The results of this experiment are more consistent with linguistic accounts of SLI and less compatible with domain-general accounts. In the following, the predictions of each theoretical accounts of SLI are compared to the results obtained in this experiment.
Domain-general accounts of SLI, such as the one formulated in Deevy and Leonard (2004) and Montgomery (1995a; 2002a) propose that limitations in working memory and processing capacity and speed can explain most of the linguistic deficits seen in this population, including the well-documented difficulties in structures that involve syntactic movement. Theories like the one formulated in Deevy and Leonard (2004) predict the following order: SVO>OVCLS>OSVCL based on the assumption that both OVCLS and the OSVCL sentences pose similar amount of processing load, though the latter could be more challenging due to the increase in distance between the object the clitic coindexed with it . However, this prediction is not borne out by the results, as these two sentences with fronted NP‟s were treated differently by the SLI group and the TD groups. There are significant differences between these two word orders as the sentences with the OVCLS word order are significantly more difficult than OSVCL sentences, especially for children with SLI. Children with SLI have an overall mean of 62.8% on OSVCL sentences that involve single movement, indicating they have good performance when they are compared to language controls, though they are significantly poorer than age controls. However, children with SLI scored 38.0% on OVCLS sentences, which shows that they have particular
difficulties with these sentences that involve two movements. These differences in the performance of children with SLI on these two types of word orders cannot be explained by processing accounts of SLI, as both are short and consist of two NP‟s , a verb and a clitic only, hence they are less demanding in terms of working memory. Moreover, both of them are noncanonical and less frequent than canonical SVO word order in Gulf Arabic. Nevertheless the differences in the test results are consistent and substantial.
Domain-specific accounts of SLI predict that both OSVCL and OVCLS sentences would be more challenging than the canonical word order of SVO, especially for children with SLI. They predict, moreover, that OSVCL would be less challenging than the OVCLS because there is less movement in OSVCL.
According to grammatical accounts of SLI (e.g. van der Lely, 2005, Friedmann and Novogrodsky, 2005), in OSVCL sentences, the object is moved to the initial position to put more emphasis on it (topicalisation) and a chain is formed between the object and its trace. The verb in these sentences assigns the thematic role of the theme to the trace of the moved NP, this is followed by another process where the thematic role is transferred via a chain to the moved NP. Therefore, to arrive at the correct interpretation of sentences with moved NP‟s, the thematic role of the trace must be linked with the moved constituent. In OVCLS sentences, there is an additional movement of the verb to a position higher than the subject and lower than the moved object. This additional movement is expected to increase the level of complexity in the sentence, especially for children with SLI whose grammatical system might be vulnerable to such structural complexity (van der Lely, 2005).
It is noteworthy, that there might be some extra processing demands involved in these complex sentences as they might require some working memory resources, but the argument taken by linguistic accounts of SLI is that grammatical complexity better explains the difficulties seen in this population. In any structure involving hierarchical dependencies, some representations should be held in memory while other information in the sentence is being processed. However, domain-specific accounts and domain-general accounts differ as to where the primary deficits lie. While the former attributes these difficulties to deficits in the grammatical system, the latter argues that general processing mechanisms, such as limited working
memory or slowed processing cause these deficits in children with SLI. The results of this experiment show that children with SLI responded in a qualitatively different way when presented with sentences with moved elements. While typically developing children performed as well on canonical SVO as they did on fronted OSVCL sentences (that have one movement), children with SLI did worse on OSVCL than they did on SVO and their performance was significantly lower than their age control group. When the level of complexity increases in OVCLS, where there are two elements being fronted, children with SLI show a dramatic decrease in their performance, which was significantly worse than their age and language controls. The findings of this experiment demonstrate that grammatical complexity is a crucial factor in the distinction between children with SLI and their typically developing peers, as children with SLI seem to perform worse as the sentences increase in complexity, a prediction put forward by domain-specific accounts of SLI, such as that of van der Lely (2005).
When children with SLI are faced with this movement complexity and due to limitations in their grammatical system, they resort to the use of the Subject-first (NP-first) strategy, which is one of the common sentence comprehension strategies available for all children. Children with SLI are known to assign the subject role to the first NP they encounter in the sentence even in passive sentences (Bishop, 1992; van der Lely & Harris, 1990). In Gulf Arabic, this strategy can be employed with SVO sentences leading to correct interpretation. When used with OSV and OVS sentences, this strategy can lead to erroneous interpretation by children with SLI. Typically developing children, on the other hand, will also have access to movement strategy, which helps them reach the correct thematic role assignment. The poor performance of children with SLI on OVCLS (NVN) sentences supports the notion that children with SLI over rely on the use of the Subject-first strategy due to deficits in their grammatical system. On OSV (NNV) sentences, these children may rely also on Subject-first strategy as their default strategy in the absence of animacy and world knowledge cues. Their better performance on OSV could be due to use of agreement cues, which is triggered by the presence of NNV configuration in these sentences.
An alternative non-structural (processing) hypothesis that assumes there is no syntactic movement in TD children and those with SLI might argue that the
performance of children with SLI is better explained by a non-movement strategy that assumes children with SLI have more difficulties with fronted sentences because these children have access to word order cues only (Subject-first strategy) in these sentences. According to this, SVO sentences are easier because parsing these sentences using Subject first strategy will yield the right interpretation. While both fronted sentences start with the object, therefore they are more difficult to interpret, especially as only word order and agreement cues are available. This hypothesis assumes that these children rely on this „default‟ strategy because they cannot use other cues, such as agreement cues. However, the results of the experiment show that children with SLI used agreement cues as their performance was not distinguished from typically developing children on agreement cues, as indicated by lack of group*agreement interaction. Moreover, the fact that children with SLI scored significantly better on OSV sentences than they did on OVS sentences shows that they must have used agreement cues in these sentences. Overall, results show that children with SLI, like their TD peers, benefited from agreement cues. Children with SLI use word order cues not because these are the only cues available to them, rather because they had limitations in their grammatical system caused by movement complexity, which lead to them relying on this Subject-first strategy.
In summary, the findings of this study demonstrate that the comprehension deficits seen in children with SLI are caused by movement complexity. They are not caused by limitations in working memory or the use of non-syntactic strategies.
4.4.3 Implications for typical and atypical sentence comprehension