Does number interference occur during sentence processing?

Does number interference occur during sentence processing? Katja Suckow (k.suckow@dundee.ac.uk) School of Psychology, The University of Dundee Dundee, DD 1 4 HN , UK Roger P.G. van Gompel (r.p.g.vangompel@dundee.ac.uk) School of Psychology, The University of Dundee Dundee, DD 1 4 HN , UK Abstract that interference is not just due to similarity in the type of noun phrase but also due to similarity of other features. Van Dyke and Lewis (2003) described how the degree of over- lap between a retrieval cue and two similar linguistic items affects the retrieval of these items. Two items (the subject and object noun phrase) have to be retained in memory in ob- ject relatives until they can be integrated with the verb. At the point of integration at the verb, retrieval cues are used to identify the target item from memory. When items in mem- ory share retrieval cues it is difficult to identify this target. Thus, a similarity based interference effect arises for items with retrieval cue similarity. The account of Lewis and Va- sishth (2005) predicts that a similarity based interference ef- fect arises at the embedded verb in object relatives because two items have to be retrieved from memory instead of only one for subject relatives. This interference effect at the verb should be larger when these two items share retrieval cues. Models of interference in sentence processing claim that ob- ject relative clauses are harder to process than subject relatives due to interference between the subject and object noun phrase. The interference effect for object relatives at the verb should be more pronounced when the two noun phrases retrieved from memory are similar. To test this, two eye tracking experiments manipulated whether the number feature of the noun phrases (singular or plural) was either the same or different. Both ex- periments showed the well-known relative clause effect. How- ever, in Experiment 1 the effect of number congruency was in the opposite direction from that predicted by interference. Experiment 2 showed the interaction predicted by similarity based interference at sentence wrap-up, but because this inter- action was observed later than the relative clause effect and only occurred in Experiment 2, it suggests that retrieval inter- ference due to cue overlap is a weak effect that might be the result of a checking procedure in syntactically complex sen- tences. Keywords: sentence processing; similarity based interference; working memory. Introduction There is much evidence that object relative clauses such as (1) are more difficult to understand than subject relative clauses such as (2) (e.g., King & Just, 1991). It is generally believed that this difficulty stems from limitations in working memory (Gibson, 1998; Gordon, Hendrick, & Johnson, 2001; Lewis, 1996; King & Just, 1991). 1. The banker that the accountant helps counted the money. 2. The banker that helps the accountant counted the money. For example, King and Just (1991) found that readers with low working memory experienced more difficulty with ob- ject than subject relatives. More recently, Lewis (1996) and Van Dyke and Lewis (2003) have proposed an account that assumes that working memory demands increase when two similar linguistic items (e.g., two noun phrases) need to be retrieved simultaneously, slowing down sentence processing. Lewis and Vasishth (2005) proposed an ACT-R account that explains how the processing of subject and object relatives is affected by the similarity of the noun phrases. Consistent with this, Gordon, Hendrick, and Johnson (2001) found that object relatives are easier to process when the embedded noun phrase is a name (Joe) and the head noun a definite noun phrase (the barber) compared to when both are definite noun phrases. However, Van Dyke and Lewis’s (2003) and Lewis and Va- sishth’s (2005) similarity based interference account claims For example, in object relatives like (1) the two subject and object noun phrases (the accountant and the banker) need to be simultaneously retrieved at the embedded verb (helps). On the other hand, only one noun phrase (the banker) needs to be accessed at the verb (helps) in subject relatives like (2). The similarity based interference effect that occurs with ob- ject relatives should be particularly strong when the two noun phrases that need to be retrieved are similar. Thus, because both the difficulty of object relatives and the difficulty of re- trieving items that share retrieval cues are effects of interfer- ence at the embedded verb, they should occur at the same time. Van Dyke and Lewis (2003) claim that number information is one of the retrieval cues for identifying a target. In relative clauses, congruency in the number feature of the subject and object noun phrase may have a strong effect on interference, because the subject has to agree in number with the verb: The retrieval cues of the target item (the subject noun phrase) have to match the number cue of the search probe (the verb). If the object has the same number as the subject and therefore also matches the number feature of the verb, this should result in interference when the subject and object are integrated with the verb. Thus, according to the retrieval cue based inter- ference account, retrieval difficulty at the verb (e.g., helps) in object relatives should be more pronounced when the two noun phrases in memory share the same number than when they do not.