The MEG data set used in the tutorials is from a language study on semantically congruent and incongruent sentences that is described in detail in Wang et al. (2012). Three types of sentences were used in the experiment. In the fully congruent condition (FC) the sentences ended with a high-cloze probability word, e.g. De klimmers bereikten eindelijk de top van de berg (//The climbers finally reached the top of the mountain//) In the fully incongruent condition (FIC) sentences ended with a semantically anomalous word which was totally unexpected given the sentential context, e.g., De klimmers bereikten eindelijk de top van de tulp (//The climbers finally reached the top of the tulip//). The third type of sentences ended with a semantically anomalous word that had the same initial phonemes (and lexical stress) as the high-cloze words from the congruent condition: initially congruent (IC). There were 87 trials per condition for each of the three conditions, and a set of 87 filler sentences were added. From the EEG literature it is known that a stronger negative potential is produced following incongruent compared to congruent sentence endings about 300-500 ms after the word onset. This response is termed the N400 effect¹ ². For more information about the materials take a look at the published EEG experiment using the same sentence materials³.
In the study applied here, the subjects were seated in a relaxed position under the MEG helmet. Their task was to attentively listen to spoken sentences. They were informed that some of the sentences would be semantically anomalous. Acoustic transducers were used to deliver the auditory stimuli. After a 300-ms warning tone, followed by a 1200 ms pause, a sentence was presented. Every next trial began 4100 ms after the offset of the previous sentence. To reduce eye blinks and movements in the time interval in which the sentence was presented, subjects were instructed to fixate on an asterisk presented visually 1000 ms prior to the beginning of the sentence. The asterisk remained on the screen until 1600 ms after the onset of the spoken sentence. Subjects were encouraged to blink when the asterisk was not displayed on the screen.
MEG signals were recorded with a 151 sensor CTF Omega System (VSM MedTech Ltd.,Port Coquitlam, Canada). In addition, the EOG was recorded to later discard trials contaminated by eye movements and blinks. The ongoing MEG and EOG signals were lowpass filtered at 100 Hz, digitized at 300 Hz and stored for off-line analysis. To measure the head position with respect to the sensors, three coils were placed at anatomical landmarks of the head (nasion, left and right ear canal). While the subjects were seated under the MEG helmet, the positions of the coils were determined before and after the experiment by measuring the magnetic signals produced by currents passed through the coils.
The MEG data are stored as epochs or trials of fixed length around each stimulus trigger, i.e. the file does not represent a continuous record of the data and the data in the inter-trial-interval is not stored. The consequence is that the data cannot be represented as a continuous record, as at the epoch boundaries there is a discontinuity. See also the background on the CTF system.
Magnetic resonance images (MRIs) were obtained with a 1.5 T Siemens system. During the MRI scans ear molds with small containers filled with vitamin E marked the same anatomical landmarks. This allows for realignment of the MRIs to the MEG coordinate system according to the anatomical landmarks.
- Kutas M, Hillyard SA. (1980) Reading senseless sentences: brain potentials reflect semantic incongruity. Science. 207(4427):203-5
- Kutas M, Federmeier KD. (2000) Electrophysiology reveals semantic memory use in language comprehension. Trends Cogn Sci. 4(12):463-470
- van den Brink D, Brown CM, & Hagoort P. (2001). Electrophysiological evidence for early contextual influences during spoken-word recognition: N200 versus N400 effects. J Cogn Neurosci. 13(7):967-985
- Wang L, Jensen O, van den Brink D, Weder N, Schoffelen JM, Magyari L, Hagoort P, Bastiaansen M. (2012) Beta oscillations relate to the N400m during language comprehension. Hum Brain Mapp. 2012 Dec;33(12):2898-912.