The Age-of-Acquisition Effect in Patients with Language Impairments

The age of acquisition (AoA) effect refers to the finding that words learned early in life are processed faster than words learned later in life. The influence of AoA has been found using many tasks, including picture naming, lexical decision, and eye-tracking methodology (Juhasz, 2005). Patients with language impairments such as aphasia, Alzheimer’s disease, dyslexia, dysgraphia, and those who have received an anterior temporal lobectomy are all influenced by AoA. The words that these patients successfully produce are often early-acquired, and when an error is committed, it is usually the result of a failure to retrieve a later-acquired word, suggesting that early-acquired words are better preserved in the mental lexicon than later-acquired words. The robust AoA effects found in these patient populations suggest that it could be an important tool for making more accurate diagnoses and constructing more effective treatment plans. The studies described in this review also shed light on a possible locus or loci of AoA in the mental lexicon. Through close examination of the performance of these patients on various tasks, it appears that AoA either resides in the semantic system, therefore supporting Steyvers and Tenenbaum (2005)’s “semantic hub” model or AoA could represent a general learning mechanism which is explained by Ellis and Lambon Ralph (2000)’s connectionist model and network plasticity. AoA in Patient Populations 2 Introduction The ability to use language to communicate is a skill that readily develops shortly after birth in humans. Children utter their first word between the ages of ten to fifteen months and rapidly develop sophisticated language by the age of five (Siegler, Deloache, & Eisenberg, 2011). From this point on, children into adolescence and adulthood continue to add more and more words to their mental lexicon, which is the term used to describe the organization of the language processing system (Siegler et al., 2011). The three domains of the mental lexicon include orthography (word form), phonology (sound), and semantics (meaning). Retrieving a word from the mental lexicon (i.e. lexical access) is a somewhat taxing process involving recognition of the word form or picture, understanding, and production. A disruption anywhere along the line can result in failure to retrieve the correct word. Kittredge, Dell, Verkuilen, and Schwartz (2008) describe one model of lexical production consisting of two major steps: First, a holistic lexical representation corresponding to a specific concept is accessed and selection of the appropriate lexical unit occurs. Second, phonological characteristics of the word are accessed. It is thought that semantic errors, such as uttering “duck” instead of “swan” when looking at a picture of a swan, result from a disruption at the first step, whereas mispronunciations are the result of a disruption in the second step (Kittredge et al., 2008). Levelt, Roelofs, and Meyer (1999) cited by Kittredge et al. (2008) suggested a model in which speech production occurs in three steps, rather than two. Their model is similar to that of Kittredge et al.’s, but it includes a “lemma” access stage between the semantic representations and phonological representations. Selected for at the lemma level are grammatical aspects