration in order for the combination of sources to be strong enough to be measured

4. What are we measuring? • Synaptic and axonal currents cancel each other out • Dendritic currents are the primary source of electrical activity that can be measured • Two electromagnetic signals arise from the primary source • Secondary (volume) electrical currents • Magnetic flux (magnetic fields)

5. Volume Currents

6. Electricity • Volume currents follow the path of least resistance in the extracellular components • EEG measures the volume currents at the surface of the head • Volume currents become distorted and attenuated • Overcoming this distortion was one major factor in the development of MEG

7. Properties of magnetic fields • An electric current will induce a magnetic field perpendicular to its direction • A magnetic field will induce a current • Magnetic fields are not distorted as they emerge from the brain source • Magnetic field strength: • is proportional to the strength of the source current (amperes). • dissipates as a function of the square of the distance from the current source.

8. Magnetometer • Loop of wire at the head surface • Magnetic field at the surface of the head creates a current in the wire by induction • Only the portion which is perpendicular to the head • Strength of the current is proportional to the strength of the magnetic field • The surface distribution can be determined with enough magnetometers

9. Amplification • The induced currents are very small • Smaller than thermal noise of conventional amplifiers • Superconductive Quantum Interference Devices (SQUIDS) • Wires are cooled to 4 K to lower the resistance and housed in a thermally insulated drum (a dewar)

10. Dipolar Distributions • If you have a current line in a sphere (e.g. a head), the magnetic field will create a dipolar distribution • On the surface of the sphere, only the perpendicular part of the vector can be measured • Two extrema points are where the field strength is the highest

11. Forward Problem • Calculating an effect or predicting a phenomenon from a set of known causes and antecedent conditions. • If we know the cause (the sources) we can uniquely predict the effect (the magnetic distribution) using the Biot-Savart Law • Strength, orientation, (x, y, z) location • Predicts surface distribution

12. Inverse Problem • If we do not know the cause (or the sources) we cannot uniquely determine the effect (or the distribution). • We can hypothesize what the sources are and determine a hypothesized distribution • We iteratively change the parameters until we are satisfied that the hypothesized distribution matches the actual distribution.

13. Dipolar Distributions • Source location must be below the midpoint • Source location must be at a depth proportional to the distance between the extrema • Source strength related to absolute intensity of flux (at a given depth) • Source orientation related to orientation of extrema

14. Coordinate system • Y-axis: line between pre-auricular points • X-axis: line between nasion and perpendicular intersection of midpoint of the y-axis • Z-axis: line perpendicular to x-axis and y-axis at the midpoint • Labeled with lipid markers and structural MRI is taken.

15. MEG Validity • How sure are we that MEG images are accurate? • Consistent when compared with lesions, intracranial electrophysiology, and behavioral tasks • Limited spatial resoultion • Excellent temporal resolution

16. Neural dynamics of reading morphologically complex words Vartiainen et al. 2009 NeuroImage

17. Research Question • How are morphologically complex word forms represented and processed in the brain? • “book+s” • ‘books’ • “koulu+i+ssa+mme+kin” • ‘even in our schools’ • Do inflected words require additional processing early or late in the assumed sequence of cognitive operations?

18. Traditional Views • Two separate lexical entries for book and books • Butterworth, 1983 • Decomposition into constituent morphemes • Taft and Forster, 1975 • Dual-route mechanism where low-frequency words are processed through decomposition and high-frequency words are processed in the full-form • Chialant and Caramazza, 1995

19. Other Factors • Frequency • Alegre and Gordon, 1999 • Type of morphological operation • Miceli and Caramazza, 1988 • Inflectional complexity of language • Lehtonen et al., 2006a • Language background of speaker • Portin et al., 2008

20. Inflectional processing • Word form level • Decomposition of stem and affixes • Lemma level • Lexical representations of stem and affixes are recombined • Evidence indicates that the increased processing is during the semantic-syntactic level • Pawel’s question

21. Lexical, Syntactic, or Semantic? • Hyönä, Vainio, and Laine 2002 European Journal of Cognitive Psychology • A lexical decision task with isolated words showed more effortful processing for inflected than monomorphemic nouns • However, this morphological complexity effect did not generalise to words within sentence contexts • Fixation durations • Response latencies

22. Finnish • Finnish is a morphologically complex language • Effects of this complexity should be evident • Inflected Finnish nouns elicit longer reaction times and/or higher error rates when compared with otherwise matched monomorphemic nouns • There is a difference between inflected words and noninflected words in high frequency words, less so in middle frequency words, and non at all in low frequency words • Lehtonen and Laine, 2003

23. Cognitive processing • Lynn’s question • Lyytinen 1987 Scand J Psychol • Correlated cognitive components with inflectional errors • Block classification and integration skill of visual-linguistic information • Significance of memory increased as a function of the relative linguistic component

24. Hemodynamic studies of reading

25. Hemodynamic Localization • Effects were typically reported in activation of the left inferior frontal gyrus, interpreted to reflect analysis of grammatical features, and the left temporal regions, thought to denote access to the semantic representations of the stem and affix

26. MEG EvidencePylkkänen and Marantz, 2003Salmelin, 2007 • Basic visual analysis • Midline occipital cortex • 100 ms • Letter-string analysis • Left occipito-temporal cortex (fusiform gyrus) • 140-200 ms • Lexical-semantic activation • Left superior temporal cortex • 200 ms to 600-800 ms • Fiorentino and Poeppel 2007 (~350 ms, English)

27. Background • EEG: Lehtonen et al., 2007 • 450 ms onwards • N400 and positive component larger for inflected words • fMRI: Lehtonen et al., 2006b • Left posterior superior temporal sulcus • Left inferior frontal gyrus

28. Subjects • 5 males • 5 females • Mean age 30 • Age range: 25-46 • Normal vision (7) • Corrected-to-Normal vision (3) • Lucy’s question • Lack of information about subjects

29. Stimuli • 22.7 million word newspaper corpus • Analyzed using WordMill Lexical Search • The frequency range was allowed to be broader for the high-frequency range (26.4-504 occurrences per million) than the low-frequency range (0.04-4.23) to find a large enough number of high-frequency monomorphemic items of sufficient length

30. Syntactic case (75%) Genitive Partitive Marianna’s question Frequency of inflectional morpheme Effect of idioms on inflectional morphemes Semantic case (25%) Inessive Elative Illative Adessive Ablative Allative Related case Essive Stimuli

31. Stimuli • 320 stimuli in four groups of 80

32. Procedure • Words were presented visually one at a time • The MEG response timing was corrected for the 34-ms delay from stimulus trigger • Each noun was shown for 400 ms, and the stimulus onset asynchrony was 3000 ms • Stimuli from the different categories were presented in a random order and divided into 4 blocks • Subjects were instructed to read the words silently

33. Procedure • 20 additional target words were presented randomly • 5 words per category were matched similarly to the actual stimuli • The word was followed by a question mark • 1500 ms after the word onset • Duration 1500 ms • Prompted the subject to read out loud the preceding word • The next trial started after a delay of 1500 ms • The target trials as well as the trials immediately following the question marks were not included in the analysis. • This is one main difference from previous studies

34. MEG • 102 triple sensor elements • Two orthogonal planar gradiometers • One magnetometer • Band-pass filter: 0.03 and 200 Hz • Digitized at 600 Hz • Averaged: 0.2 s to 1 s after stimulus

35. MEG • Epoch rejected if EOG signal > 150 μV • Average 67 artifact-free trials from 80 trials • Israel’s question • Rejecting artifact

36. Epoch rejection

37. Left frontal Right frontal Left temporal Right temporal Parietal Left rolandic Right rolandic Left occipito-temporal Right occipito-temporal Occipital Areal mean signal (AMS)

38. Time windows • 50-170 ms • 170-330 ms • 330-500 ms

39. Equivalent Current Dipole • ECDs identified individually • ECD location and orientation fixed while amplitude varied • Head coordinate system set by nasion and ear canals • Head Position Indicator coils • Maximum peaks during the time windows were compared (repeated measures ANOVA) • Word frequency • Morphological complexity

40. Results • The response was stronger to inflected than to monomorphemic words at 330–500 ms • Left temporal region • Parietal region • The response was stronger to the low- than high-frequency words at 330–500 ms • Left temporal region • The response was stronger to the low- than high-frequency words at 170–330ms • the parietal region • There were no significant effects in the 50–170 ms time window over any region

41. Results • ECDs from individual subjects, each representing the center of an active cortical region, were grouped according to similarity in location and time course of activation • Duplicated results from previous studies

42. MEG EvidencePylkkänen and Marantz, 2003Salmelin, 2007 • Basic visual analysis • Midline occipital cortex • 100 ms • Letter-string analysis • Left occipito-temporal cortex (fusiform gyrus) • 140-200 ms • Lexical-semantic activation • Left superior temporal cortex • 200 ms to 600-800 ms

44. Results

45. Results • Inflected low-frequency words • Strongest activation • Monomorphemic high-frequency words. • Weakest activation

46. Results • Inflected low-frequency words • Longest duration (35 ms) • Monomorphemic high-frequency words. • Shortest duration (10 ms)

47. Results • No significant interaction was found between Morphological complexity and Word frequency in any of the measures.

48. No activation in frontal cortex No activation in Visual Word Form Area Rapid changes of highly synchronized neural activation Silent reading task Activation in inferior frontal cortex Activation of VWFA Changes in blood flow, oxygenation, and glucose uptake Lexical decision task MEG fMRI/PET

49. Question • The neural response was stronger and longer-lasting to the inflected than to the monomorphemic words, suggesting decomposition of all the inflected words throughout the frequency range used in the study. • However, there was no statistically significant interaction between morphology and word frequency, which implies that morphological decomposition occurred for inflected words throughout the frequency range employed.

50. Conclusions • All inflected words in Finnish are decomposed • Very high frequency inflected words may be an exception to this rule • Only those inflected words that are of very high frequency in the Finnish language may acquire full-form representations. • Laine et al., 1994 • Niemi et al., 1994 • Soveri et al., 2007