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The potential statistics for Event Related Potentials (ERPs)

Laura S. Bos

Center for Language and Cognition, Groningen (CLCG)

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Overview

› Extremely quick ERP-course

› The Time Reference ERP study

� Most frequent way of ERP analysis:Repeated measures ANOVA

› When ANOVA might not be right

› An alternative: non-parametric testing

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Extremely quick ERP course

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Extremely quick ERP course

› ERP effectsDe ober die nu de peper maalt krijgt geen fooi.The waiter who now the pepper grinds gets no tip.

* De ober die zonet de peper maalt krijgt geen fooi.The waiter who just before the pepper grinds gets no tip.

P600 effectPz electrode

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Dragoy, Stowe, Bos & Bastiaanse (subm.)From time to time: Processing Time Reference Violations in Dutch

› Time Reference

� Agrammatic aphasia: reference to the past particularly problematic (cf. present)

� Specific for aphasia? Theory indicates no:

- Reference to past: discourse linking Zagona, 2003

- Present tense: locally bound

- Parallels with e.g. pronouns vs. reflexives

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Introduction: discourse linking

Time line› Present: evaluation time = event time (Zagona, 2003)

De man drinkt koffie (onvoltooid tegenwoordige tijd)The man drinks coffee (simple present)

__________________________________________________

› But:De man dronk koffie (onvoltooid verleden tijd)The man drank coffee (simple past)

___________________________________________________

sentence

sentence

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Method ERP experiment:

Materials

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Method ERP experiment:

Materials and procedure› 380 sentences

� 80 (20 x 4) Time Reference (TR)� 300 other sentences- control, filler, other experiment

› 4 lists with 8 subjects per list › word-by-word reading

� 500 ms fixation point *� 240 ms word, 240 ms blank� 1750 ms blinking interval *****

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Method ERP experiment:

Procedure› 3 time windows:

� 300-500ms; 500-700ms; 700-1000ms� Comparisons: target V the same- PresPres vs. *PstPres and PstPst vs. *PresPst

Pz electrode

PresPres

*PstPres

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Data analysis› Repeated measures ANOVA (N-way/factorial)

� Every subject saw sentences in 4 conditions. - one of every quadruplet- Ho: ERPs violation sentence = ERPs correct sentence- H1: ERPs violation sentence ≠ ERPs correct sentence

� Factors:- TR (present, past)- Violation (correct, violated)- Midline: posteriority (3)- Lateral: posteriority (6) and laterality (2)

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In SPSS› Organised by ROI per condition (left-right) › Every subject one line› Time windows analysed seperately � filter data

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In SPSS

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In SPSS

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Use syntax function (paste instead of ‘ok’)

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In SPSS

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Results: overall

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Time window

Midline ROIs Lateral ROIs

300-500 500-700 700-1000 300-500 500-700 700-1000

Violation n.s. F(1,31)=6.55

p<.02

F(1,31)=4.84

p<.05

n.s. F(1,31)=6.17

p<.02

F(1,31)=1.31

p<.02

Violation *targetV

F(1, 31) = 6.34 p < .02

F(1, 31) = 7.74, p < .01

F(1, 31) = 12.33, p <

.001

(F(1, 31) = 7.35, p < .01

F(1, 31) = 6.36, p <

.02

F(1, 31) = 13.52, p < .001)

› Interpretation:

� 500ms after TR violation by V an effect occurs

� 300ms after TR violation by V, present and past tense verbs are processed differently

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Results: present reference V

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Time window

Midline ROIs Lateral ROIs

300-500 500-700 700-1000 300-500 500-700 700-1000

Violation F(1, 31) = 7.97, p < .01

F(1, 31) = 16.46,

p < .001

F(1, 31) = 17.19, p < .001

F(1, 31) = 8.19,

p < .01

F(1, 31) = 12.25, p < .001

F(1, 31) = 17.83, p < .001

Violation* posteriority

n.s. F(2, 62) = 4.83, p < .02

F(2, 62) = 9.62, p < .001

n.s. F(5, 155) = 2.87, p < .08

F(5, 155) = 7.29, p < .01

› Interpretation: see waveforms

� 300ms after Vpresent a violation of time reference elicits a positive wave, peaking at 600-700ms.

� larger effect at central and posterior ROIs

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Results: past V

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Time window

Midline ROIs Lateral ROIs

300-500 500-700 700-1000 300-500 500-700 700-1000

Violation n.s. n.s. n.s. n.s. n.s. n.s.

Violation* posteriority

n.s. n.s. n.s. n.s. n.s. n.s.

› Interpretation:

� no effect for a violation of time reference by Vpasttime-locked to the verb

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Results

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› Interpretation:

� violations of the temporal context by past tense verbs are processed differently from violations by present tense verbs.

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When ANOVA might not be right

› McCarthy and Wood (1985)

› ERP is multiplicative:� twice ERP source strength

� twice voltage at ROIs

› ANOVA (+mixed model) is additive:� twice ERP source strength

� plus X μV at every ROI� y = I + ax1 + bx2 + …

� Assumes Stotal = Serror + Smanipulation

� Possible: sign. interaction violation*location even though the distribution is the same (but different source strength).

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When ANOVA might not be right

› Maris (2004) and Maris & Oostenveld (2006)� If locus & time of the effect are not predicted?

� running several tests on the data causesproblems; the family-wise error rate increases

› Nonparametric testing

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When ANOVA might not be right

› Maris & Oostenveld (2006): Nonparametric testing� permutation tests

1.put all trials (correct + violation) into 1 set � trial: time bin of e.g. 100 ms

2. draw as many samples as you had items (so: 20)�this is the ‘random partition’

3. calculate a test statistic4. repeat 2 and 3 many times (e.g. 1000x)5. calculate the proportion of random partitions with a

more extreme value � the ‘p-value’ (Monte Carlo)6. if p < α then the distributions are different

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When ANOVA might not be right

› Maris & Oostenveld (2006)

� Visual inspection:

- Cluster the significant trials bytemporal adjacency

- calculate sum of t-statistics per cluster

- take the cluster with the largestt-statistic

� Uncorrected not conservativeenough

� Bonferroni correction (α/samples) more conservative

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Baggio (2008)› Similar study analysed with Maris & Oostenveld’ method� also a P600 effect for PsPr violations (present verb)

� more precise indication when and where

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Results and conclusions

� Time reference violations by a past tense verb are processed differently from by a present tense verb

� In ERPs, effects are not additive over the eletrodesites, as ANOVA assumes for interactions

� Nonparametrical testing of ERP data: especially iflocation and time point of effect are not predicted

� Different results? � to be tested

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Questions?› Thank you for your attention!

› References› Baggio, G. (2008) Processing temporal constraints: An ERP study. Language

Learning, 58, 35-55

› Maris, E. (2004). Randomization tests for ERP topographies and whole spatiotemporal data matrices. Psychophysiology, 41, 142–151.

› Maris, E. & Oostenveld, R. (2007). Nonparametric statistical testing of EEG- and MEG-data. Journal of Neuroscience Methods 164, 177-190.

› McCarthy, G. & Wood, C.C. (1985). Scalp distributions of event-related potentials: an ambiguity associated with analysis of variance data. Electroencephalography and clinical neurophysiology 62, 203-208.

› Zagona, K. (2008) Tense and anaphora: Is there a tense-specific theory of co-reference? In Barrs, A. (ed.) Anaphora: A reference guide. (pp140-171). Oxford: Blackwell Publishing.

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Results: present reference V

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