Neighbourhood density effects in spoken word production

Most theories of speech production postulate that in the process of producing words, representations that are similar to the target word with respect to its meaning and, for some theories, in terms of form, also become active. However, in previous speech production research, findings are still inconclusive with regards to the influence of the number of these neighbours, with data supporting the full range of possibilities - no effect, inhibitory and facilitatory effects. This thesis investigates what constitutes a word’s meaning and form neighbourhood in the lexicon and whether and how this neighbourhood might affect the production of the intended target word. Paper 1 is a comparison of several measures of semantic neighbourhood density, and an investigation of their influence on the picture naming performance of English monolinguals and individuals with aphasia. While no effect was observed in monolinguals, a facilitation effect of semantic neighbours was observed in aphasic speakers. Paper 2 follows this up with a case study investigating the effects of different semantic neighbourhood measures on a facilitated naming paradigm involving two aphasic participants with different levels of impairment. Although inhibitory effects of semantic neighbourhood variables were observed for both participants at baseline, the change in each participant’s performance at post-test was affected differently and by distinct measures in each participant. In Paper 3, the picture naming performance of English monolinguals and late French-English bilinguals is investigated with respect different types of phonological neighbourhood density and frequency measures , including a novel phonological neighbourhood density metric adapted to these bilingual speakers. Results showed, for both speaker groups, an interaction between phonological neighbourhood measures and the target word’s frequency, with inhibitory effects on low frequency targets and facilitatory effects on the most frequent targets, frequency being affected, for bilinguals, by language experience and the overlap between the target and its translation equivalent. The complex patterns of facilitation and inhibition coming from neighbours that were found across this thesis are discussed in light of the main theories of lexical access in speech production, and are best explained within theories that assume interactivity between the different levels of representation.