The developmental approach taken here combines what we already know about morpho-syntactic deficits in DLD with recent developments in the fields of linguistics and language acquisition. First, it is possible to divide phonological and morphological patterns into three pattern types (Single Feature, OR/Disjunction, Family Resemblance/Prototype), with these types having a long history of study in visual pattern learning. Importantly, children with DLD appear to have maximum difficulty with morpho-syntactic patterns of the OR type (e.g., regular past tense; add a "t" if the final consonant is voiceless OR a "d" if the final consonant is voiced). In contrast, studies using artificial grammars show that infants who are typically developing are highly adept at learning Single Feature and OR pattern types; Family Resemblance patterns may be weaker. Family resemblance patterns relate to similarity, and may be more closely tied to the lexicon than to grammar (e.g., beagles and terriers are similar and they are both dogs; the words "goat" and "boat" are similar). Typical adults are adept at Single Feature and Family Resemblance patterns, but appear to be, at least superficially, more like children with DLD in their performance on the OR pattern.

With these intriguing findings as a starting point, the proposed research links phonological and morphological sequence learning in children with developmental language disorder (DLD). In the studies focused on children, we ask if 4- to 6-year-olds who are typically developing as well as those with DLD (both with and without co-occurring speech sound deficits) and speech sound disorder (SSD; without co-occurring morpho-syntactic deficits) are sensitive to input examples that fit one of these three patterns. Our hypothesis is that, consistent with their long-documented morpho-syntactic deficit, children with DLD will have particular difficulty with the OR pattern. Consistent with their intact morpho-syntactic skills, children with SSD should show no deficits in the OR pattern, revealing a link between OR pattern learning, phonology, and morpho-syntax. Our endpoint measure is that children with DLD (both with and without an accompanying SSD) do not show sensitivity to the OR pattern, as measured by sound pattern (e.g., phonetic accuracy, syllable sequence stability) and motor learning (e.g., articulatory and acoustic variability) measures.

This hypothesis is assessed by indexing the learning trajectory across the training period and during generalization to new nonwords that are consistent or inconsistent with the training pattern. In the generalization phase, children are asked to name new novel words that are either consistent (i.e., that fit the training rule) or inconsistent (i.e., that do not fit the training rule) nonwords. It is predicted that children with TD and SSD, who are still in a period of language learning plasticity (they can still learn the sound patterns and grammar of a second language) will be sensitive to all three patterns. However, children with DLD (both with and without accompanying SSD) are predicted to show sensitivity to the family resemblance and single feature patterns, but not to the critical OR pattern-the one associated with sound pattern and grammatical learning, This would reveal an important connection between sound pattern and grammatical learning, which could lead to earlier diagnosis (phonological patterns emerge earlier than grammar) and to more general intervention practices that focus on sequential pattern learning rather than specific language goals may be efficacious.

A second aim explores whether the inclusion of a semantic subcategory cue facilitates learning the OR pattern. The hypothesis is that children with DLD (as well as typical adults) who initially are predicted to have the most difficulty with the OR pattern, will show increases in sensitivity following the addition of a semantic category cue (e.g., animals for the voiced consonant rule OR tools for the voiceless consonant rule). This result would be especially important for learners with DLD, as it may provide a pathway for intervention of a core learning deficit.

Critically, these studies are developmental in nature. In the adult component of this research, we ask if adults are sensitive to input examples that fit one of these three patterns. Our hypothesis, based on previous findings from the Gerken lab, is that adults, like children with DLD, do not show sensitivity to the OR pattern. However, adults are sensitive to single feature and Family Resemblance patterns. Adults still learn words readily and are sensitive to family resemblance prototype relationships. For adults, perceptual measures index sensitivity. Similar to the children with typical development, DLD and SSD, we explore whether the inclusion of a semantic subcategory cue facilitates learning the OR pattern in adults. The hypothesis is that all learners, including adults, will show increases in sensitivity following the addition of a semantic category cue (e.g., animals for voiced rule OR tools for voiceless rule). The findings from adults contribute to a dynamic developmental framework. Because of the dramatic developmental changes in sensitivity to phonological sound patterns and the presumed linkage of such patterns to morpho-syntax, the data from adults are essential in identifying the underlying mechanism(s) of DLD and to suggesting possible intervention strategies, such as employing semantic cues to the OR pattern and strengthening lexical organization.

From previous work, we know that infants are sensitive to the OR rule. In the studies focused on toddlers, we ask if dependence on the associatively organized lexicon can account for the infant-to-adult developmental changes observed for the OR pattern (which is not associatively organized) and thereby explores the possibility that children with DLD rely on their lexicons to compensate for their sequential pattern learning deficit. The hypothesis is that 20-month-old children, who are just entering a period of rapid vocabulary learning, will show increased sensitivity (based on looking time to words that are consistent vs. inconsistent within the training set) in the family resemblance condition when a visual referent (i.e., semantic cue) is added, but decreased sensitivity in the OR condition when a visual referent is added. These results will also be related to vocabulary size as reported on the CDI, with the prediction that robust patterns are smaller for children who received referents or who have smaller vocabularies for the OR pattern and greater for children who received referents or who have larger vocabularies for the family resemblance pattern. The results from toddlers, who are just entering a period of rapid vocabulary learning, provide an essential framework for understanding how different types of phonological patterns apply to lexical and morpho-syntactic learning, The results of the proposed study provide a critical developmental framework for identifying the underlying mechanism(s) of DLD. These findings will also lead to the development of intervention strategies, such as strengthening lexical organization, to support problematic aspects of sequential pattern learning.