A Review Paper on the Syntactic Abilities of Individuals with Down Syndrome

This systematic review aims to synthesize current findings on the syntactic abilities of individuals with Down Syndrome from childhood into adolescence and adulthood and discuss them in terms of the delayed or deviant pattern of development as well as in terms of the critical period for syntactic development. This literature search was conducted using research articles written only in the English language, but concerning syntax in any language, after a thorough search in the web databases, following the inclusion criteria set for this review. Studies which examine any syntactic domain of language via particular and targeted materials were included. The findings show that individuals with Down Syndrome lag behind typically developing and present a delayed pattern of syntactic development. The Down Syndrome population presents difficulties with both comprehension and production of syntax, a fact which is observed in various syntactic structures and becomes apparent in more complex ones, such as subordinate clauses, passivisation and pronouns.


Introduction
Down syndrome (DS) is the most common genetic cause of intellectual Intellectual disability (ID), occurring in approximately 1 in 700 to 1 in 800 live births of both sexes throughout the world (Rondal, 1988(Rondal, , 1998Rogers et al., 1996;Nadel, 1999). A diagnosis of DS is given when an error in cell development results in an extra copy of chromosome 21, so there are 47 chromosomes than the usual 46.
DS can also be the result of mosaicism, when only some cells include an extra copy of this chromosome whilst the remainder of cells are normal, or transloca-terest in this cognitive domain derives from the fact that language is one of the most prominent indicators of intellectual impairment, which affects development in many ways. Language is divided into the following five aspects: pragmatics, phonology, semantics, morphology and syntax. The syntax, commonly known as grammar, encompasses the rules that govern how words are put together to form sentences. This includes elements such as word order, parts of speech, sentence organization, and word relationships.
Language development in DS has caught the attention of early studies such as Lenneberg (1967) who argued that children with DS are able to develop language but at a reduced level compared to unimpaired children and described the developmental course of language in children with DS as "stretched but normal". Despite the fact that there is an ongoing debate regarding the characterization of syntactic abilities of people with DS as either delayed (Bridges & Smith, 1984;Rutter & Buckley, 1994;Fabbretti et al., 1997;Thordardottir et al., 2002;Eriks-Brophy et al., 2003Schaner-Wolles, 2004, among others) or deviant (Perovic, 2004(Perovic, , 2006a(Perovic, , 2006bRing & Clahsen, 2005a;Penke, 2018), most of these studies lead to the conclusion that syntactic development in individuals with DS is highly problematic. This finding was observed in studies on both English and on morphologically richer languages.
Various studies, particularly the developmental literature, using different theoretical and methodological approaches have shown that one of the main characteristics of individuals with DS is language impairments which are greater than would be expected from their level of cognitive development, with expressive delays more severe (Vicari et al., 2000;Chapman et al., 2002;Laws & Bishop, 2003;Yoder & Warren, 2004). This delay in expressive performance is evident from infancy and becomes even more pronounced when syntax becomes more complex and vocabulary more demanding. Besides, more than half of the individuals with DS did not achieve the vocabulary increase that is usually observed in TD individuals at 24 months of age. For example, data from parental reports suggest that linguistic expression using gestures lags behind normal development (Miller, 1992). Also, Oliver and Buckley (1994) used parental records assessing first words and two-word phrases and they found that individuals with DS used approximately the same type of vocabulary as TD infants but with a delay up to 18 months, as well as presented remarkable individual differences in the development of the first ten words learnt, which ranged from 19 to 38 months. Particularly, the area of morphosyntax seems to be more impaired than other domains within the language system, such as lexical abilities (Chapman et al., 1991;Chapman & Hesketh, 2000;Kernan & Sabsay, 1996;Fabbretti et al., 1997;Schaner-Wolles, 2004). In summary, DS people have difficulty with the comprehension and production of syntax, with production more severely impaired than comprehension.
The aim of this article is to review studies on the syntactic abilities of individuals with DS from childhood into adolescence and adulthood as compared with those of typical population. More specifically, these studies are discussed in relation to a) the delayed or deviant pattern of development b) the critical period for syntactic development in this impaired population and c) the syntactic phenomenon they examined.

Method
Studies included in this review were selected based on the following criteria: 1) All studies were published in journals and written in English.
3) Recent studies as well as older ones, without chronological limitation, included.
4) There was no geographic limitation or limitation in the spoken language of the participants.
5) The materials used in the studies concerned syntax alone or in combination with other linguistic or cognitive domains.

Search Procedures
The research was conducted electronically through databases mainly via Google Scholar and PubMed, which provide the opportunity to find and read all published articles online. The initial search included studies from the last decade, however the limited number of published studies during that period of time led the authors to extend the chronological limitation. In all databases, the combination of the terms Down syndrome, syntax, syntactic abilities, syntactic impairment, was used.
The search process yielded 44 studies that met the inclusion criteria and were further analyzed. The selected studies fell under the area of syntax, and were further analyzed in terms of syntax development plateaus and in terms of characterization as delayed or deviant compared to typical population. The related studies were also classified as to which syntactic phenomenon they examined and their results are discussed comparatively and presented below. The notion of a critical period for language development was initially proposed by Lenneberg, who in a study with his colleagues (Lenneberg et al., 1964) reported data supporting the hypothesis of a freeze in language acquisition in DS after around 14 years old. Sixty-one individuals with DS were followed over a three-year period. Authors observed that those who had reached adolescence failed to make further progress in language structures, in contrast to younger subjects for whom some progress was observed. However, judging from the fact that only 4 subjects were beyond 14 years when tested, which was a very limited sample, a generalization was not considered safe. In addition, Fowler (1990) maintained that no notable language improvement is possible beyond early adolescence. In another study, Fowler et al. (1994) reported no further modification in MLU over a 2 to 4 years following initial measurement in four adolescents with DS with mean chronological age (CA) 12.7 years at the beginning of the study. MLU remained in the range 3, that is 3.50 words plus grammatical morphemes. However, they claimed that the language ability of individuals with DS did not improve, and that the syntactic language skills plateau in adolescence. Rondal & Comblain (1996) conducted several cross-sectional studies and one longitudinal of 4 years in the language abilities of various age groups of persons with DS aged between 14 -50 years. They used the Batterie pour l'Evaluation de la Morpho-Syntaxe (BEMS) to examine various morphosyntactic structures. The authors concluded that morphosyntax and phonology continue to be areas of relative weakness for adults with DS, whereas semantics and pragmatics remain areas of relative strength. In addition, they claimed that receptive and expressive morphosyntax and lexical skills remain stable from late adolescence through mature adulthood, at least until 50 years of age. Similarly, Iacono et al. (2010) investigated the relationship amongst age, language and related skills in adults with DS. They used measures of receptive and expressive language from studies of younger individuals with DS in exploring the relationship between linguistic and associated skills and age in young to older adults. The results of the study indicated that the group of adults with DS was heterogeneous in their ability to complete tasks and their performance on these tasks also varied. It is worth mentioning the finding that increasing CA was associated with lower performance in all measures. However, when they removed scores on the Raven's Coloured Progressive Matrices, a measure of non-verbal cognition, and on the Adaptive Behaviour Dementia Questionnaire, which measures the change in daily functioning that may be associated with AD, they observed that decrease appeared only in the test of auditory short-term memory (Digit span), and in the measure of expressive language that included morpho-syntax (MLU-50). Interestingly, a relationship between receptive language and ageing was not found. In this line, Witecy & Penke (2017) examined whether receptive syntactic skills change from childhood/adolescence to adulthood. The results of their study indicated that the development of receptive syntactic skills comes to an end in the  Koizumi et al. (2019) examined the characteristics of syntactic development in native Japanese-speaking children with IDs compared to MA-matched TDC. They indicated that the development of syntax in children with ID, including DS, was significantly delayed than in TDC and a temporal plateau of 1 -3 years appears from a mental age (MA) of 5 -6 years, during which the development of syntax comprehension stops, while by reaching the MA of 7 -9 years, grammar comprehension abilities start developing remarkably. Probably, the comprehension of basic grammar forms in children with ID might be promoted/facilitated when the MA of these children reaches this level. However, in general, the type of disability might affect the development of syntax.

Plateau-Critical Period in Syntactic Development of Down Syndrome's Individuals
On the contrary, there are studies in the recent literature on the continued development of language in late adolescent and early adulthood. Thordardottir et al. (2002) claimed that syntactic development in DS does not stop at late adolescence and is not limited to simple syntax. They investigated the use of complex syntax in narrative language samples of children and adolescents with DS and a group of TDC matched on MLU and found that the examined groups did not differ significantly in either the proportion of utterances containing complex sentences or in the variety of complex sentence types used. However, the group of individuals with DS actually had a higher mean and higher upper range than the control children. More specifically, the results demonstrated that the individuals with DS, as a group, use complex sentences and their use of complex syntax is commensurate with their MLU. Interestingly, individuals with DS had a higher proportion of complex sentences for short utterances, however, this is reversed for the longer utterances. Only 42% -68% of utterances of 10 -13+ morphemes produced by the individuals with DS were complex, compared to 69% -89% for the control group. The authors argued that the use of complex syntactic constructions and the increase in MLU of the DS group were interrelated and, consequently, there was a syntactic development in the individuals with DS throughout adolescence. Andreou (2013) examined Greek-speaking individuals with DS and she concluded that their syntactic development does not reach a "ceiling" in adolescence but continues to grow especially in the expressive domain. Also, Kernan & Sabsay (1996) claimed that the lack of any significant correlation between chronological age and language measures in their study indicates that the linguistic ability of the adults with DS does not deteriorate with age before the age of thirty-five years in terms of expressive lexical, morphological, or syntactic ability. Similarly, in a more recent study, Facon & Magis (2019) examined receptive syntax and vocabulary in French children, adolescents and young adults with DS using a cross-sectional developmental approach. They compared the DS group to participants matched on CA and cognitive level with ID of undifferentiated etiology. The authors found that chronological age was significantly related to both vocabulary and syntax comprehension with a larger effect for vocabulary than for syntax. For both measures, participants with undifferentiated etiology performed better than those with DS. However, authors observed that syntax and vocabulary had a continuous progress between childhood and adulthood for both groups. However, Chapman's work on this issue yielded different outcomes. Chapman and his colleagues have examined language of adults and adolescents with DS in several studies and the results are presented below. In a study of 1991, they investigated the variation in receptive vocabulary and syntactic comprehension in a sample of children and adolescents with DS aged from 5 to 20 years. They observed differences within the group of DS increasing with age, between lexical and syntactic comprehension skills, while vocabulary comprehension was relatively more advanced than syntax. They also observed that the differences increased with age between nonverbal cognitive subtests of pattern analysis and short-term memory for bead arrangements. Chapman et al. (1991Chapman et al. ( , 2002 indicated that limitations in short-term memory contribute to limited growth in both receptive and expressive language beyond single words, as children move to adolescence and adulthood, indicating a plateau. Overall, adolescents with DS were described as having an advanced vocabulary comprehension, and also appeared to have mild deficits in syntax comprehension. Chapman et al. (1998) investigated cross-sectionally the hypotheses that children and adolescents with DS show a "critical period" for language acquisition and a "simple sentence syntactic ceiling" in production. The study utilized MLU from a 6-minute conversational and 12-minute narrative language samples to determine if a plateau in syntax occurred. The participants were divided into four age groups ranging from age 5 to 20. The two younger groups, who had not yet reached adolescence, exhibited an MLU of 3.0 or less, indicating that they were not yet able to use complex syntactic forms. However, several participants in the adolescent age groups (12.6 -16.5 years and 16.6 -20.5 years) showed a mean MLU greater than 3.0. MLU increased with CA in both conversational and narrative language samples, although increases were larger in narrative than in conversational context, most notably after the age of 16, while the individual variability became also larger at this point. The authors concluded that the findings of their study were not consistent with the hypothesis of a simple syntax ceiling. There was no evidence for a slowing of lexical or syntactic development from age group 2 (8 -12 years) or from age group 3 (12 -16 years). Overall, the MLU of the DS group was shorter than the controls, whereas narrative samples contained more word tokens, more word types, and longer MLU than conversation samples, for both groups. The analysis of the narrative language sample by age sub-group showed no evidence of a critical period for language development ending at adolescence, nor of a "syntactic ceiling" after which the acquisition of syntactic structures stops.
Moreover, Chapman et al. (2002)  Because of the lack of systematic data little is known about what happens after 50 years of age in individuals with DS. Das et al. (1995) recorded little or no change in nonverbal reasoning, memory, language, planning and attention, and adaptive skills up to 60 years. However, authors stated that participants beyond 60 years performed more poorly than younger individuals particularly in planning and attention. Also, Prasher & Chung (1996) suggest the existence of age-associated functional decline in approximately 20% of the people with DS (50 -71 years) in short-term memory, speech, activity, practical skills and general interests. Therefore, grammar eventually deteriorates together with the progressive breakdown of conceptual aspects of language. Accordingly, language profiles associated with individuals with Alzheimer's disease only and with individuals with DS in the first stages of Alzheimer's disease would be characterized by major dissociations between morphosyntax on the one hand, and language semantic and pragmatic aspects, on the other hand.

Delayed or Deviant Pattern of Development?
As mentioned above, the syntactic abilities of people with DS may be characterized as delayed or deviant compared to typical population. These studies are presented below in order to provide a framework on abilities of individuals with DS in this domain.
There are two major theoretical approaches in the literature on DS: 1) the delay hypothesis (Quantitative Variation) which assumes that language development in individuals with DS passes through the same stages as those observed in TDC and the only difference is that more time is needed, thus the differences  (Bridges & Smith, 1984;Bol & Kuiken, 1990;Fowler, 1990;Rutter & Buckley, 1994;Fabbretti et al., 1997;Vicari et al., 2000;Eriks-Brophy et al., 2003Estigarribia et al., 2012;Polišenská et al., 2018) and 2) the difference hypothesis (Qualitative Variation) according to which language development in individuals with DS does not follow the same stages as those observed in TDC, thus the observed differences between the two populations are a matter of qualitative variation (Perovic, 2001(Perovic, , 2006a(Perovic, , 2006bRing & Clahsen, 2005b;Tsakiridou, 2006;Sanoudaki & Varlokosta, 2014;Penke, 2018). Bridges and Smith (1984) conducted a series of experiments in children with DS to investigate their syntactic comprehension compared to TDC using tests on comprehension of active and passive sentences. The results reported no difference between groups. Language comprehension in children with DS was fundamentally the same as those of TDC, but with a delay of 6 months in the case of the passive sentences and a delay of one year in the case of active sentences. Also, Bol & Kuiken (1990) adopted the delay hypothesis of language development in DS in their study, as they observed that the difference in performance between the children with DS and the normal controls was mainly with respect to "the degree to which the language is affected" and not with respect to the patterns observed. They emphasized that since the patterns were similar between the two groups the language of children with DS could be characterized as delayed rather than deviant. In this line, Rutter & Buckley (1994) found that individuals with DS were rather delayed in acquiring the morpheme rules in comparison with TDC. In addition, Fowler (1990) in her research which aimed to discover maximum limits acquisition of syntactic skills in people with DS and to explain the reasons that great deficiencies are observed in this area, was also in favor of the delay hypothesis.
Similarly, Eriks-Brophy et al. (2003 conducted experiments on comprehension and production abilities of high-functioning individuals with DS using act-out tasks and true-value judgement tasks. They examined the comprehension of active and passive constructions, wh-questions and particularly the sensitivity to A-and A'-dependencies of subjects with DS. According to the results, the delayed-but normal view of language development for subjects with DS was evident. In addition, Fabbretti et al. (1997) examined lexical and morphosyntactic abilities of children and adolescents with DS and those of typical development using a story description task. They investigated a number of complex sentence structures such as relative clauses and gerund sentences and postulated that syntax does not pose great difficulties on people with DS. They found that the subjects of the two groups used similar types of clauses, and they had a comparable repertoire of conjunctions and clitic pronouns as well. However, the majority of the DS group presented a restricted syntactic profile which could not be indicative of people with spared syntactic abilities. Vicari et al. (2000) also supported the delay hypothesis via the examination of production and comprehension of morphosyntactic elements. Another portion of studies indicated an alternative view of results on the development of morphosyntax in individuals with DS who present a particular developmental trajectory that deviates from that of normal development. Perovic (2001, 2006a, 2006b) was one of the researchers who supported the deviant syntactic behavior and suggested a specific syntactic deficit in DS due to an unusual pattern of performance in the interpretation of reflexive pronouns. Perovic (2001) examined the comprehension of reflexives and pronouns in English-speaking subjects with DS, indicating that the DS subjects had specific difficulties assigning appropriate interpretation to reflexives, that according to standard Binding Theory reflexives are governed by Principle A. On the contrary, they had no difficulties in sentences with pronouns that are governed by Principle B in the same framework 1 . Those findings led her to propose a specific syntactic deficit in the language of DS's participants, which is not observed in control group, hence, her findings support the difference hypothesis.
Moreover, Perovic (2006a) made an experimental investigation into the knowledge of binding in a group of 4 girls with DS and a group of 4 TD children. Similarly, the participants with DS were found to have difficulties comprehending reflexives, but not pronouns. In contrast with pronouns, which are interpreted by invoking extra-syntactic mechanisms, the interpretation of reflexives depends on a syntactic relation between the reflexive element and its antecedent. Such a pattern is exactly the opposite to the one found in TD English children, who had trouble applying the co-reference rule that regulates the interpretation of the pronouns. This result provides evidence that language in DS is not merely delayed, but also deficient. In her study of 2006b examined 6 Ser-1 Binding theory concerns syntactic restrictions on nominal reference. It particularly focuses on the possible coreference relationships between a pronoun and its antecedent. In standard Binding Theory, Binding Principle A governs the distribution and interpretation of reflexives, whereas Binding Principle B is concerned with pronouns (Chomsky, 1981). Open Journal of Modern Linguistics bo-Croatian speaking adults with DS and TD controls matched on non-verbal MA using a truth-value judgment task for testing full and clitic forms of pronouns and reflexives. Results revealed that adult speakers of Serbocroatian with DS had difficulties forming the syntactic dependency of anaphoric binding. They showed particular difficulties in those involving the anaphor "sebe" which can only be interpreted as a deficit in establishing the syntactic relation between the anaphor and its antecedent. Furthermore, she noticed that the parallel between the Serbo-Croatian speakers' pattern here and that of English speakers in Perovic (2001) is observable in the mismatch name reflexive conditions only, perhaps due to different language-particular strategies that the speakers of English and Serbocroatian use to interpret the anaphor.
Ring and Clahsen (2005b)  Regarding the Greek language, Stathopoulou (2007) investigated the production of Greek relative clauses in eight adolescents with DS compared to sixteen typically developing children of similar mental age. The results of the study revealed a highly significant difference between the mean correctness scores of the examined groups. The children with DS exhibited significantly lower performance than that of mental aged controls across all types of relative clauses, while both groups exhibited the same pattern of performance on the four relative clauses types (SS > OO = OS > SO 2 ). Therefore, this finding provides evidence that language development in Greek-speaking individuals with DS could be characterized as "delayed". Nevertheless, a significant difference in performance between SS and OS relatives, as well as between SO and OO relatives in the control group, could provide evidence for the deviance hypothesis, instead of a simple delay. However, further research is needed. Tsakiridou (2006)
Considerable evidence points to productive syntax deficits in young individuals with DS that cannot be explained by cognitive level, for example the emergence of two-word combinations is delayed in young children with DS, and children and adolescents with DS continue to produce shorter, less complex 3 Table 1 presents a summary of the studies included in this review and concerns the syntactic ability of DS's population. Open Journal of Modern Linguistics noun and verb phrases, sentence structures, questions, negations and omissions of grammatical words than TD individuals of the same nonverbal ΜΑ as they get older (Rosin et al., 1988;Chapman et al., 1998Chapman et al., , 2002Vicari et al., 2000;Chapman & Hesketh, 2000;Tsakiridou, 2006;Fabbretti et al., 1997;Stathopoulou, 2007;Price et al., 2007Price et al., , 2008Caselli et al., 2008;Zampini & D'Odorico, 2011).
However, contrary to the above studies, Thordardottir et al. (2002) found that individuals with DS and those with typical development had parallel performance in conjoined and subordinate sentence forms, in the proportion of utterances containing complex sentences and in the variety of used types of complex sentences. The analysis of developmental patterns suggested a similar order acquisition across groups, that is conjoined sentences, infinitive clauses with equivalent subjects, non-infinitive wh-clauses, relative clauses, gerund clauses, full propositional complements, multiple embeddings, infinitive clauses with a different subject (infrequent), unmarked infinitive clauses (infrequent), and wh-infinitive clauses (infrequent).
More specifically, the studies included in this review relied on various syntactic constructions and their results are a point of interest. For example, subordinate and relative clauses, as well as negated constructions were not observed or observed much less frequently in DS individuals' language (Rondal & Comblain, 1996;Fabbretti et al., 1997;Joffe & Varlokosta, 2007;Stathopoulou, 2007;Witecy & Penke, 2017). Regarding syntactic structures, one domain that appears to be particularly disadvantaged is the interpretation of passives (Bridges & Smith, 1984;Eriks-Brophy et al., 2004;Ring & Clahsen, 2005b;Joffe & Varlokosta, 2007;Price et al., 2007;Witecy & Penke, 2017). Bridges & Smith (1984) assessed the ability of Down's syndrome children to act out active and passive semantically biased and neutral sentences in a comprehension task and found that although Down's syndrome children closely resembled controls both in the percentage of correct responses and in individual patterns of error, their performance on active sentences was better than on passive ones. They concluded that there was evidence of a slight (6 to 12 months) delay in the appearance of syntactic strategies of comprehension by the Down's syndrome children compared to TDC. The results of the study of Eriks-Brophy (2004) support the view of Bridges and Smith (1984) that construction develops in a normal but delayed manner in DS. Individuals with DS benefit from the elimination of the by-phrase in non-actional passives, while the greater difficulty of non-actional passive with a by-phrase could be attributed to inability to transmit a non-agentive theta role through the by-phrase. Also, Ring & Clahsen (2005b) found that the DS individuals encounter difficulties in the comprehension of active and passive sentences. The accuracy scores on actives were higher than the accuracy scores on full and short passives for both the DS and TD children, although the DS participants performed significantly worse than the  (2007) revealed that DS group understood significantly less passives than the TD controls, with significantly better performance in active sentences and they also obtained significantly higher transitive responses with full and short passives than ambiguous passives. In addition, Witecy & Penke (2017) found that German children, adolescents, and adults with DS presented particular difficulties in the comprehension of subordination and coordination, in sentences with passive voice or topicalization, and that relative clauses with pronouns in accusative or dative case proved to be problematic. Further, sentences with the negative element "nicht" (not) were understood best, whereas sentences with the disjunctive conjunction "wedernoch" (neither nor) were most error-prone. Meanwhile, the authors pointed out that difficulties increased with sentence length and grammatical complexity, but were also apparent in simple sentences.

Performance in Active/Passive Structures, in Subordination and in Pronouns
As reported above, in the study of Stathopoulou (2007) the children with DS encountered difficulties across all types of relative clauses. Interestingly, they adopted an avoidance strategy by producing responses, such as simple active sentences and coordinated constructions, as well as elliptical responses, instead of the targeted relative clauses, as indicated from the error analysis. In addition, Tsakiridou (2006) proved the difficulty of DS's subjects to produce wh-questions. Their correct answers in all four tested structures were low, whereas children with TD performed at ceiling. Similarly, Joffe & Varlokosta (2007) investigating the production, comprehension and repetition of a range of wh-question types (wh-subject/object, which NP-subject/object) of individuals with DS, observed that DS group as well as WS group and TDC performed better on comprehension than elicitation and also significantly better on repetition than elicitation. On the contrary, Fabbretti et al. (1997) using a story description task investigated a number of complex sentence structures such as relative clauses and gerund sentences. The examined groups used similar types of clauses, more often simple clauses than complex, with DS subjects produced significantly larger number of simple clauses. Most of the participants with DS used more verbs and half of them produced more coordinated clauses than their normal matches, but these differences between the two groups were not significant. Similarly, there were no significant differences concerning the production of complex clauses considered as a whole, or the use of embedded clauses, which were very rare in both groups.
Regarding the examination of complex syntactic structures, Polišenská et al. (2018) observed that the participants of all groups in her study presented similar order of difficulty, with simple Subject-Verb-Object (SVO) active sentences producing the best performance, whereas comprehension of relative clauses proved the most difficult. The analysis revealed no significant difference between the groups on function words and relative clauses, but there were significant differences on negation and the SVO category with the group of children with ID finding those structures more difficult. Grela (2003) found that there were no  (2017) observed that complex sentences were in general understood poorly. With respect to performance on simple sentences, they found that as the number of constituents increases, comprehension becomes more difficult. Sentences with three elements were understood worse than those with two elements and double object constructions were especially challenging for the participants with DS. Grela (2002) found that children with DS infrequently used sentence complements, sentential embeddings, or compound sentences. Also, Galeote et al. (2013) noticed that the morphosyntactic complexity increased very gradually in both groups of children up to the 300-word level. From that level on, significant differences appeared between the two groups of children, TD and DS. Accordingly, even though the children of both groups showed no differences in the combining of words, nor in MLU of the three longest phrases, the syntax of children with DS was less complex. Moreover, Zampini & D'Odorico (2011) conducting a qualitative analysis of the complex sentences uttered found that there were significant differences in the use of simple and complex sentences.
Furthermore, they observed a lower number of subordinate clauses in children with DS. Particularly, the children with DS produced only non-finite clauses, whereas 28% of the complex sentences produced by the TDC were finite clauses.
Also, 59% of the utterances produced by the children with DS contained at least one verb, whereas the TDC produced 88% utterances with verbs. Additionally, Price et al. (2008) highlighted that DS group produced not only shorter, less complex utterances overall, but less complex noun phrases, verb phrases, and with DS had considerable difficulties with the comprehension of sentences containing personal pronouns, either it was subject or object pronouns. Fabbretti et al. (1997) noticed that the two examined groups of their study had a comparable repertoire of conjunctions and clitic pronouns.

The Use of Verbs and Nouns
It is suggested that verbs are more complex than nouns, carrying both semantic and syntactic information and consequently, they are more difficult to learn them on average, even for TD children. Verbs are, however, fundamentally linked to nouns because they require arguments, or additional words, to help complete their meanings. Also, verbs are responsible for linking words within a sentence, they play a key role in syntax and possible disorders and may impact the syntactic development. Kernan and Sabsay (1996) comparing the linguistic and cognitive abilities of adults with DS to those with mental retardation of unknown etiology found that adults with DS performed significantly lower on syntax in all subcategories except for nouns and simple sentence structure. Also, in the study of Polišenská et al. (2018) examined groups showed significantly better performance for nouns compared with verbs, particularly 81% versus 59% correct answers for the ID group, compared with 88% versus 65% for the TD group. Moreover, Galeote et al. (2018) aimed to analyze the acquisition of different classes of words in Spanish-speaking children with DS, with special emphasis on nouns and verbs. Their results indicated considerable similarity between children with DS and their TD peers. Although the post hoc comparisons showed no differences between the two groups of children for any of the word classes, children with DS tended to produce fewer predicates and closed-class words. Nevertheless, the specific analysis of verbs showed that children with DS produced fewer words of this kind, even though this result presented statistically significant value only at a higher lexical level of 251 -400 words. At the first three levels (≤10, 11 -50, and 51 -100 words) no differences were observed between the proportions of nouns, predicates and closed-class words, all of which were very low. Nouns began to emerge at the level of 101 -250 words and their proportion became distinguishable from that of predicates and closed-class words. Predicates emerged at the level of 251 -400 words, although their proportion was still below that of nouns. Importantly, at the final level (≥401 words) the proportion of predicates was equivalent to that of nouns. The study of Checa et al. (2016) analyzed the composition of early vocabularies in a large sample of 108 Spanish-speaking children with DS and compared it with that of children with TD. The categories examined were nouns, predicates, closed-class words, and social words. The performance of children with DS was similar to that of children with TD with the same vocabulary size. The only significant difference was the larger production of nouns by children with DS. The most surprising among their findings was the absence of any differences between the two groups of children in the production of closed-class words. The development of the other classes of words was also similar. Social words were the most frequently produced category at the first lexical levels (<100 words), with a linear decline as vocabulary size increases. Nouns showed continuous growth across the first levels, and by the level of 101 -250 words, they were more numerous than all the other classes of words, whereas predicates showed a slow but continuous growth and their presence become evident after the level of 101 -250 words. Closed-class words showed minimal production, especially at the initial levels, although they showed a slow and constant growth. Bello et al. (2014) investigated the lexical comprehension and production abilities as well as gestural production and they found that nouns are understood and produced in higher percentages compared to predicates. Overall, both groups produced significantly more correct answers in comprehension than in production task. Zampini & D'Odorico (2011) found no significant differences in the proportion of common nouns and verbs-adjectives, however the children with DS produced a significantly higher proportion of simple terms, as routines-people, and a significantly lower proportion of adverbs-function words. The analysis of the argument structures of verbs with two or more arguments showed that subject-verb-indirect object and subject-verb-direct object were the most frequent multi-argument structures for both groups. It was of particular interest that children with DS seemed to have greater difficulties in expressing sentences in a grammatically correct form, though they were able to combine words.
As it is reported, verb production is also affected by the syndrome in many ways. Some studies show that individuals with DS produce fewer verbs overall relative to TD children, whether matched on MLU (Hesketh & Chapman, 1998), receptive vocabulary (Michael et al., 2012), or nonverbal cognitive ability levels. Loveall et al. (2019) examined verb production by individuals with DS relative to both TD peers matched by nonverbal cognitive ability level and to individuals with mixed-etiology ID of other origins matched by chronological age. Results of this study indicated that participants with DS produced narratives with less verb density than participants with TD, they had smaller verb type-token ratios than participants with intellectual disability and they demonstrated relatively strong verb diversity. Although individuals with DS may have a large number of verbs in their vocabularies, they did not use them as regularly in their narrations. Hesketh & Chapman (1998)  that the individuals with DS produced significantly fewer lexical or grammatical verbs per utterance, but with greater diversity of lexical verbs. Also, individuals with DS produced a significantly smaller percentage of lexical verbs that were metacognitive or metalinguistic in nature and a significantly greater number of utterances that did not include a verb. Authors suggested that the syntactic deficit in DS did not arise from a failure to construct syntactically complex utterances, but may reflect their difficulty in accessing verbs when constructing utterances as a result of deficits in auditory short-term memory. However, Grela (2002) failed to find significant differences between individuals with DS and TD children matched on MLU in their production of verbs, probably due to differences in the elicitation materials used across studies. Hesketh & Chapman (1998), Michael et al. (2012) and Loveall et al. (2019) all used narrative tasks, but Grela's (2002) data included language transcripts. Also, differences may occur due to different CA, as the participants in the study of Grela (2002) may not have had sophisticated enough expressive language to reveal differences in verb use. Also, studies have found that participants with DS may produce a greater variety of verbs than TD controls of similar developmental level, specifically lexical verbs (Grela, 2002;Hesketh & Chapman, 1998).
Studies examining verb comprehension in DS have not found significant differences between groups with DS and TD (Loveall et al., 2016;Michael et al., 2012) but have found differences between individuals with DS and mixed-etiology ID (Loveall et al., 2016). More specifically, Loveall et al. (2016) were especially interested in the comprehension of verbs, because of their importance to later syntactic development. They compared groups with DS, TD, and mixed-etiology ID and found that, relative to age-matched peers with ID, youth with DS performed lower on verb items. Further, this difference was maintained even when they were compared for overall receptive vocabulary and phonological memory. The two groups, however, did not perform differently on noun or attribute items, and there were not observed group differences between the groups with DS and TD. Also, Michael et al. (2012) examined the use and comprehension of verbs differing in argument structure. DS and TD groups performed similarly on single-word tasks, but the DS group omitted verbs from targeted sentences in their narratives and showed poor grammaticality judgment abilities. Interestingly, they did not differ significantly from the TD group in comprehension of isolated nouns or verbs, and in naming of single nouns or verbs, as well. Both groups had difficulty retrieving verbs to label stimulus pictures. Individuals with DS performed significantly worse when asked to judge sentence grammaticality and omitted verbs in elicited narratives significantly more often than TD individuals, specifically when productions of 2-place and 3-place verbs were attempted. Individuals with DS also omitted other necessary elements of argument structure, such as subjects, in sentences containing 2-place and 3-place verbs significantly more often than individuals with typical development. Authors concluded that performance was not related to working memory skills and individuals with DS display a specific expressive deficit in verb and argument struc-G. Andreou, E. Chartomatsidou DOI: 10.4236/ojml.2020.105029 500 Open Journal of Modern Linguistics ture retrieval (but not comprehension) that varies as a function of verb type (1-place, 2-place, and 3-place).

Conclusion
The developmental literature has shown that syntax constitutes a particular linguistic challenge for individuals with DS who are relatively more impaired in the domain of language than in other areas of cognition. The syntactic processing abilities of the DS population as compared to typically developing individuals were explored in this review paper. Also, the purpose of this paper was to discuss the findings of published studies on the syntactic abilities with reference to the deviant or delayed pattern of development, as well as to the critical period of language development, and more specifically of syntactic development, in the DS population.
In the studies included, researchers used a plethora of elicitation materials, methods of analysis and also different groups of subjects to make comparisons. Chronological age, mental age, developmental age, MLU, or receptive vocabulary size were among those measures used for matching the examined groups. This variability among matching measures probably accounts for the differences in the studies' outcomes. Additionally, the different numbers of the examined samples may not guarantee the generalization of the findings. Furthermore, different results in the studies on the syntactic abilities of the DS population from different countries may have emerged from the specific characteristics of the language spoken by the samples, in terms of morphosyntax. Most studies in the relevant literature have been carried out with English-speaking individuals, and an important question that arises is whether their results apply for other languages. For this reason, this review paper included studies from various countries with different speaking languages, thus giving information on the language abilities and more specifically on the syntactic abilities, of the DS population in English (Bridges & Smith, 1984;Laws & Bishop, 2003;Joffe & Varlokosta, 2007), German (Witecy & Penke, 2017;Penke, 2018), Italian (Fabbretti et al., 1997;Vicari et al., 2000;Caselli et al., 2008;Zampini & D'Odorico, 2011;Bello et al., 2014), Spanish (Checa et al., 2016;Galeote et al., 2013Galeote et al., , 2018, French (Facon & Magis, 2019), Slovak (Polišenská et al., 2018), Greek (Tsakiridou, 2006;Stathopoulou, 2007;Andreou, 2013;Sanoudaki & Varlokosta, 2014), Serbo-Croatian (Perovic, 2006b), American (Hesketh & Chapman, 1998;Thordardottir et al., 2002;Chapman et al., 2002;Chapman, 2006;Michael et al., 2012;Loveall et al., 2016Loveall et al., , 2019; among others) and Japanese (Koizumi et al., 2019). No restriction was mooted during the selection process of the studies and therefore this review provides evidence from a variety of languages in addition to English, giving the opportunity for comparisons and further knowledge on the syntactic performance of this population in various languages.
The data from the studies presented, showed a disadvantage of the DS population in syntactic processing, at both production (Hesketh & Chapman, 1998;Vicari et al., 2000;Ring & Clahsen, 2005a;Tsakiridou, 2006 Stathopoulou, 2007;Caselli et al., 2008;Zampini & D'Odorico, 2011;Andreou, 2013;Bello et al., 2014;Koizumi et al., 2019;Loveall et al., 2019) and comprehension (Chapman et al., 1991Rondal & Comblain, 1996;Vicari et al., 2000;Perovic, 2001Perovic, , 2006aRing & Clahsen, 2005a, 2005bAndreou, 2013;Bello et al., 2014;Joffe & Varlokosta, 2007;Sanoudaki & Varlokosta, 2014;Loveall et al., 2016;Witecy & Penke, 2017;Polišenská et al., 2018;Koizumi et al., 2019) levels, compared to typically developing individuals. This disadvantage has been interpreted by the researchers as either delayed or following a completely different pathway. From the examination of the published studies, we conclude that the pattern of syntactic development in the individuals with DS seems to be more delayed than deviant (Bridges & Smith, 1984;Rutter & Buckley, 1994;Fabbretti et al., 1997;Thordardottir et al., 2002;Eriks-Brophy et al., 2004;Schaner-Wolles, 2004, among others) or deviant (Perovic, 2004(Perovic, , 2006a(Perovic, , 2006bRing & Clahsen, 2005a;Penke, 2018). In a remarkable number of studies there is a considerable individual variability, but the great majority of the studies examined, revealed that the language characteristics of individuals with DS follow a common profile with TDC. Receptive language is typically stronger than expressive language, and vocabulary is stronger than syntax in both receptive and expressive domains. With regard to the critical period for language development in the DS population, some authors concluded that there was a plateau in the syntactic development, that ended near adolescence, and therefore, it was thought that little progress in the syntactic domain was possible after that period (Lenneberg et al., 1964;Fowler, 1990;Fowler et al., 1994;Rondal & Comblain, 1996;Iacono et al., 2010;Witecy & Penke, 2017;Koizumi et al., 2019). However, the majority of the reported outcomes supports the opposite view (Kernan & Sabsay, 1996;Chapman et al., 1998Chapman et al., , 2002Thordardottir et al., 2002;Andreou, 2013;Facon & Magis, 2019). Language learning continues in both processes, that is in production and in comprehension, for older adolescents and young adults. The appearance of plateaus in syntax development may be partly linked to the nature of the sample, particularly their age, or to the materials used. For example, individuals with DS tend to use more complex syntactic structures in the narrative tasks used in some studies than in different tasks used by others.
Particularly, concerning their syntactic ability DS individuals tend to produce shorter and less complex utterances compared to typically developing population (Rosin et al., 1988;Chapman et al., 1998;Price et al., 2008;Zampini & D'Odorico, 2011;Galeote et al., 2013;Frizelle et al., 2018), although advances in syntactic complexity may continue into late adolescence and young adulthood. It is also reported that DS group produced not only shorter, less complex utterances overall, but less complex noun phrases, verb phrases, and sentence structures, like questions and negations than did the TD group (Rosin et al., 1988;Chapman et al., 1998Chapman et al., , 2002Vicari et al., 2000;Chapman & Hesketh, 2000;Tsakiridou, 2006;Fabbretti et al., 1997;Stathopoulou, 2007;Price et al., 2007Price et al., , 2008Caselli et al., 2008;Zampini & D'Odorico, 2011). Moreover, difficulties emerged in subordination and in coordination, in relative clauses, in adverbial clauses, in  (Rondal & Complain, 1996;Joffe & Varlokosta, 2007;Stathopoulou, 2007;Zampini & D'Odorico, 2011;Witecy & Penke, 2017;Polišenská et al., 2018). Additionally, they tended to use infrequently sentence complements, sentential embeddings, or compound sentences (Grela, 2002;Thordardottir et al., 2002;Frizelle et al., 2018). Obviously, the above difficulties increased with sentence length and grammatical complexity, but were also apparent in simple sentences. Overall, the Subject-Verb-indirect/ direct Object was the most frequent multi-argument structure for DS and TDC groups (Zampini & D'Odorico, 2011;Frizelle et al., 2018;Polišenská et al., 2018). On the contrary, results from the Italian language show that syntax does not pose great difficulties on people with DS, since while DS subjects produced significantly larger number of simple clauses, they are able to produce a number of complex sentence structures such as relative clauses and gerund sentences (Fabbretti et al., 1997). However, it was of particular interest that children with DS seemed to have greater difficulties in forming grammatically correct sentences. Similarly, results from English-speaking people indicate that there were similar omission patterns of subject arguments for both TD and DS children when comparisons were made across verb categories with no difference as argument structure complexity increased (Grela, 2003). However, children with DS were presented more likely to omit arguments in the subject position than in the direct object position, whereas TD controls used a greater number of anomalous arguments (Grela, 2003;Zampini & D'Odorico, 2011;Michael et al., 2012). Regarding complex syntactic structures, it is worth mentioning that the Slovak DS population was observed to have similar order of difficulty with those of TD, with simple Subject-Verb-Object active sentences producing the best performance, whereas comprehension of relative clauses proved the most difficult (Polišenská et al., 2018). The results from the studies examined showed that DS children exhibited significantly lower performance than that of MA controls across all types of relative clauses, with the exception of intransitive subject relative clauses, and they adopted an avoidance strategy by producing other types of responses instead of the targeted relative clauses (Stathopoulou, 2007;Frizelle et al., 2018;Polišenská et al., 2018). Also, relative clauses with pronouns in accusative or dative case proved to be problematic (Witecy & Penke, 2017). In addition, DS individuals had severe problems with producing wh-questions, while similarly to TDC performed better on comprehension than elicitation and also significantly better on repetition than elicitation (Eriks-Brophy et al., 2004;Tsakiridou, 2006;Joffe & Varlokosta, 2007).
Moreover, another domain that appears to be particularly disadvantaged in the DS population is the interpretation of passives (Bridges & Smith, 1984;Eriks-Brophy et al., 2004;Ring & Clahsen, 2005b;Joffe & Varlokosta, 2007;Witecy & Penke, 2017;Koizumi et al., 2019). Their performance on active sentences was better than that on passive ones and they also obtained significantly higher transitive responses with full and short passives than ambiguous passives (Joffe & Varlokosta, 2007). In addition, they appeared to encounter difficulties in G. Andreou, E. Chartomatsidou DOI: 10.4236/ojml.2020.105029 503 Open Journal of Modern Linguistics the comprehension and interpretation of sentences with pronouns, mainly with reflexive pronouns instead of personal pronouns, probably due to their inability to establish a certain syntactic dependency, namely the binding relation between an anaphor and its antecedent (Bol & Kuiken, 1990;Perovic, 2001Perovic, , 2002Perovic, , 2006aPerovic, , 2006bRing & Clahsen, 2005b;Sanoudaki & Varlokosta, 2014;Witecy & Penke, 2017). It is worth mentioning that TDC also present difficulties with personal pronouns and that the performance of DS individuals did not reveal any differences from TDC in their interpretation of other elements, such as pronominal clitics (Perovic, 2006b;Fabbretti et al., 1997;Sanoudaki & Varlokosta, 2014). It must be noted that there are conflicting conclusions in the studies examined concerning pronouns, as in some of them the DS population presented difficulties with personal (either it was subject or object pronouns) and possessive pronouns, as well as poor control over demonstrative and interrogative pronouns (Witecy & Penke, 2017) and in others DS individuals presented comparable performance in clitic pronouns (Fabbretti et al., 1997;Perovic, 2006b). Regarding the Greek language difficulties are not observed in the DS cohort with pronouns, except for reflexive pronouns (Sanoudaki & Varlokosta, 2014). This is an important finding because of the different pattern in the comprehension of pronouns in the Greek language compared to other languages. It is worth pointing out that DS individuals presented significantly better performance on nouns with continuous growth from the first lexical levels compared with verbs (Kernan & Sabsay, 1996;Hesketh & Chapman, 1998;Michael et al., 2012;Checa et al., 2016;Polišenská et al., 2018). Nouns were also understood and produced in higher percentages compared to all the other classes of words. It is documented that DS individuals produced fewer or no utterances with verbs overall relative to TD population, although this result was found statistically significant only at higher lexical levels (of 251 -400 words) (Galeote et al., 2018). At this point, we should mention that although individuals with DS may have a large number of verbs in their vocabularies, with greater diversity of lexical verbs, however, they did not use them regularly (Loveall et al., 2019). Moreover, they were able to produce transitional forms and made significantly greater use of some of them, for example formulas and dummy element productions, than TDC in some cases (Zampini & D'Odorico, 2011).
It must be noted that since we find impaired syntax and syntactic processing in the DS cohort, it is important to consider possible intervention strategies that may be used in the education context mainly, to enhance syntactic performance on the part of this population. There is an urgent need for setting up more targeted and effective intervention programs, which focus on specific syntactic phenomena with the aim of improving syntactic understanding and production in individuals with DS. The improvement of language abilities of this impaired population via syntactic therapy may also improve considerably their communicational abilities.
Moreover, although a large body of research examining language skills for individuals with DS is available, the majority focuses only on the language abilities   (2013) 12 GreekDS (CA: 6 -7.11 years)/12 Greek DS adolescents (CA: 14 -15.11 years)/12 Greek TDC (CA: 6 -7.11 years) PiNG Italian MB-CDI DS: general weakness in lexical comprehension and production. Significantly higher percentage of errors than TD, as well as no-responses. Also, more representational gestures + more unimodal gestural answers. Nouns are understood and produced in higher percentages than predicates.
3. Bridges & Smith (1984) 24 English DS (mean CA: 11.1 years, VCA 2.5 -5.2)/24 TD (mean CA: 3.0 years, VCA 2.5 -5.2) matched on verbal comprehension Comprehension task of active/passive/neutral sentences Better performance on active sentences than on passive for both groups. Also, similarities in terms of percentage correct responses and patterns of errors. DS: a slight delay (6 -12 months) in the appearance of syntactic strategies of comprehension compared with those non-retarded children.

Stanford-Binet Scale
Leiter PPVT Boston Naming Test (BNT) Linguistic Comprehension Test (LCT) Phrase Repetition Test (PRT) DS + SLI: worse performance than TD. Although no significant differences in lexical and morphosyntactic comprehension abilities, significant differences did emerge in morphosyntactic production capacities. DS: more errors than SLI children, who, in turn, made more errors than TDC. DS: more omissions, a significantly higher number of articles, verbs, and prepositions than SLI children, whereas no difference was found for nouns and for modifiers. DS + SLI (but not with TD) omitted more articles in sentences than in syntagms. Qualitative analysis of the morphosyntactic errors revealed strong similarities between the DS + TD groups. 5. Chapman (2006) 20 American DS (CA: 12 -21 years)/16 with cognitive impairment of unknown origin (CA: 12 -21 years) 3-hour protocol: hearing screening, the Bead Memory and Pattern Analysis subtests of the Stanford Binet, PPVT-3, the vocabulary subtest of the Test of Auditory Comprehension of Language-3, interview language samples, narrative language samples, Kaufman-ABC number recall task (digit span) and the Nonword Repetition Test DS: The auditory-verbal working memory deficit appears to be part of its specific phenotype, as well as the loss of comprehension skills in adolescence, and is poorer for both syntax comprehension and vocabulary comprehension than the group with cognitive impairment of unknown origin. The significantly better performance of the DS and cognitively impaired groups on the PPVT-3, relative to syntax comprehension, appeared attributable to CA and the additional life experience. Deficits in auditory-verbal working memory, syntax and vocabulary comprehension, and narration of picture-books without an opportunity to preview them are all specific to the adolescent group with DS. Open Journal of Modern Linguistics Individual differences in expressive language syntax at study start are best predicted by syntax comprehension. Syntax comprehension across the 6 years of study participation is best predicted by age at study start and measures of short-term memory (auditory + visual), while change in syntax comprehension across this time is predicted by age at study start. The expressive language acquisition continues in adolescence for most individuals with DS, and is predicted by syntax comprehension and its growth trajectory. The rate of MLU increase is greatest for those for whom comprehension declined less. However, no evidence of a critical period. DS speakers: more advanced content than is typical for their average utterance length, thus need for more complex expressive syntax. Measures of syntax development which focused on comprehension rather than on production might depict different longitudinal outcomes of loss, plateauing, or gain depending on age. Chapman et al. (1998) 47 American DS (CA: 5.6 -20.6 years)/47 TDC (CA: 2.2 -6.1 years) 3-hour protocol: hearing screening; picture descriptions; story retelling; Form L of the PPVT-R; 6 min. conversation and 12 min. narration with the examiner; an object hiding task evaluating fast mapping for a novel noun (Chapman et al., 1990); the Expressive Vocabulary, Bead Memory, and Pattern Analysis subtests of the Stanford-Binet, 4 th edition; conversation and snack with a parent; a speech motor evaluation; delayed story recall; event narration; the TACL-R; the delay condition of the object-hiding task administered earlier.

7.
No evidence for a slowing of lexical or syntactic development from age group 2 (8 -12 years) on or from age group 3 (12 -16 years) on, no evidence of a critical period for language development ending at adolescence, nor of a "syntactic ceiling" at MLU corresponding to simple sentences for the DS group. DS: specific language impairment compared to control children, in number of different words and total words (in the first 50 utterances) and in MLU. Clear evidence of a deficit in the DS group's expressive language performance across measures of syntactic complexity, word frequency, diversity in a fixed number of utterances, and rate of word production, despite more frequent utterances per minute, in both conversational and narrative samples.
8. Chapman et al. (1991) 48 American DS (CA: 5.6 -20.6 years)/48 TDC (CA: 2 -6 years) 3-hr protocol: hearing screening, picture descriptions, story retelling, Form L of the PPVT-Revised, conversation and narration with the examiner, an object hiding task (Chapman et al., 1990), the Expressive Vocabulary, Bead Memory, and Pattern Analysis subtests of the Stanford-Binet, 4th ed., conversation and snack with a parent, a speech motor evaluation, delayed story recall, event narration, the Test for Auditory Comprehension of Language Revised, and the delay condition of the object hiding task Differences within the group of DS between lexical and syntactic comprehension skill increasing with age, as well as between nonverbal cognitive subtests of pattern analysis and short-term memory for bead arrangements. CA and mean MA, collectively, accounted for 80% of the variability in syntax comprehension and hearing status predicted an additional 4%. Overall, adolescents with DS can be described as having advanced vocabulary comprehension, and also have appeared to have mild deficits in syntax comprehension. Their results are consistent with findings of sequential processing deficits or visual storage deficits. Open Journal of Modern Linguistics In line of Bridges and Smith (1984) construction develops in a normal, but delayed manner in DS. DS: benefit from the elimination of the by-phrase in non-actional passives. Standardized tests are not always good predictors of grammatical ability, as emerged from the comparison in the comprehension and production abilities of two subjects. Fabbretti et al. (1997) 10 Italian DS (CA: 6. 16. Grela (2003) 7 American DS (CA: 6.2 -12.2 years)/7 TDC (CA: 2.4 -2.8 years)

11.
Language transcripts DS children omitted subject arguments as frequently as TDC. Both groups were likely to omit subject arguments in intransitive as in transitive verb constructions. DS omitted more subject arguments than arguments in the direct object position. TDC more likely to produce anomalous arguments than children with DS.

17.
Grela ( No difference between groups on syntactic complexity for utterances with lexical and grammatical verbs. The syntactic deficit in DS did not arise from a failure to construct syntactically complex utterances, but may reflect difficulty in accessing verbs when constructing utterances due to deficits in auditory short-term memory.

19.
Joffe & Varlokosta (2007) 10 English WS (CA: 6.9 -13.10 years, mean MA: 4.8 years)/10 DS (CA: 5.11 -14.0 years, mean MA: 4.6 years)/10 TDC (CA: 3.3 -6.5 years, mean MA: 5.0 years) WISC III/WPSSI-R TROG2 Test of Active and Passive Sentences (TAPS) A wh-question elicitation task based on Thornton's (1990) elicitation technique and on Varlokosta (2004) assessed knowledge of wh-movement A wh-question comprehension task based on Varlokosta (2004) assessed understanding of wh-questions A wh-question repetition task WS + DS similar performance on the standardized measure of grammatical ability and on the experimental tasks that tapped comprehension of passives, and production and comprehension of wh-questions, however, both groups scored significantly below their MA-matched peers mostly in the syntactic tasks. DS: poorer performance than other groups on the repetition of wh-questions. WS + DS: difficulties with the comprehension and production of past tense, significantly better performance in active sentences, no difference in the understanding of ambiguous sentences, poorer performance on both full and short passives (worse on sentences with irregular verbs), while experienced difficulties in wh-question interpretation and production. DS: significantly more reversal responses. Wh-subject questions were the easiest for all groups and which NP-object the most difficult. 20. Kernan & Sabsay (1996) 1 -3 years), and a temporal plateau appears from an MA of 5 to 6 years, during which the development of syntax comprehension is halted. Children with ID with MA of 7 -9 years old: comprehension of basic grammar forms. DS: significant delay in syntactic development (conjunction particles, particle strategies, sentence structures). Syntax production abilities of ID children: lower than expected based on their MA and considerably delayed than comprehension abilities. MA of 7 -8 years for ID children for development of syntax production and comprehension abilities. Most errors in the transformation of the passive + causative voice to the active voice. DS children: more difficulties in comprehending morphologically and syntactically complex aspects than other impaired children.

22.
Laws Grammar understanding (TROG) was equally impaired in both groups, but receptive vocabulary (BPVS-II) was more problematic for the children with SLI than for the DS group. SLI+DS: have language skills that significantly lag behind nonverbal mental level. Both groups were impaired on tests of grammatical morphology and phonological memory. Both groups present similarities: vocabulary was an area of relative strength, whereas syntax was poor. Expressive language was more severely affected than receptive. Deficits in production of grammatical markers of verb tense. Poor performance on tests of word and nonword repetition. DS: did not differ from MA-matched controls in receptive and expressive vocabulary and in production of irregular past tense morphemes. They tended to use the wrong verb ending in elicited responses. From MLU of 4.5 or more the number of regular and irregular past tense forms produced correctly did not significantly differ from controls.
35 American DS (CA: 11 -21 years)/27 with ID (CA: 13 -20 years)/29 TDC (CA: 4 -6 years) Narrative story generation task DS: narratives with less verb density than participants with TD and smaller verb type-token ratios than participants with intellectual disability. Although individuals with DS may have a large number of verbs in their vocabularies, they did not use them although the mean productivity of multiword utterances increased over the three time points (36, 42 and 48 months). Different growth patterns of early syntactic development could be identified. Significant relationships between early syntactic skills and both the child's vocabulary size and developmental age. The number of functions expressed by word combinations appeared to be related to both the vocabulary size and developmental age for all three levels of CA. Open Journal of Modern Linguistics PPVT-4 Leiter-R Nonword Repetition subtest of the CTOPP ID performed significantly better than both DS + TD on verb items but not on nouns or attributes. DS + TD: same pattern of lexical knowledge, performing better on nouns than both verbs and attributes, while ID participants performed similarly on nouns and verbs, but worse on attributes. CA and cognitive ability are significant correlates of receptive vocabulary for participants with DS. The pattern of results in the DS-TD contrast changed after adding phonological memory as a covariate, DS performed significantly better than the group with TD on overall receptive vocabulary but there were no group differences on verb knowledge.
9 American DS (CA: 11.11 -32.10 years)/9 receptive vocabulary age-matched and gender TDC (CA: 3.2 -13.6 years) 1st session: PPVT-4, digit-span task, a word-span task, a sentence-repetition task, a single word-naming task, and the hearing screening. 2 nd session: digit-span task with nonverbal response, a word-span task with nonverbal response, a spatial-memory task, a single-word comprehension task, a grammaticality judgment task, and a narrative task.
Three memory tasks: a digit span task, a word-span task, and a spatial-memory task (TAPS-3) DS: significantly worse than the TD group on the sentence memory task, while performed similarly to the TD group on all other measures of memory skills. So, their memory deficits are not tied to linguistic stimuli or verbal responses, per se, but may instead be tied to language processing. DS: significantly worse than TDC in judging sentences grammatically. DS: significantly more omissions of verbs in elicited narratives. Specific expressive deficit in verb and argument structure retrieval (but not comprehension) that varies as a function of verb type.

26.
Penke ( Perovic (2006a) 4 English DS (CA: 17 -21 years)/4 TDC (CA: 5.11 -7.10 years) Picture Truth Value Judgement task DS: difficulties comprehending reflexives, but not pronouns indicating a selective grammatical deficit in DS which is syntactic in nature. The opposite pattern from TDC. DS's language: not merely delayed, but also deficient due to inability to establish the syntactic relation between the anaphor and its antecedent. Modules of the computational system, such as morphosyntax, are relatively more impaired than those associated with the general processing system, such as lexical knowledge or pragmatics.
28. Perovic (2006b) 6 Serbo-Croatian DS (mean CA: 23.3 years)/TDC (CA: 5 -6.11 years) Picture Truth Value Judgement task No statistically significant differences between the two groups. Control participants performed at ceiling on all conditions, as well as participants with DS also performed well on a number of test conditions. They were able to answer both yes (match) and no (mismatch) questions appropriately. They also scored high on experimental match conditions with full forms of the pronouns and the reflexive and match conditions with clitic forms. DS differed significantly to those of the TD controls for name reflexive and for quantifier-reflexive, both mismatch conditions. DS: particular difficulties with the anaphor "sebe". 29. Perovic (2002) 4 English DS (CA: 17 and 21 years)/TDC in various groups from Chien & Wexler (1990) Picture Truth Value Judgement task adapted from Chien & Wexler (1990) DS: specific difficulties assigning appropriate interpretation to reflexives (Principle A of standard Binding Theory), as opposed to pronouns, constrained by Principle B. This pattern is the reverse of the well-known "Delay of Principle B" effect confirmed in typical acquisition. On conditions involving pronouns (NPM, NPX, QPM, QPX), the subjects performed at ceiling but their performance is strikingly different on conditions that involve reflexives. Specific syntactic deficit in the language of DS, related to the inability to establish a certain syntactic dependency. 30. Perovic (2001) 4 English DS (CA: 17 -21 years)/TDC in various groups from Chien & Wexler (1990) Picture Truth Value Judgement task adapted from Chien & Wexler (1990) The process of acquisition of Binding in DS qualitatively different compared to typical linguistic development. Specific syntactic deficit in DS, inability to establish a certain syntactic dependency (an anaphor and its antecedent). Performance at ceiling on conditions involving pronouns, but below chance on at least one (match or mismatch) condition with reflexives. DS: significantly worse performance on anaphors as opposed to pronouns, revealing a pattern opposite to the well-known "Delay of Principle B Effect". Open Journal of Modern Linguistics Four elicitation tasks examining the past tense (use of existing regular and irregular past tense verbs/distinction between existing irregular verbs and homophonous denominal verbs), noun plurals (production of existing regular and irregular plurals), and comparative adjectives DS: significantly higher percentage of unmarked forms in both regular and irregular conditions. A similar pattern is seen in the Past Tense 2 task, the DS group produced significantly more unmarked forms in both conditions. Similarly, in the comparative adjective task the DS group used significantly more uninflected forms in two of the three conditions than the control group. The results are parallel for non-tense related morphemes indicating that the linguistic impairment in DS is broader than in SLI and not restricted to the finiteness cluster. No significant difference of examined groups in the use of corrected forms for regular or denominal verbs, nor in the correct production of irregular forms for irregular verbs.

Wechsler Intelligence
Scale for Children TROG STOP (Syntactic Test of Pronominal Reference) TAPS (Test of Active and Passive Sentences) No significant differences between the different age subgroups, either for the DS or the control participants and no significant differences between the match and the mismatch conditions. DS: particular difficulties in the interpretation of sentences with reflexive pronouns, whereas more accurate performance on sentences with non-reflexive pronouns. DS performed significantly worse than the controls in all conditions, but they had more difficulties interpreting passive than active sentences. Distinct patterns of linguistic impairment indicating that different genetic etiologies are associated with different specifically linguistic patterns of impairment and no with low levels of general intelligence.
36. Rondal & Comblain (1996) 11 DS (mean CA: 9.10 years)/16 DS (mean CA: 18.4 years)/15 DS (mean CA: 30.8 years)/11 non intellectually impaired children (mean CA: 3.8 years) MLU TVAP TVP BEMS (8 receptive subtests) The language of most DS adults formally restricted morphosyntactically. Their utterances were short, mono-propositional with limited and inconsistent use of grammatical morphology. Infrequent use of articles and verbs were not regularly inflected. DS adults: 50% or less correct responses in sentence comprehension (personal pronouns, articles, verbal inflections, subordinate clauses, negative + passive sentences). Slightly better receptive performance in relative clauses. No evidence for progress in receptive-expressive morphosyntactic aspects of the language of the DS adults compared to DS adolescents. 37. Rosin et al. (1988) 10 American DS (CA: 10.6 -17.5 years, mean MA: 6.2 years)/10 MR (CA: 12.5 -18.7 years, mean MA: 6.3 years)/Normal 1 (CA: 5.1 -6.11 years, mean MA: 6.7 years)/Normal 2 (CA: 12.  Intelligence Test (SON-R 2.5 -7) TROG-D Repetition subtest of the SETK 3 -5 Number recall subtest of the K-ABC The development of receptive syntactic skills comes to an end in the transition from adolescence to adulthood. Difficulties increased with sentence length and grammatical complexity, but were also apparent in simple sentences. More grammatical than lexical errors. No difference in comprehension of nouns, verbs and adjectives. The comprehension of sentences in perfect tense more error-prone than the comprehension of noun plural morphology. Difficulties in comprehension of function words, disjunctive conjunction, personal pronouns, subordination and coordination, the interpretation of subject relative clauses, passive voice, topicalization and relative clauses with pronouns in accusative or dative case.

44.
Zampini and D'Odorico Although no significant differences in the proportion of common nouns and verbs-adjectives, DS produced a significantly higher proportion of simple terms, as routines-people, and a significantly lower proportion of adverbs-function words. DS did not produce all the kinds of transitional forms, but only forms with a lower level of complexity, such as formulas and dummy element productions. Although, they were able to use word combinations, they produced a low number of morphologically complete sentence (only non-finite clauses), and they had greater difficulties in expressing sentences in a grammatically correct form. Subject-Verb-Indirect Object and Subject-Verb-Direct Object were the most frequent multi-argument structures for both groups.