2.3 Event-Related Potentials
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20 2. BACKGROUND Which word accent a word is associated with depends to a large extent on which suffixes it includes (Bruce, 1977; Riad, 1998). Thus whereas the stem mink in the definite singular mink–en ‘the mink’ is associated with a L* tone (Accent 1), the same stem has a H* tone (Accent 2) in the plural indefinite mink–ar ‘minks.’ 2.3 Event-Related Potentials To gain more insight into the specific processes involved in the interaction of prosodic, pragmatic, syntactic, and morphological information, electroencephalog- raphy (EEG) was used together with the Event-Related Potential (ERP) method to monitor the online brain activity during language processing. The EEG equip- ment used in the experiments, a Synamps 2 differential amplifier and a Neuroscan QuikCap, measures changes in the electric potential between sixty-four active electrodes connected to the scalp and a frontal cap-mounted ‘ground’ electrode (see Figure ). In order to cancel out activity stemming from electrical changes at the ground electrode, the amplifier further subtracts the potential between a reference electrode and the ground from the active–ground potentials (see Luck, 2005). During recording, a cap-mounted electrode was used as reference, but off- line, the data was re-referenced to the average signal from electrodes located over the mastoid process at both sides of the head. 3 The potential changes measured in this way stem partly from postsynaptic electrochemical processes involved in the transmission of information between groups of neurons. However, other sources of electrical activity might change the potentials much more than the brain activity. Thus, muscle activity from e.g. tensing the jaws creates high-frequency voltage fluctuations, and changes in head position as well as in the ‘skin potential’ between different layers of the skin due to e.g. sweating give rise to large, slow voltage changes. Similarly, eye-movements and blinks produce large peaks that might dominate the signal. Further, the electric sources stemming from the AC line network present in cables and equipment in the laboratory increase the amplitude at 50 Hz. Another source of noise is the digitization procedure. In the experiments reported on here, the EEG data were sampled at a rate of 250 Hz, meaning that activity of frequencies higher than that was not directly recorded. However, activity at higher frequencies might appear in the data as artifactual waves at lower frequency, a phenomenon known as ‘aliasing.’ 3 The mastoid process is a protruding part of the temporal bone situated behind the ear.
Transcript of 2.3 Event-Related Potentials
20 2. BACKGROUND
Which word accent a word is associated with depends to a large extent on whichsuffixes it includes (Bruce, 1977; Riad, 1998). Thus whereas the stem mink in thedefinite singular mink–en ‘the mink’ is associated with a L* tone (Accent 1), thesame stem has a H* tone (Accent 2) in the plural indefinite mink–ar ‘minks.’
2.3 Event-Related Potentials
To gain more insight into the specific processes involved in the interaction ofprosodic, pragmatic, syntactic, and morphological information, electroencephalog-raphy (EEG) was used together with the Event-Related Potential (ERP) methodto monitor the online brain activity during language processing. The EEG equip-ment used in the experiments, a Synamps 2 differential amplifier and a NeuroscanQuikCap, measures changes in the electric potential between sixty-four activeelectrodes connected to the scalp and a frontal cap-mounted ‘ground’ electrode(see Figure ). In order to cancel out activity stemming from electrical changesat the ground electrode, the amplifier further subtracts the potential between areference electrode and the ground from the active–ground potentials (see Luck,2005). During recording, a cap-mounted electrode was used as reference, but off-line, the data was re-referenced to the average signal from electrodes located overthe mastoid process at both sides of the head.3
The potential changes measured in this way stem partly from postsynapticelectrochemical processes involved in the transmission of information betweengroups of neurons. However, other sources of electrical activity might changethe potentials much more than the brain activity. Thus, muscle activity from e.g.tensing the jaws creates high-frequency voltage fluctuations, and changes in headposition as well as in the ‘skin potential’ between different layers of the skin due toe.g. sweating give rise to large, slow voltage changes. Similarly, eye-movementsand blinks produce large peaks that might dominate the signal. Further, the electricsources stemming from the AC line network present in cables and equipment inthe laboratory increase the amplitude at 50 Hz. Another source of noise is thedigitization procedure. In the experiments reported on here, the EEG data weresampled at a rate of 250 Hz, meaning that activity of frequencies higher thanthat was not directly recorded. However, activity at higher frequencies mightappear in the data as artifactual waves at lower frequency, a phenomenon knownas ‘aliasing.’
3The mastoid process is a protruding part of the temporal bone situated behind the ear.
2.3. EVENT-RELATED POTENTIALS 21
Figure 2.1: Synamps 2 amplifier and NeuroScan Quik-Cap. From left to right:The safety-providing Power Unit, the System Unit, used to synchronize data sam-pling with stimulus onset, the Head Box, which is the actual amplifier, and theelectrode cap.
Several methods for dealing with these problems have been proposed, most ofthem involving filtering the waveform. In the analyses presented in the presentdissertation, an online high-pass filter with a cutoff frequency of 0.05 Hz anda low-pass filter of 70 Hz reduced most artifactual slow waves and the aliasingeffect, respectively. An offline low-pass filter of 30 Hz attenuated the 50 Hz linenoise, as well as muscle artifacts. Independent Components Analysis filtering wasused to compensate for the eye movements and blinks (Delorme & Makeig, 2004).
The signal remaining after filtering is partly the result of the perception ofstimuli and partly of brain activity from background processes pertaining to stim-ulus-unrelated thoughts or bodily functions, but also from non-regular artifactsfrom e.g. sudden movements during the recording. The ERP method has beendeveloped in order to find regular patterns corresponding to the processing of spe-cific stimuli rather than to random brain activity. In the experiments reported onhere, ERPs are obtained by time-locking the EEG-waveform to the presentationof stimuli, such as the beginning of words, morphemes, or tones. Since brainactivity gives rise only to small voltage changes, remaining larger artifacts werereduced by rejecting all trials exceeding a pre-specified voltage, ±75µV in thestudy using visually presented stimuli,±100µV in the auditory experiments. EEG
22 2. BACKGROUND
from approximately forty different trials per condition were averaged and statis-tically analyzed for between sixteen and twenty-two participants per experiment.In the averaged ERP-waveform, recurrent responses to stimuli in the form of posi-tive (plotted downwards) or negative potential peaks, referred to as ‘components,’emerge.
2.4 ERP in language research
The method used in the experiments to monitor the prosodic, semantic, pragmatic,syntactic, and morphological processing of sentences involved breaking expecta-tions assumed to arise at the different levels of linguistic analysis. Using thismethod, earlier research has found effects at three stages of language processing(Friederici, 2002), summarized below in Table 2.1.
Table 2.1: ERP effects of word processing in three phases following Friederici(2002), extended with findings from Steinhauer et al. (1999), van den Brink et al.(2001), Heim & Alter (2006), and Bornkessel & Schlesewsky (2006).
ERP effectsPhase Time window Processes grammar semantics sound/prosody0 up to 100 ms Acoustic and pho-
neme analysisN100
1 100 to 300 ms Word category re-cognition
ELAN N200 P200
2 300 to 500 ms Lexical semantics,morphosyntax
LAN N400
3 from 500 ms Generalized map-ping, reanalysis
P600 P600 CPS, P600
2.4.1 Word category recognition
At a first stage, between 150 and 200 ms following the presentation of a word,its category is identified and the fit with the activated syntactic structure can bedetected. If, for example, a preposition is perceived instead of an expected noun,such as the underlined of in Max’s of proof the theorem, the result is an Early LeftAnterior Negativity (ELAN) (Friederici et al., 1993; Neville et al., 1991).
2.4. ERP IN LANGUAGE RESEARCH 23
In a similar time window, between 150 and 250 ms following stimulus onset,phonetic patterns differing from the semantically activated word have been foundto give rise to a broadly distributed negativity (Connolly & Phillips, 1994; Hagoort& Brown, 2000), termed ‘N200’ (van den Brink et al., 2001). The N200 is thoughtto reflect lexical selection based on semantic expectations combined with a rapidacoustic analysis.
2.4.2 Semantics and morphosyntax
At a second stage, between 300 and 500 ms, the semantic and morphosyntacticproperties of the word are evaluated. The integration of the word in the semanticcontext corresponds to a mainly centro-posterior negative peak at 400 ms, the‘N400’ (Kutas & Hillyard, 1980; Holcomb & Neville, 1990; Kutas & Van Petten,1994). The more difficult the word is to integrate in the semantic context, thelarger the N400. The underlined word in He spread the warm bread with socksthus produces a relatively large peak (Kutas & Hillyard, 1980).
Problems in the morphosyntactic evaluation of the word give rise to a LeftAnterior Negativity around the same time, i.e. between 300 and 500 ms (Barber& Carreiras, 2005; Coulson et al., 1998; Frisch et al., 2004) or 300 and 600 ms(Friederici & Frisch, 2000; Friederici et al., 1996; Munte et al., 1997).
2.4.3 ERPs of prosody
The presence of phrase-initial high pitch accents and early pitch peaks due to de-accenting in German have been found to increase the ‘P200’ (Friederich et al.,2001; Heim & Alter, 2006). In MEG studies, the source of the P200 has beenlocated to the planum temporale and Area 22 (Godey et al., 2001; Hari et al.,1987), but generators in the brain stem have also been suggested (Crowley &Colrain, 2004; Rif et al., 1991). Considering these potential generators, it is notsurprising that pitch processing would modulate the P200. Thus, the auditoryassociation cortex of Area 22 in the right hemisphere is involved in processing ofprosodic features such as pitch (Zatorre et al., 1994) and sound intensity (Belinet al., 1998).
The processing of right-edge boundary tones (Steinhauer et al., 1999) and fo-cus (Toepel et al., 2007) has given rise to a late posterior positive shift in the ERPs,termed the ‘Closure Positive Shift.’
24 2. BACKGROUND
2.4.4 Integration of information
At a third stage, starting at around 500 ms, syntactic structures that have bro-ken expectations are reprocessed, giving rise to a sustained positivity peaking ataround 600 ms, the ‘P600.’ (Hagoort et al., 1993; Neville et al., 1991; Osterhout& Holcomb, 1992, 1993). More recently, however, not only syntactic structureshave been found to give rise to late positivity, but also syntactically incongru-ent prosodic structures (Eckstein & Friederici, 2005, 2006), unexpected thematicrole-assignment (Bornkessel et al., 2002; Hoeks et al., 2004; van Herten et al.,2005), and even question intonation mismatching a preceding statement intona-tion (Astesano et al., 2004). Taking into account the spectrum of processing con-flicts that might lead to a late positivity, Bornkessel & Schlesewsky (2006, 2008)proposed that the P600 is caused by problems in ‘generalized mapping,’ the con-fluence of information related to different sources contributing to the pragmaticinterpretation of a sentence, and subsequent reanalysis.
