Human listeners achieve quick and effortless speech comprehension in daily life and can adopt new words easily into their vocabulary. However, the underlying mechanisms under spoken word recognition and learning remain to be better understood. This thesis examines the neural and functional mechanisms of spoken word recognition and memory encoding by using a competitor priming paradigm - prior presentation (priming) of a competitor spoken word (e.g. hijack) is followed by the presentation of a similar sounding word sharing the same initial segments (e.g. hygiene). Consistent with the Bayes rule, the prior probability of the competitor word has been increased due to the earlier exposure, which can in turn change the perception or memory encoding of the target word.

The MEG study described in Chapter 2 examined the neural implementations of spoken word recognition by testing two distinct implementations of Bayes perceptual inference. Competitive-selection accounts (e.g. TRACE) propose direct competition between lexical units such that inhibition of irrelevant candidates leads to selection of critical words, while predictive-selection accounts (e.g. Predictive Coding) suggest that computations of prediction error by comparing heard and predicted speech sounds drive the update of lexical probabilities that are crucial to word recognition. The study results indicated that MEG signals localised to the superior temporal gyrus (STG) showed greater neural responses evoked by competitor primed words than unprimed words after the point at which they were uniquely identified (after /haidʒ/ in hygiene) and these stronger neural signals also correlated with the longer response times caused by competitor priming. These findings were more in line with the predictive neural mechanisms.

Chapter 3 reports studies that investigated lexical and sub-lexical processing during spoken word recognition, specifically whether changes in lexical prediction that give rise to the competitor priming effect (longer response times) continue to be observed even when word recognition is not required for task performance. Here, the pause detection task was compared with the lexical decision task in a set of experiments to direct participants’ attention to phonological processing or lexical processing respectively during the perception of prime or target items. The findings showed opposite effects of these two kinds of processing, with the competitor priming effect observable only when participants’ attention was on lexical processing, while phonological facilitatory effect was observed when the pause detection task was used, and that prime item was presented with pause inserted. These results were in accordance with the Predictive Coding account and the Distributed Cohort Model, as both of which support inhibitory lexical processing and facilitatory sub-lexical processing in their respective structures.

Chapter 4 describes tasks and analyses that examined the effect of competitor priming on spoken word memory encoding by using additional recognition memory data collected from the same experiments as reported in Chapter 2 and 3. Participants’ memory performance was measured by how accurately they could distinguish previously heard items from foils. The findings indicated that enhanced prediction error caused by competitor priming facilitated memory encoding of words when the encoding was repeated multiple times and involved deeper lexical-semantic processing. These findings were consistent with the PIMMS framework, which proposes that prediction error caused by the competitor priming effect should improve memory encoding. Moreover, subsequent memory analyses of the MEG data (as reported in Chapter 2) showed pseudoword encoding localised to the medial temporal lobe, consistent with the initial rapid encoding stage of novel word learning in the complementary learning systems.

In conclusion, the thesis provides evidence for a unified account of computations of prediction error which supports spoken word recognition and memory encoding while also shows that the effects of lexical and sub-lexical processing are dissociated during these two processes.