Presentation on theme: "Hagoort, P., Brown, C.M., Groothusen, J. (1993) The Syntactic Positive Shift (SPS) as an ERP measure of syntactic processing."— Presentation transcript:
Hagoort, P., Brown, C.M., Groothusen, J. (1993) The Syntactic Positive Shift (SPS) as an ERP measure of syntactic processing
Introduction The data in this study may prove useful for resolving an ongoing debate in parsing research about the nature of processing of the parser: two approaches
Debate: Autonomous approach: computation is based on syntactic principles. Lexical, semantic and pragmatic information have their influence only after syntactic parse is delivered Separate Separate level for the syntactic structure of a sentence is required.
Debate: Interactive approach: syntactic information is directly integrated with lexical and semantic/pragmatic information, i.e., non syntactic sources are used either to direct the parser initial analysis or to evaluate immediately the outcome of syntactic analysis on a word-by- word basis. No separate level of representation is required.
How to resolve this debate? There is no sufficient empirical evidence Existing chronometric techniques may not pick on the small effects of the parsing process Re-analysis can be so fast, that it is difficult to observe
ERP method - advantages Empirical evidence from reaction times Sensitivity of ERP’s to the representational level (qualitatively different cognitive processes show up in different waveforms) If there are separate processing components producing output at different level of representation there are distinct brain correlates to semantic and syntactic processing
Previous studies dealing with the ERP responses to syntactic violations Kutas & Hillyard (1983): first study to investigate ERP responses to syntactic errors Osterhout & Holcomb (1992): ERP responses to the syntactic violations (verb sub-categorization and phrase structure constraints) Neville et al. (1991): violations of constraints on the movement of wh-phrases
Summary of the previous research Syntactic violations do not show N400 effect shown for semantic anomaly Syntactic violations show P600 family positivities However, the results are not robust as the studies have not resulted in the global ERP index of parsing syntactic operations
Problems with the previous studies The number of studies is too limited to draw conclusions regarding the existence and nature of syntactic ERP responses Studies on parsing have been done on English only The question is whether specific syntactic violations in English result in similar ERP effect as the same syntactic violation in other languages (i.e., Dutch)
Current study Its goal is to investigate ERP manifestations of syntactic parsing (electrophysiological response that is qualitatively different from semantic parsing) – Syntactic Positive Shift (SPS) Although it may not provide the answer, it opens the way for a novel empirical investigation Syntactic processing is studied based on the following three syntactic violations in Dutch:
Choice of syntactic violation Should help optimize the likelihood that the subjects detect the violations All violations can be locally processed by detecting a mismatch between syntactic specifications of immediately adjacent elements or elements separated by a determiner (size of syntactic buffer is kept to a minimum) All three types are different at the level of the grammar and semantics Will reflect ERP responses to different types of syntactic information
Types of syntactic violations in Dutch Agreement: subject-NP and finite verb do not agree in number (*On a rainy day the old man buy a life insurance.)
Types of syntactic violations in Dutch Subcategorization: verb that does not take an object NP is followed by a noun which has a role of grammatical object (*The tired young man elapsed the book on the floor.)
Types of syntactic violations in Dutch Phrase structure: obligatory word order violation-in Dutch, in NPs consisting of Adj and Adv, the Adv should precede the Adj (*Most of the visitors like the colourful very tulips in Holland.)
SVO and VSO agreement violation Violations between verbs and nouns: on either finite verbs or subject nouns within the same sentence (i.e., the subject noun could be singular in combination with plural form of the verb)
Examples: SVO and VSO agreement violation (the CW is italicized)
Subcategorization violation Involves obligatory intransitive verbs, which take a noun as a direct object In Dutch, unlike English, no continuation is possible following the noun in object position
Method Total of 360 sentences – 50% grammatically correct and 50% contain grammatical violation Each incorrect sentence is derived from the companion correct sentence such that words preceding and following the word string that makes the sentence ungrammatical are the same as the companion correct sentence The sentences are matched in number of words Other than specific violations, the sets of 180 correct and incorrect sentences are closely matched well controlled
Criteria for violations Immediate (restricted to two adjacent words in a clause, or by a triplet of words) The Critical Word has a counterpart in its companion correct sentences The CW is either verb or noun, max length = 9 and min length = 4 At least 3 words preceding the CW and at least two words following the CW
Experiment design Two experimental lists 180 sentences each (90 correct and 90 incorrect, each made of 30 sentences of each violation type) Order: pseudo-randomized for each list (no > than 3 incorrect or correct sentences occur) The experiment included two parts: Grammaticality Judgment Pretest: to ascertain that the syntactic violations were perceived by the subjects as such The ERP experiment
Grammaticality Judgment Pretest: Results The subjects correctly identified the violations The majority of responses were either to CW or the word immediately following the CW Choosing a CW as a point of ungrammaticality in a sentence is a valid method
The ERP experiment Subjects: 34 University students, all native speakers of Dutch, mean age=23 years old, range 18-28 years Procedure: The stimuli were displayed in the same manner as in a pretest (word by word, each word was presented for 300 ms with an ISI=300 ms, the ITI depended on number of words in a sentence)
The ERP experiment EEG activity has been recorded using an Electrocap with seven scalp electrodes, each referred to the left mastoid Sampling started 150 msec before the presentation of the first word of each sentence Total sampling epoch = 8550 msec Task: comprehending the whole sentence No other task assigned The subjects were told that there were incorrect sentences, but no information on violations was provided
Average waveforms were computed by subject for correct and incorrect sentence for each of the violation types Calculations are done separately for each of the seven electrodes The baseline for CW and its preceding position was chosen Each violation analyzed separately Results
Grand average waveforms by electrode site for the critical word in the correct and incorrect agreement condition (see page 454-455) Results by violation: Agreement
CWs have widely distributed positive shift (the waveforms for the incorrect CW have a widely distributed positive shift in comparison to the correct words, which starts at around 500 msec following the onset of the CW and continues throughout the following word ) Analysis on each position (to test if the shift occurs on both grammatical words and CWs): Positions preceding the CW Penultimate and sentence-final positions the shift is different on those positions (p. 456-457) Agreement: Data analysis
The waveforms do not show the same positive shift to the CW However, the waveforms show a sustained positive shift preceding and on the CW This shift is most prominent at the Fz and right anterior sites, where it is in a position following the CW Frontal positivity is absent or marginal at the Cp or P lateral sites (p. 459-461) Subcategorization: Data analysis
The waveforms at the CW are characterized by a positive shift with the broad scalp distribution for the incorrect CW in the latency window of 500-700 msec The positivity is present from the immediate onset of the CW (p. 463-465) The negative shift for the incorrect condition is very similar to subcategorization and agreement conditions Phrase structure: Data analysis
The CW in the incorrect agreement and phrase-structure conditions have a positive shift compared to the correct conditions Most probably it results from the syntactic violation on the CW There is no significant positive shift in the subcategorization condition Summary of the results
All three violations have a significant negative shift for the incorrect condition on the sentence-final, and penultimate positions The negativity most probably results from the semantic analysis problems originating in syntactic violations Summary (continued)
Major result – the widely distributed positivity elicited by two of three syntactic violations: starts at about 500 msec, with a centro-parietal maximum (very similar to the P600 reported by Osterhout and Holcomb) SPS is different from N400 resulting from semantic violations Brain seems to process syntactic information differently from semantic one Discussion
The SPS is not an ERP response to violations only: in phrase structure violations, it occurs one word BEFORE the syntactic violation (due to the parser’s attempt to entertain the possibility of statistically infrequent but nevertheless grammatical construction “Determiner-Adj-Adv-Adj-Noun”) This is consistent with other empirical evidence that the parser avoids keeping all the possibilities open until the disambiguating information is received Discussion (continued)
The parser operates on the basis of the principle of computational economy or on the basis of the frequency of alternative syntactic constructions The SPS occurs in the absence of other tasks the “surprise” account of the observed positivity is less likely Both open- and closed- class of words trigger SPS the possibility that SPS is related to a word class is ruled out Discussion (continued)
The subcategorization violation results which differ from two other violation types are not only syntactic, but also semantic by necessity (verb meaning and its syntactic aspect are closely intertwined) it is possible that “semantic” negativity and “syntactic” positivity occurred on the same word However, it may also be a methodological fallacy: it should be also tested on the closed class of words or using “syntactic prose” Discussion: problems encountered
It is hardly possible to avoid the constellation of semantic and syntactic violation The obtained results do allow for strong claims regarding temporal relation between parsing operations and semantic integration processes It is unclear if SPS is time-locked to the initial structural assignment or to the processor’s rejection of this first assignment Discussion: problems encountered (continued)
Although the results do not allow for conclusions that can resolve this debate, they show that there is some intermediate level of syntactic representation, which is computed, or a process of syntactic re-analysis initiated upon encountering a structural violation. At the level of language processing there is a relation between syntactic processing and “syntactic” positivity in the ERP waveform. Autonomous or interactive parsing?
Positive shift is a manifestation of a functionally distinct process in computation of syntactic structures A new label is assigned to this ERP response: SPS The obtained positivity can be related to the domain of syntactic parsing It is a common brain response to very different types of syntactic violations Autonomous or interactive parsing?
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