Verbal communication relies heavily on the ability to effortlessly produce intended words to express a meaning. This capacity is frequently impaired in individuals with aphasia, and impairment often lasts well into the chronic stages. However, the nature of anomia can vary. Phonological neighborhood density (PND) is one feature of words which has been shown to impact the ease of retrieval in speakers with aphasia; words with more similar-sounding neighbors are easier to retrieve because the neighbors help activate the target. However, it is unclear how different types of lexical access breakdowns affect the impact of PND. The aim of this project was to analyze the relationship between word retrieval accuracy, speech error patterns, and PND in individuals with aphasia. Twenty-two participants with various types and severities of aphasia named 200 single-syllable line drawings. WebFit, an online software program designed to fit naming data to a theoretical model of word retrieval, was used to characterize participants' error patterns by calculating the strength of connections within the lexicon, as well as the rate of decay. Analyses confirmed previous findings that participants with all types of breakdown achieved lower rates of overall accuracy. Weaker connections between semantic knowledge and words resulted in a more errors that were close to the target, relative to errors with no relationship to the target. Individuals with more severe impairments of the semantic-lexical connections and the lexical-phonological connections produced words with many neighbors more accurately than words with fewer neighbors. Implications for initial therapy target selection and directions for further research are discussed.

A central issue in word recognition is how readers retrieve and select the right representation among others in the mental lexicon. Recently, it has been claimed that recognition of individual words is influenced by the degree to which the words possess unique vs. shared letters or sounds relative to other words, that is, whether the words have few or several neighbors. Research on so-called neighborhood density effects advances understanding of the organization and operation of the mental lexicon. Orthographic neighborhood effects have been claimed to be facilitative, but recent studies of visual word recognition have led to a revised understanding of the nature of the orthographic neighborhood density effect. Through a reexamination of orthographic and phonological neighborhood density effects, the specific objective of the present research is to understand how orthographic and phonological representations interact across two different writing systems, i.e., English (an alphabetic orthography) and Chinese (a morphosyllabic orthography). The phenomena were studied using a joint behavioral (lexical decision) and neural imaging approach (near infrared spectroscopy, or NIRS). Orthographic and phonological (more, specifically, homophone) neighborhood density were manipulated in three lexical decision experiments with English and three with Chinese readers. After different sources of facilitative inter-lexicon connections were controlled, orthographic and phonological neighborhood density effects were found to be inhibitory in both writing systems. Inhibitory neighborhood density effects were also confirmed in two NIRS experiments of English and Chinese. The present research provided a better control of lexical characteristics than was the case in previous research on neighborhood effects and found a clear and consistent pattern of neighborhood density effects. This research supports interactive-activation models of word recognition rather than parallel-distributed models, given the evidence for lateral inhibition indexed by inhibitory neighborhood density effects. As such, the present study furthers the understanding of the organization and operation of the mental lexicon.