J Psycholinguist Res DOI 10.1007/s10936-012-9224-4
Sentence Complexity and Working Memory Effects in Ambiguity Resolution Ji Hyon Kim · Kiel Christianson
© Springer Science+Business Media, LLC 2012
Abstract Two self-paced reading experiments using a paraphrase decision task paradigm were performed to investigate how sentence complexity contributed to the relative clause (RC) attachment preferences of speakers of different working memory capacities (WMCs). Experiment 1 (English) showed working memory effects on relative clause processing in both offline RC attachment preferences and in online reading time measures, but no effects of syntactic complexity. In Experiment 2 (Korean), syntactic complexity due to greater distance between integrating heads, as measured by the dependency locality theory (Gibson in Cognition 68:1–76, 1998), significantly increased the proportion of attachment to NP1. However, no effects of working memory were found. The difference in results between English and Korean is proposed to be due to head-directionality effects. The results of our study support the conclusion that working memory-based accounts provide a better explanation than previous language-dependent accounts for differences in RC attachment preferences. We propose that previous language dependent-accounts of cross-linguistic differences in RC processing have overlooked the interaction between individual WMC and a language’s general structure, which is a central factor in RC attachment. Keywords Working memory · Sentence complexity · Relative clause processing · Ambiguity resolution · Paraphrase decision task
J. H. Kim (B) Department of English Linguistics, Hankuk University of Foreign Studies, 270 Imun-dong, Dongdaemun-gu, Seoul 130-791, Korea e-mail:
[email protected] K. Christianson Educational Psychology, Beckman Institute for Advanced Science and Technology, University of Illinois, Urbana-Champaign, 226A Education Building, 1310 S. 6th St. MC 708, Champaign, IL 61820, USA e-mail:
[email protected]
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Introduction The globally ambiguous relative clause (RC), e.g., Someone shot the servant of the actress who was on the balcony, is one of the most widely studied syntactic structures in the psycholinguistic literature. In this structure, the RC who was on the balcony can be attached to either the noun phrase (NP) the servant (NP1, or high attachment (HA)) or the actress (NP2, or low attachment (LA)). Many previous studies have reported cross-linguistic differences in the RC attachment preferences of this structure. Languages shown to have a HA preference to the NP1 include Dutch (Brysbaert and Mitchell 1996), French (Pynte 1998), German (Konieczny and Hemforth 2000), Greek (Papadopoulou and Clahsen 2003), Italian (Frenck-Mestre and Pynte 2000), Japanese (Kamide and Mitchell 1997) and Korean (Jun 2003; Lee and Kweon 2004). On the other hand, languages that have been reported to display a LA preference to the NP2 include Basque (Ziardegi et al. 2004), Brazilian Portuguese (Miyamoto 1998), English (Carreiras and Clifton 1999; Cuetos and Mitchell 1988), Norwegian, Romanian and Swedish (Ehrlich et al. 1999). These cross-linguistic differences in RC attachment preferences have been interpreted as evidence for differences in processing strategies across languages. Recent studies, focusing on the role of individual working memory capacity (WMC) on RC ambiguity resolution, have shown that individual working memory is a factor that may affect RC attachment preferences within a language. Working memory refers to the mechanisms or processes that involve the storage of information in the face of interference. Working memory differs in principle from short term memory (STM), which refers to the storage of information for a short period of time with no interference at all (Just and Carpenter 1992). The pool of resources referred to as WMC differs from individual to individual and has been shown to cause differences in the processing and interpretation of both temporarily ambiguous and globally ambiguous sentences (Just and Carpenter 1992; King and Just 1991). Several offline studies have investigated the effects of individual WMC on the disambiguation strategies of the ambiguous RC construction. Mendelsohn and Pearlmutter (1999) found that low-span subjects showed a significant preference for HA, but high-span subjects did not show a particular attachment preference. Swets et al. (2007) reported a negative correlation between WMC and RC attachment preference, such that subjects with higher working memory spans showed a higher proportion of LA preference, and vice versa. The results of these studies point in the same direction, suggesting an inverse relationship between WMC and RC attachment preference. However, a few studies investigating WMC and RC attachment and using online measures of processing, such as eye movement data, have reported results in the opposite direction. Traxler (2007) carried out an eye-tracking experiment with native English speakers and found a significant interaction between WMC and RC attachment, such that an increase in regression path time and total time for NP1 attachment was associated with a decrease in working memory, suggesting that low-span readers preferred a LA interpretation. In a self-paced listening study, Felser et al. (2003) reported that native English-speaking children also showed a tendency for a positive relationship between working memory and RC attachment. In their study, the high-span group showed a HA preference, and the low-span group showed a LA preference. There are two possible reasons behind these mixed results: the experimental methodology that was used, and the type of RC sentences that were used as experimental items. The mixed results of the previous studies can be grouped based on whether online or offline methodology was used. The offline studies (Mendelsohn and Pearlmutter 1999; Swets et al. 2007) showed a negative relation between WMC and RC attachment, while the online studies (Felser et al. 2003; Traxler 2007) showed a positive relation between the two factors.
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Theories of WM suggest that the online and offline methodology used above may actually be examining different stages of processing. The Separate Sentence Interpretation Model (SSIR) by Caplan and Waters (1999) places a distinction between the interpretive and postinterpretive stages of processing. The interpretative stage of processing refers to the extraction of meaning from linguistic input, i.e., assigning syntactic structure to a sentence and using that structure to arrive at a correct interpretation. Post-interpretive processing occurs when the reader uses the meaning that is extracted from the syntactic structure to accomplish other tasks. Therefore, previous studies examining relative clause attachment through online methodology may have been focusing on the interpretive stage of processing, while studies using the offline forced-choice paradigm may have focused on the post-interpretive stage. The mixed results in the previous studies examining RC attachment may have been due to the examination of different stages of processing (interpretive vs. post-interpretive). The second possible explanation for the mixed results in previous studies is the type of RC construction that was used in the studies. The offline studies discussed above (Mendelsohn and Pearlmutter 1999; Swets et al. 2007) used globally ambiguous sentences in a forced-choice question paradigm, which requires participants to determine the NP to which the ambiguous RC should be attached. This paradigm inevitably draws the attention to the potential ambiguity in the RC structure, which may lead participants to process the structure differently than they would in a more natural reading context. On the other hand, the online studies (Felser et al. 2003; Traxler 2007) used the relative clause in an unambiguous context, by using either gender or number to disambiguate the sentence when examining the participants’ processing of HA and LA sentences. One advantage of the online methodology used in these studies (e.g., eye-tracking and self-paced listening) is that they allow for a more natural processing of the test sentences than the offline forced-choice paradigm, as well as providing data from the interpretive stage of processing. However, because the test items used were disambiguated toward either a HA or LA reading, it was not possible to examine the online processing of globally ambiguous sentences, per se. Whatever the cause or causes of the previous divergent results, it is important to note that the WM-based accounts are fundamentally different from accounts that are predicated upon differences across languages in several ways. The language-dependent accounts propose either that syntactic differences across languages predict attachment preferences (Frazier and Clifton 1996; Gibson et al. 1996), or that frequency of attachment within a given language predicts attachment preferences in said language (Cuetos et al. 1996). In other words, both of these accounts predict relative consistency across speakers within a given language with respect to attachment preferences. This view is in stark contrast to the WM-based accounts, which predict more variability within a given language, with attachment preferences potentially varying from speaker to speaker based on an interaction of the complexity and content of any given sentence and the WMC of any given speaker, possibly along with language-specific syntactic characteristics of a given structure. We propose that the purported cross-linguistic differences are not necessarily wrong, but rather that they are too simplistic. What might be driving them is an interaction between individual WMC and a language’s general structure, such as head-directionality. In addition, this interaction might also be modulated by other factors, such as sentence complexity and sentence content, i.e., whether a sentence is temporarily or globally ambiguous. In the current study, we developed a paraphrase decision task paradigm and paired it with a traditional self-paced reading task in order to enable the examination of both the interpretive and post-interpretive stages of processing of the same ambiguous target sentence. This paradigm asks participants to judge whether a paraphrase of RC sentence is correct or not,
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and in this way probes the participants’ RC attachment preference without drawing attention directly to the ambiguity of the RC structure, as the forced-choice questionnaire task often used in offline studies might do. At the same time, this paradigm also allows for the examination of the processing of globally ambiguous sentences while using online experimental methods, which has not been possible in previous online studies. For these reasons, the paraphrase decision task allows for a more indirect and subtle window into natural human processing mechanisms, which we hope will lead to clearer results regarding the processing of the ambiguous RC construction. In order to investigate more closely the interaction between WMC and RC attachment, we used the paraphrase decision paradigm to examine the online and offline processing of ambiguous RC sentences. In addition, we also investigated how this interaction was modified by the factor of syntactic complexity—specifically, the distance between subjectverb integrations as predicted by the Dependency Locality Theory (DLT) (Gibson 1998). Finally, the effects of language structure on RC attachment were also investigated by examining RC processing in in a head-initial language, English, and a head-final language, Korean. The predictions of our study are relatively straightforward: If attachment preferences are simply language-dependent, we would expect to find differences in attachment preference between English and Korean, based on previous observations of preferences in these languages (Carreiras and Clifton 1999; Cuetos and Mitchell 1988; Jun 2003). If, however, attachment preference depends more on the interaction between individual WMC and a language’s general structure, we should find variability in attachment preferences within languages. Importantly, these differences should also be modulated by the relative syntactic complexity of the structures, specifically, the distance and intervening content between critical syntactic heads and dependents, and also language-specific properties, such as headdirectionality.
Experiment 1 Previous studies investigating RC attachment preferences used either subject-modifying RCs (Omaki 2005; Swets et al. 2007; Traxler 2007) or object-modifying RCs (Felser et al. 2003; Mendelsohn and Pearlmutter 1999) as test sentences. The DLT (Gibson 1998, 2000) predicts clear differences in the relative processing difficulties of these two sentence types. Subjectmodifying RCs in English are predicted to be more difficult in terms of processing load than object-modifying RCs because the distance between the integration of the main subject and verb of the sentence is greater in subject-modifying RCs. This difference is shown in (1). (1) a. Subject-modifying RC The agent of the star who met me at the party last night was poor. b. Object-modifying RC The police arrested the agent of the star who met me at the part last night. In the sentences above, the main subject is italicized and the main verb is underlined. In (1a), there are two referents intervening between the integration of the main subject and verb, i.e., the relative pronoun ‘who’ and the verb ‘met.’1 The object-modifying RC in (1b) 1 1st and 2nd person pronouns are not counted as referents in Gibson (2000) dependency locality theory, but relative pronouns are, as are main verbs.
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has no referents intervening between the subject-verb integration, as the two are adjacent. According to the DLT, then, in English the subject-modifying RC condition is predicted to impose a greater processing load than the object-modifying RC condition. The present study investigated whether the difference in processing difficulty between subject-modifying and object-modifying RCs, as predicted by the DLT, had an effect on the RC attachment preferences of native English speakers. Monolingual native English speakers were tested on their RC attachment preferences through a paraphrase decision task that aimed to probe the preferred interpretation of the ambiguous RC construction without drawing attention to the global ambiguity of the structure. In addition, the paraphrase decision task was paired with a self-paced word-by-word reading task in order to yield both online data (i.e., reading times) and offline data (i.e., RC attachment preference), allowing us to examine both the interpretive and post-interpretive stages of human sentence processing (Caplan and Waters 1999). The results of this study were therefore expected to aid in accounting for the mixed results in previous studies, which were either online or offline. We had the following predictions for the effects of processing difficulty and working memory on RC attachment. First, because low span readers have been shown to insert a pause (even in silent reading, due to normal sub-vocalization during reading, cf. Rayner and Pollatsek 1989) between the NP and the modifying RC, leading to a HA interpretation (Jun 2003; Swets et al. 2007), we expected both the online and the offline data to show a significant difference between the low span and high span groups, with low span readers preferring HA, and the high span readers preferring LA. Second, because sentences higher in complexity as defined by the DLT (Gibson 1998, 2000) are predicted to impose a greater processing load, we expected the more difficult subject-modifying RC condition to minimize differences between the two groups by causing the high span group to behave like the low span group due to the relative drain on processing resources in both groups. Method Participants Sixty native speakers of American English at the University of Illinois at Urbana-Champaign participated in this experiment. All had normal or corrected-to-normal vision. They received either course credit for their participation or $5 payment. To prevent language transfer effects (Cuetos et al. 1996), participants who indicated they had spent more than one year in a nonEnglish speaking country or those who were fluent in a second language other than English were not included in the experiment. Materials and Design Forty sets of sentences with the ambiguous RC construction were constructed. Each set contained four versions, crossing RC position (subject-modifying, object-modifying) with referential load (full noun, 1st/2nd person pronoun). While RC position was previously discussed as a factor that is predicted to cause differences in processing difficulty by the DLT (Gibson 1998, 2000), the factor of referential load also plays a role in the predicted processing difficulty of a sentence. According to the Givenness Hierarchy (Warren and Gibson 2002), 1st/2nd person pronouns consume less resources than full nouns, which in turn leaves more resources available to devote to processing the rest of the sentence. A sample item set is shown in (2).
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(2) a. Subject RC, High referential load The lawyer of the client who insulted the witness during the trial was intelligent. b. Subject RC, Low referential load The lawyer of the client who insulted me during the trial was intelligent. c. Object RC, High referential load The judge reprimanded the lawyer of the client who insulted the witness during the trial. d. Object RC, Low referential load The judge reprimanded the lawyer of the client who insulted me during the trial. Regarding the factor of RC position, the Subject RC condition is predicted to be more difficult to process than the Object RC, due to greater distance between subject-verb integrations, according to the DLT (Gibson 1998, 2000). Regarding the factor of referential load, the high referential load condition is predicted to be more difficult to process than the low referential load condition, as the sentences in the high referential condition include a full NP instead of a 1st/2nd person pronoun as the object of the embedded RC, which may contribute to similarity-based interference. Each item consisted of two parts: a target sentence containing the ambiguous RC construction, and a paraphrase of the target sentence which was disambiguated toward either HA or LA. A sample item with both paraphrase versions is given in (3). (3) a. Target sentence The lawyer of the client who insulted the witness during the trial was intelligent. b. Paraphrase HA: The lawyer of the client was intelligent. The lawyer insulted the witness during the trial. LA: The lawyer of the client was intelligent. The client insulted the witness during the trial. The paraphrases were counter-balanced across lists so that each participant saw only one version of the paraphrase for each experimental item. Note that both paraphrases are accurate, depending on whether one has made a high or low attachment while reading the globally ambiguous sentence. For this reason, one dependent offline measure will be response time to endorse the paraphrase as correct, operating under the assumption that longer response latencies signal incongruence with the initial attachment preference. The second offline measure will be paraphrase endorsement accuracy; because all responses should be “yes,” incorrect “no” responses are taken to signal very strong attachment preferences that do not match the paraphrase. For example, if a participant answers “no” to a HA paraphrase, his/her attachment preference is taken to be strongly LA; if a participant answer “no” to a LA paraphrase, his/her preference is taken to be strongly HA. Longer paraphrase “yes” responses are taken to indicate relatively weaker attachment preferences that are inconsistent with the given paraphrase. In addition to the experimental sentences, 88 filler items with various syntactic structures were also included. The four versions of each experimental sentence were counterbalanced across four lists in a Latin square design, such that each participant saw only one of the four versions. The presentation of the stimuli was pseudo-randomized so that at least one filler was presented between two target sentences.
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Procedure Reading Span Task The reading span task (Conway et al. 2005) was designed to measure the participants’ working memory capacity and was implemented using E-prime. In this version of the reading span task, participants were tested on sets of sentences ranging from two to five sentences per set. There were three trials for each set size, resulting in a total of forty-two sentences for the entire task (3 × (2 + 3 + 4 + 5) = 42). Each item consisted of a complete sentence followed by a question mark and then a capital alphabet letter. Participants were instructed to read each sentence aloud, pause at the question mark, and decide whether the sentence ‘made sense in the real world’ or not. Participants were to answer ‘yes’ or ‘no,’ depending on the semantic plausibility of the sentence, and their responses were recorded by the experimenter. After the semantic plausibility decision, they were instructed to read the capital letter aloud also. By pressing the space bar, the participants could proceed to the next item. When the participant reached the last sentence in a set, three question marks (‘???’) were presented on the screen. The participants were instructed to stop at this point and write down each of the letters in the order in which they had appeared in the set. A sample set of three items is shown in (4). (4) a. No matter how much we talk to him, he is never going to change.? J b. The prosecutor’s dish was lost because it was not based on fact.? M c. Every now and then I catch myself swimming blankly at the wall.? F ??? The correct responses to the semantic plausibility condition are ‘yes, no, no,’ and one point was given for every letter correctly written in the correct order on the answer sheet (J, M, F). Paraphrase Decision Task The paraphrase decision task was designed to probe the RC attachment preferences of the participants in a more indirect manner than the forced-choice questionnaire used in previous off-line studies (Mendelsohn and Pearlmutter 1999; Swets et al. 2007), while at the same time providing online data, through the reading times for each word. Participants read the target sentence word-by-word in a non-cumulative self-paced reading task. At the beginning of each trial, underlines corresponding to each word in the upcoming sentence appeared on the computer screen. When the participant pressed the space bar, the first word appeared. Upon each subsequent button-press, the next word would appear and the previous one was replaced by the underlines. After the last word of the target sentence, the next screen showed a paraphrase of the target sentence, disambiguated toward either HA or LA. The participant was to decide whether the paraphrase correctly matched the target sentence or not by pressing Y or N on a button box. The reading times for each word of the target sentence, the Y/N answer to the paraphrase decision task, and the response time for the paraphrase decision were all recorded automatically using E-Prime experiment presentation software. A practice session consisting of five practice items preceded the test session, and the entire task took less than 30 min. Results Reading Span Task One point was given for each letter written in the correct position on the answer sheet. There were forty-two sentences total, so the possible score ranged from 0 to 42. The mean reading
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span score was 28.68 (SD = 5.74), with a range of 16–40. The semantic plausibility questions were answered correctly 97.6 % of the time, with none of the participants falling below 95 % accuracy. Paraphrase Decision Task RC Attachment Preference The analysis was done as follows. When the given paraphrase for an experimental item was the HA version, RC attachment preference was coded as HA if the answer was ‘yes’ and LA when the answer was ‘no.’ When the LA version of the paraphrase was given, RC attachment preference was coded as LA when the answer was ‘yes’ and HA when the answer was ‘no.’ For statistical analysis, HA answers were coded as 1 and LA answers were coded as 0. The mean attachment preferences for each participant in each condition were calculated. The mean RC attachment preference was 0.48. This was not significantly different from 0.5 ( p = .27, t = 1.11), indicating that the mean attachment preference of the participants was neither HA or LA, but at chance level. Arcsine square root transformations were performed on the coded data to eliminate spurious effects that might arise from performing ANOVAs on categorical data (Hogg and Craig 2005). A repeated measures ANOVA was performed with the two factors of RC position and referential load. The ANOVA revealed no main effects or interaction (Fs < 1.93, ps > .17). A correlation analysis performed between the participants’ mean RC attachment and reading span scores showed a significant negative correlation (r = −.27, p < .05); the larger a participant’s WMC, the more she or he preferred LA. Paraphrase Task Reaction Times For the analysis of the paraphrase reaction time (RT) data, RTs less than 1,000 ms and greater than 10,000 ms were excluded, resulting in an exclusion of 7.25 % of the data. Repeated measures ANOVAs were performed, one with participants as a random effect (F1) and one with items as a random effect (F2), over the remaining RTs. The analyses revealed a significant effect of referential load (F1(3, 59) = 25.14, MSE = 356,623.27, p < .001; F2 (3, 59) = 15.47, MSE = 293,026.40, p < .001), with longer reaction times for the heavy referential load condition. There was no significant effect for RC position nor was there an interaction between the two factors (Fs < 1.89, ps > .18). The RT data was also analyzed separately for the low WM and high WM groups. The participants were divided into low WM and high WM groups based on their reading span scores. The mean reading span score for the low WM group was 23.93 (SD = 3.30), and the mean reading span score the high WM group was 33.43 (SD = 3.07). The mean RT for the high WM group was 4,869.27 ms (SD = 916.49), and the mean RT for the low WM group was 4,607.84 ms (1,048.27). The difference between these two means was not statistically significant (t = 1.03, p = .31). Reading Times The reading times for each region in the target sentence for the low WM and high WM groups are presented in Fig. 1. The low WM group showed numerically longer reading times than the high WM group for every word in the target sentence. T-tests showed that the difference between reading times in the two groups was significant only for the second NP region ‘N2’ (t = 2.31, p < .05). The differences between the two groups in the first four regions excluding N2 (N1, of, who, RCV) were only marginally significant (ts > 1.87, ps < .10).
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reading times (ms)
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Fig. 1 Reading times by WM group (Experiment 1)
Discussion: Experiment 1 In this experiment, we investigated WMC and sentence complexity effects on ambiguous RC processing by native English speakers. The results showed significant effects of WMC in both the online and offline data. In the offline data, i.e., RC attachment preference, a significant negative correlation was found between WMC (measured as reading span score) and RC paraphrase decision, indicating that individuals with higher WMC preferred to attach the RC to the lower noun (NP2) more often. As the paraphrase decision was made after the target sentence had been read, and thus occurs in the post-interpretive stage of processing, these results are consistent with previous offline studies investigating the effects of WMC on RC attachment (Mendelsohn and Pearlmutter 1999; Swets et al. 2007). In the online reading time data, the low WMC group had significantly longer reading times at the second, or lower NP (NP2), than the high WM group. Previous studies investigating WMC effects on RC disambiguation in the post-interpretive stage (Mendelsohn and Pearlmutter 1999; Swets et al. 2007) postulated a reason for the negative correlation between WMC and RC attachment. The ‘chunking’ hypothesis, proposed by Swets et al. 2007, attributes the negative correlation between WMC and RC attachment to a pause in processing between NP2 and the RC for low span readers. Swets et al. (2007) suggest that low span readers do not have sufficient resources to perform a non-stop incremental parsing of the sentence, which causes them to pause at this point. This pause in processing at the boundary of the complex NP and the modifying RC serves to chunk NP1 and NP2 into a single unit, leading to HA. Swets et al. simulated the effects of this pause by changing the segmenting of the presented sentences so that NP1 and NP2 were presented as a single segment, forcing both high WM and low WM readers to ‘chunk’ the NP. The results of this change in segmenting resulted in the high WM readers changing from a previous LA preference to a HA preference, supporting the ‘chunking’ hypothesis. The online reading time results of our first experiment indicate that the low WMC readers do in fact pause at the boundary of the complex NP and the modifying RC during online processing. These results provide evidence in support of the chunking hypothesis (Swets et al. 2007). Also, the results of Experiment 1, showing significant effects of WM on online and offline measures of RC processing, demonstrate the advantages of the paraphrase
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decision task paradigm, allowing a more detailed investigation into both the interpretive and post-interpretive processing stages. The second goal of Experiment 1 was to investigate sentence complexity effects on RC attachment. Contrary to the predictions that the difference in processing difficulty due to the distance between subject-verb integrations (Gibson 1998) in the Subject-modifying and Object-modifying RC conditions would lead to a significant difference in RC attachment preferences, there was no significant effect of RC position or referential load in the paraphrase decision data. The lack of a significant effect of RC position may be due the fact that the experimental items were quite syntactically complex to begin with, due to the presence of a transitive verb in the embedded RC. The overall complexity of the test items may have washed out any observable effects of sentence complexity due to RC position. Regarding the factor of referential load, the analysis of reaction times (RT) in the paraphrase decision task showed a significant effect of referential load, suggesting that it took longer for participants to decide whether a paraphrase was correct or not when the embedded object was a full NP compared to when it was a 1st or 2nd person pronoun. Therefore, it is possible that while the additional load of processing a full NP as opposed to a 1st/2nd person pronoun did show up as longer RTs for the paraphrase decision task, the difference was too small to be reflected in the actual yes/no answers to the paraphrase decision task. Experiment 1 investigated the effects of WMC and sentence complexity on the processing of ambiguous RCs by native English speakers. WM effects were found to interact with RC processing in both the online and offline data, but the factor of sentence complexity failed to show significant effects. This experiment was also designed to test the predictions of WM-based accounts of ambiguous RC processing as opposed to language-dependent accounts. The mean attachment preference of the participants was not different from chance level, indicating that there was no significant preference of HA or LA. These results differ from previous studies showing a weak but significant LA preference in English RC attachment (Carreiras and Clifton 1999; Cuetos and Mitchell 1988). The lack of a significant attachment preference in a purported LA language (English), in addition to the significant WM effects on RC attachment obtained in both online and offline measures provide partial support for WM-based accounts on RC processing. However, in order to compare the predictions of WM-based accounts with language-dependent accounts, an examination of a HA language is also required. In Experiment 2, we investigate WM and sentence complexity effects in Korean, a head-final language, in order to provide additional evidence supporting WM-based accounts of differences in RC attachment.
Experiment 2 The goal of this experiment was to examine whether individual WMC interacted with RC attachment preferences within a head-final language. Sentence complexity was investigated as a factor that might modulate this interaction. The effects of head-directionality on RC processing allowed for a comparison of the results of Experiment 1 (English) with the results of Experiment 2 (Korean). The DLT (Gibson 1998, 2000) makes different predictions about the processing difficulty of Subject vs. Object RCs in Korean, a head-final language, as compared with a head-initial language such as English. In English, the Subject RC was predicted to be higher in processing difficulty due to greater distance between integrating heads. However, in a head-final language such as Korean, the DLT predicts that the Object RC will be more difficult to process,
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as the distance between subject-verb integration is greater when the RC modifies the object than when the RC modifies the subject. The difference between the two structures is shown in (5). (5) a. Subject RC Cayphan-cwung-ey cungin-ul moyok-han pyunhosa-uy kokayk-un ttoktttok-hayss-ta. Trial-during-LOC witness-ACC insult-MOD lawyer-POSS client-TOP intelligent-PSTDECL ‘The lawyer of the client who insulted the witness during the trial was intelligent.’ b. Object RC Phansa-nun caypahn-cwung-ey cungin-ul moyok-han pyunhosa-uy kokayk-ul thail-ess-ta. Judge-TOP Trial-during-LOC witness-ACC insult-MOD lawyer-POSS client-ACC rebukePST-DCL ‘The judge rebuked the lawyer of the client who insulted the witness during the trial.’ The main subjects in the sentences in (5) are in bold, and the main verb is underlined. There are no intervening referents between the subject and verb in the Subject RC (5a), whereas there are three nominal referents between the subject-verb integration in the Object RC (5b): witness, lawyer, and client. Therefore the DLT predicts that in Korean, the Object RC is heavier in processing load than the Subject RC. In addition to the predicted differences in sentence complexity between the Subject-RC and Object-RC by the DLT (Gibson 1998, 2000), presentation method is also a factor that was predicted to modulate RC attachment preferences in Experiment 2. Because the self-paced reading presentation of the experimental sentence was word-by-word, the head-final property of Korean results in the presentation of the RC before the complex NP. Therefore, each participant will read the experimental sentence in RC-NP2-NP1 order. This contrasts with the NP1-NP2-RC presentation order of English, in which participants were presented with the entire complex NP before the modifying RC. This word-by-word presentation method, interacting with the head-final property of Korean may also modulate RC attachment preferences. The predictions for Experiment 2 were as follows. Korean is a language previously reported to have a strong HA preference (Jun 2003; Lee and Kweon 2004). If language-based accounts are correct, the Korean participants are predicted to show an overall HA preference (Jun 2003; Lee and Kweon 2004) in both the online and offline data. If, however, WM-based accounts are correct, and RC attachments are driven by an interaction between WMC and general language structure, Korean native speakers are predicted to show fluctuations in their RC attachment preferences as effects of individual WMC (Swets et al. 2007; Traxler 2007) or sentence complexity effects (Gibson 1998, 2000), or both. The word-by-word presentation method of the online part of the paraphrase decision task is also expected to modulate the interaction between WMC and RC attachment. As the participant is presented with the modifying RC and the lower NP (NP2) before seeing the higher NP (NP1), it is possible that the results may show RC attachment preferences that differ from previous studies showing a strong HA preference (Jun 2003; Lee and Kweon 2004).
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Method Participants Thirty-two native speakers of Korean participated in this study. The participants were new students at the University of Illinois at Urbana-Champaign who had just arrived from Korea, and tended to be students in academic programs that did offer extra credit for experiment participation.2 This pool of participants was less accessible than the native English participants in Experiment 1. Thus, most of them received a monetary incentive of $5 for their participation in the experiment. As most experiments in university labs draw from pools of both paid and course-credit participants, this variation in samples between Experiment 1 and Experiment 2 was not considered to be a significant confound. Participants who had spent more than one year in an English-speaking country and those that indicated they were fluent in a language other than English were excluded from the experiment to avoid language-transfer effects (Cuetos et al. 1996). Materials and Design Forty sets of sentences were constructed. The experimental sentences were direct translations of the English sentences in Experiment 1, to exclude semantic/pragmatic factors that could affect the RC attachment preferences between the two experiments. As in Experiment 1, each set contained four versions, crossing RC position (subject-modifying position, objectmodifying position) and referential load (full NP, 1st/2nd person pronoun). A sample item set is presented in (6). (6) a. Subject RC, High referential load Cayphan-cwung-ey cungin-ul moyok-han pyunhosa-uy kokayk-un ttoktttok-hayss-ta. Trial-during-LOC witness-ACC insult-MOD lawyer-POSS client-TOP intelligent-PSTDECL ‘The lawyer of the client who insulted the witness during the trial was intelligent.’ b. Subject RC, Low referential load Cayphan-cwung-ey na-lul moyok-han pyunhosa-uy kokayk-un ttoktttok-hayss-ta. Trial-during-LOC me-ACC insult-MOD lawyer-POSS client-TOP intelligent-PST-DECL ‘The lawyer of the client who insulted me during the trial was intelligent.’ c. Object RC, High referential load Phansa-nun caypahn-cwung-ey cungin-ul moyok-han pyunhosa-uy kokayk-ul thail-essta. Judge-TOP Trial-during-LOC witness-ACC insult-MOD lawyer-POSS client-ACC rebukePST-DCL ‘The judge rebuked the lawyer of the client who insulted the witness during the trial.’ d. Object RC, Low referential load Phansa-nun caypahn-cwung-ey na-lul moyok-han pyunhosa-uy kokayk-ul thail-ess-ta. 2 The participants had been in the United States for less than two months at the time of the study.
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Judge-TOP Trial-during-LOC witness-ACC insult-MOD lawyer-POSS client-ACC rebuke-PST-DCL ‘The judge rebuked the lawyer of the client who insulted me during the trial.’ As in Experiment 1, each item in this experiment consisted of two parts: a target sentence containing the ambiguous RC construction, and a paraphrase of the target sentence which was disambiguated toward either HA or LA. The paraphrases were counter-balanced across lists so that each participant saw only one version of the paraphrase for each experimental item. In addition to the forty experimental sentences, eighty-eight filler items with various syntactic structures were also included. The four versions of each experimental sentence were counterbalanced across four lists in a Latin square design, so that each participant saw only one of the four versions of each item. The presentation of the stimuli was pseudo-randomized so that at least one filler was presented between two target sentences. Procedure Reading Span Task The reading span task was a Korean version of the task that was used in Experiment 1 (Conway et al. 2005). The two versions of the reading span task were matched in the total number of items as well as the number of items per set and presentation order, in order to reduce variability stemming from external factors other than WMC itself. Paraphrase Decision Task The paraphrase decision task was also the same task that was used in Experiment 1, but used the Korean items described above. The entire experimental session lasted about 40 minutes. Results Reading Span Task The grading criteria for the reading span task was identical to Experiment 1, with a possible score range from 0 to 42. The mean reading span score was 30.34 (SD = 5.70), with a range of 17 to 42. The semantic plausibility questions were answered correctly 96.4 % of the time, with none of the participants falling below 95 % accuracy. In contrast to the native English group in Experiment 1, the correlation analysis performed between the native Korean participants’ overall mean RC attachment and reading span scores did not show a significant correlation. Paraphrase Decision Task RC Attachment Preference The mean RC attachment preference for the Korean participants was 0.41 (SD = 0.13). T tests showed that this was significantly lower than chance (t (1, 31) = 3.91, p = .005), indicating that the native Korean speakers had a clear LA preference. This mean RC preference was also significantly different from the mean attachment preference for the native English speakers in Experiment 1, which was only at chance level (=0.48). Similar to the native English group, the Korean speakers also showed a wide range of individual variation, with mean attachment preferences ranging from 0.15 (almost exclusively LA) to 0.80 (almost exclusively HA). After arcsine square root transformations were performed on the coded paraphrase verification data (Hogg and Craig 2005), a repeated measures ANOVA was performed with
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the two factors of RC position and referential load. The ANOVA revealed a main effect for RC position that was significant in the within-subjects analysis and marginally significant in the within-item analysis (F1(3, 31) = 8.98, MSE = 0.03, p = .005; F2(3, 39) = 3.42, MSE = 0.09, p = .07), with a higher percentage of HA in the Object-modifying RC condition. The factor of referential load was not significant in either the within-subjects or within-items analysis. Paraphrase Task Reaction Times In the analysis of the paraphrase reaction time (RT) data, RTs less than 1,000 ms and greater than 10,000 ms were excluded, as in Experiment 1, resulting in an exclusion of 6.44 % of the data. The mean RT for the remaining data was 4,434.26 ms (SD = 1,977.26). A repeatedmeasures ANOVA over the remaining RTs revealed a main effect of RC position, which was significant in both the subject and item analyses (F1(3, 31) = 52.58, MSE = 211,117.03, p < .001; F2(3, 39) = 56.95, MSE = 440,934.04, p < .001), with longer RTs for the Object RC condition. The factor of referential load was also significant (F1(3, 31) = 5.86, MSE = 34,009.56, p < .05; F2(3, 39) = 5.78, MSE = 611,402.64, p < .05), with longer reaction times for the heavy referential load condition. There was no significant interaction between the two factors. Reading Times As in Experiment 1, the participants were divided into low WM and high WM groups based on their reading span scores. Reading times for both groups are presented in Fig. 2. Numerically, the high WM group showed longer reading times than the low WM group for all regions except RCO. However, when submitted to t-tests, the difference between reading times in the two groups was not significant for any of the regions (all ts > 0.26). Discussion This experiment investigated how ambiguous RC processing in Korean was affected by WM and sentence complexity. No significant effects of WMC were found in the online or offline data for the Korean participants. There was no correlation between WMC and RC attachment data. Although the low span group had a slightly higher percentage of HA (0.42) than the 900
reading times (ms)
800
low high
700 600 500 400 300 200 RCO (witness/me)
RCV (insult)
NP2 (client)
Fig. 2 Reading times by WMC group (Experiment 2)
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high span group (0.39), this difference was not significant. This may have been due to the difference in head-directionality between English and Korean, which was discussed earlier. In a head-initial language, such as English, both NPs that may be modified by the ambiguous RC are presented before the RC. However, in Korean, the modifying RC precedes the two NPs. Because the sentences were presented in a word-by-word SPR task, it is likely that this order of presentation, i.e., RC-NP2-NP1, pushed the participants toward a LA interpretation, which reduced the difference between low span and high span readers. Immediately attaching the RC to the first possible NP, i.e., NP2, would seem a good processing strategy in a word-by-word segmented task like this, as it would lower WM costs by effectively emptying the WM of the RC and its unfilled attachment site. In Experiment 1, the low span group had consistently longer reading times for all regions numerically, and the low span group’s reading times were significantly longer than the high span group at NP2, supporting the chunking hypothesis (Swets et al. 2007). However, the results of Experiment 2 were in the opposite direction, with the high span group displaying longer reading times for all target regions except the embedded object of the RC. In order to analyze these seemingly paradoxical results further, we conducted a correlation analysis of individual WMC and the paraphrase decision RTs. This analysis showed a significant positive correlation (r = .34, p < .05), suggesting that not only did readers with higher WMC take longer to read the ambiguous RC, they also took longer to make a disambiguation decision. These results suggest that it is possible that the high WMC readers were aware of the ambiguity in the RC construction, while the low WMC readers, who have a smaller pool of WM resources to begin with, were not. As the low WMC readers do not have sufficient resources to maintain both interpretations, they appear to stick to just one preferred interpretation, which shows up in shorter word-by-word reading times as well as shorter RTs for the paraphrase decision task. Similar results were found in a study by MacDonald et al. (1992), who found that high span readers showed longer reading times than low span readers for locally ambiguous sentences resolved in favor of the preferred interpretation. They suggested that the high span readers maintain both interpretations in mind until the ambiguity is resolved, whereas the low-span readers lack the resources to do so. The results for Experiment 2 are consistent with MacDonald et al.’s (1992) results and suggest that the same may be true for globally ambiguous sentences. As discussed above, it seems that the difference in WM effects for English and Korean is due to the difference in head-directionality, which causes the modifying RC to be presented before the complex NP, pushing the participants toward an overall LA interpretation, and at the same time making the overall ambiguity of the structure less conspicuous. The factor of sentence complexity did show effects: significant effects of RC position were obtained in both paraphrase decision answers and paraphrase decision RTs. RCs modifying the object resulted in a higher percentage of HA than RCs modifying the subject. In Korean, the object-modifying RC condition was predicted to be higher in terms of processing difficulty by the DLT (Gibson 1998, 2000), and therefore, these results suggest that higher processing difficulty pushed the participants toward a HA interpretation. In addition to RC position effects in the paraphrase decision answers, a main effect of RC position was also obtained for the paraphrase decision RTs, with longer RTs for the Object RC condition. These results suggest that the participants found it more difficult to arrive at a disambiguation for the object-modifying RCs, supporting the predictions of the DLT that in Korean, the Object RC will require more processing resources than the Subject RC. The factor of referential load did not cause a significant difference in RC attachment preferences. However, the referential load was significant in the analysis of paraphrase decision RTs, with longer RTs for the heavy referential load condition. These results suggest that, as
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in Experiment 1, although the difference in processing difficulty caused by referential load was reflected in the time required to make a decision, this difference may be too slight to be reflected in the RC attachment interpretation itself. To summarize, the results of Experiment 2 showed that WMC and sentence complexity are factors that may affect RC processing. Although WMC did not significantly affect RC attachment preferences (as in Experiment 1), which was attributed to the head-final property of Korean and the word-by-word presentation method, WMC did affect disambiguation times. Regarding the factor of sentence complexity, RC structures predicted to be higher in sentence complexity showed a higher percentage of HA in general. These results are consistent with Swets et al. (2007). Manipulating the presentation method so that even the high span readers were forced to ‘chunk’ the complex NP (NP1 of NP2) led to ‘low-span-like’ behavior for even the high span readers. In our experiment, we suggest that higher sentence complexity is doing the same thing: using more WM resources pushes the high-span readers toward low-span processing strategies, i.e., ‘chunking,’ and therefore, more HA. Finally, the overall RC attachment preference of the Korean participants in this experiment showed a significant LA preference, contrary to previous studies, which report Korean as a HA language (Jun 2003; Lee and Kweon 2004). These results provide additional support for our claim that although language-dependent accounts are not necessarily incorrect, they may be too simplistic. The general structure of a language, e.g., head-directionality, may interact with factors such as WMC and sentence complexity to result in different patterns of RC attachment across different languages and individuals.
General Discussion Although the ambiguous RC construction has been the target of many experimental studies in the field of psycholinguistics, the focus of these studies has changed over time. The seminal study by Cuetos and Mitchell (1988), which discovered that English and Spanish had different RC attachment preferences, started the first stage of investigation on the ambiguous RC construction, focusing on cross-linguistic differences in attachment preferences. Research on RC disambiguation strategies then moved on to WM effects on RC processing within a language (Felser et al. 2003; Mendelsohn and Pearlmutter 1999; Swets et al. 2007; Traxler 2007). These studies yielded mixed results, which we proposed to be due to differences between interpretive vs. post-interpretive processing confounded with online vs. offline tasks. In this study, we developed a new paradigm, the paraphrase decision task, in order to enable the examination of both interpretive and post-interpretive stages of RC processing. Using this paradigm, we tested the predictions of WM-based accounts and language-based accounts of ambiguous RC attachment. Experiment 1 examined how sentence complexity (as predicted by the DLT(Gibson 1998, 2000)) and individual WMC interact with RC attachment preferences for native English speakers. Significant effects of WMC were found in both online and offline measures, providing evidence in support of WM-based accounts of RC attachment. In the offline data a significant negative correlation between the participants’ reading span scores (WMC) and their mean RC attachments, supporting previous offline studies (Mendelsohn and Pearlmutter 1999; Swets et al. 2007). The paraphrase decision task enabled us to observe the online processing of the target RC structures, in which we found a significant difference in reading times for NP2 of the ambiguous RC construction, with low WMC readers showing longer
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reading times. These results not only help explain the negative correlation between WMC and RC attachment, they also provide evidence in support of the chunking hypothesis, which was suggested by Swets et al. (2007). The longer reading times at NP2 by the low WMC readers show that they are pausing between the lower NP and the modifying RC, which serves to ‘chunk’ NP1 and NP2 into a single unit, thus leading to more HA interpretations than the high WMC readers. The results of Experiment 1 also provide an explanation for the mixed results regarding WMC effects on RC attachment preferences from previous studies. A negative relation between these two factors was obtained for offline studies, which used globally ambiguous RCs (Mendelsohn and Pearlmutter 1999; Swets et al. 2007), but a positive relation was found for online studies, which used the RC construction which was disambiguated toward either HA or LA (Felser et al. 2003; Traxler 2007). We propose that the reason for the confound in results may be due to the fact that the previous online studies were not able to use globally ambiguous RCs, but used a HA or LA disambiguated version of the RC structure, and examined the subjects’ changes in behavior through eye movements or reading times to probe RC attachment preference. In Experiment 2, we investigated the effects of WMC and sentence complexity in Korean. The absence of WMC effects in online and offline measures was attributed to the difference in head-directionality between English and Korean, which resulted in a presentation order which swayed the Korean participants toward an overall LA, which was significantly different from chance level (=.41). Sentence complexity effects on RC attachment were obtained over all, however. Even though the head-final property of Korean caused the participants to be biased toward a LA interpretation in general, overriding individual WMC effects, sentences with heavier processing load, i.e., the object-modifying RCs, requiring more WM resources, resulted in a higher proportion of HA compared to the subject-modifying RCs. In both experiments, the factor of referential load seemed to be more of a post-interpretive effect that affected paraphrase decision times only, and not the paraphrase decision per se. Longer decision times for the heavy referential load condition in both English and Korean suggested that participants found it more difficult to decide on a disambiguation interpretation when processing nominals that required more WM resources, but this did not affect the final interpretation itself. The mean attachment preferences across participants in Experiments 1 and 2 contrast with previous reports. Although English is purportedly a LA language (Carreiras and Clifton 1999; Cuetos and Mitchell 1988), the mean RC attachment preference of the native English speakers in Experiment 1 did not reach beyond chance in either direction. In Experiment 2, we obtained a significant LA preference in Korean RC attachment, which contrasts with previous studies reporting Korean to be a HA language (Jun 2003; Lee and Kweon 2004). These results, in conjunction with the WM and sentence complexity effects we obtained, support our claim that although cross-linguistic differences may exist in RC attachment, individual WMC and sentence complexity are factors that may interact with the language-specific properties that have been proposed to be underlying cross-linguistic differences. Finally, the data obtained from the paraphrase decision task in this study provided a significant contribution to the field of research on RC processing by yielding online and offline data were able to be obtained from the same participants and material, which has not been possible in the methodology employed for previous studies examining RC attachment. Through the paraphrase decision paradigm, we were able to show online and offline evidence in support of the ‘chunking’ hypothesis proposed by Swets et al. (2007). In conclusion, this paper has provided evidence in support of WM-based accounts of ambiguous RC processing. We propose that although general cross-linguistic differences in
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RC attachment may exist, WM and sentence complexity are factors that may override these differences within a given language.
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