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Purpose: Child language acquisition is marked by an optional infinitive period (ages 2-4 years) during which children use nonfinite (infinitival) verb forms and finite verb forms interchangeably in grammatical contexts that require finite forms. In English, children's errors include omissions of past tense /-ed/ and 3rd-person singular /-s/. This language acquisition period typically ends by the age of 4 years, but it persists in children with language impairments. It is unknown if adults still process optional infinitives differently than other kinds of morphosyntax errors.
Method: We compared behavior and functional brain activation during grammaticality judgments across sentences with developmental optional infinitive tense/ agreement errors ("Yesterday I play the song"), nondevelopmental agreement errors ("He am tall") that do not occur in typical child language acquisition, and grammatically correct sentences.
Results: Adults (N = 25) were significantly slower and less accurate in judging sentences with developmental errors relative to other sentences. Sentences with developmental errors yielded greater activation in bilateral inferior frontal gyri relative to nondevelopmental error sentences in both auditory and visual modalities.
Conclusions: These findings suggest that the heightened computational demands for finiteness extend well beyond early childhood and continue to exert their influence on grammatical mental and brain function in adulthood.
During the course of language acquisition, children go through periods when certain grammatical computations seem exceedingly complex, resulting in systematic errors in child language production. While it is generally assumed that periods of language acquisition might depend on the child's learning experiences, cognitive abilities, and brain maturation (Newport, 1990; Wexler, 2003, 2011), little is known about the neural bases of these developmental periods and whether they leave a lasting impact on adult language processing. Child language acquisition in many languages, including English, Dutch, German, French, and others, is marked by an optional infinitive (OI) period (ages 2-4 years) during which children use nonfinite (infinitival) verb forms and finite (marked for tense) verb forms interchangeably in grammatical contexts that require finite forms (Schütze & Wexler, 1996; Wexler, 1994, 1998, 2003). In the English sentence, "Yesterday we baked cookies," the finite verb form baked is used appropriately, whereas in the sentence, "Yesterday we bake cookies," the nonfinite form bake is used erroneously. During the OI period of development, children interchangeably use both the correct...