Rapid learning of new words is crucial for language acquisition, and frequent exposure to spoken words is a key factor for the development of vocabulary. More frequently occurring (and thus more familiar) words can, in turn, be expected to have stronger memory representations than less frequent words. The neural mechanisms underlying these representations are, however, largely obscure. Even less is known about the mechanisms related to the initial acquisition of new word-forms and build-up of lexical representations. The current thesis investigated how the neural traces are activated when known and novel spoken words are perceived, and how they can be formed when novel words are first encountered and repeated. The neural processes of word memory-trace activation and rapid formation were studied in adults and children using event-related potentials. In adults, words with high frequency of occurrence elicited greater neural responses than low frequency words or meaningless pseudo-words already at ~120 ms after the time when they could be identified. Higher frequency words activated predominantly left frontal and anterior temporal cortices while the low frequency and pseudo-words showed a more bilateral temporal cortex activity. Neural dynamics during brief exposure to novel word-forms showed a rapid response increase at ~50 ms. This enhancement was associated with behaviourally-established memory performance on the novel words, confirming the relation of this neural dynamics to word learning. The enhancement, originating in the left inferior frontal and posterior temporal cortical sources, was specific to phonologically native word-forms and, furthermore, independent of whether the spoken sounds were ignored or attended to, suggesting a high degree of automaticity in native word-form acquisition. For novel word-forms with non-native phonology, such a response enhancement was not significant, while the response to known words attenuated over exposure, likely reflecting repetition-related suppression. Furthermore, individual language experience influenced the neural learning dynamics such that greater number of previously acquired non-native languages with earlier average age of acquisition predicted larger response enhancement to novel non-native word-forms whereas later average age of acquisition predicted greater increase to attended novel native words. Finally, a rapid response increase to an ignored novel native word-form in brief exposure was also observed in school-age children, and was underpinned predominantly by left prefrontal cortex and associated with writing accuracy. Remarkably, children with dyslexia failed to show such neural dynamics, suggesting deficient mechanism for automatic spoken word acquisition in dyslexia, a finding potentially relevant for further clinical research. In sum, the results suggest that exposure is key in defining the strength of perisylvian memory traces for words that can be formed rapidly and automatically in adults and typically developing children.
|Place of Publication||Helsinki|
|Publication status||Published - 2017|
|MoE publication type||G5 Doctoral dissertation (article)|
Fields of Science
- 6162 Cognitive science
- 515 Psychology
- 3112 Neurosciences