Several definitions exist that offer to identify the boundaries between languages and dialects, yet these distinctions are inconsistent and are often as political as they are linguistic (Chambers & Trudgill, 1998). A different perspective is offered in this thesis, by investigating how closely related linguistic varieties are represented in the brain and whether they engender similar cognitive effects as is often reported for bilingual speakers of recognised independent languages, based on the principles of Green’s (1998) model of bilingual language control. Study 1 investigated whether bidialectal speakers exhibit similar benefits in non-linguistic inhibitory control as a result of the maintenance and use of two dialects, as has been proposed for bilinguals who regularly employ inhibitory control mechanisms, in order to suppress one language while speaking the other. The results revealed virtually identical performance across all monolingual, bidialectal and bilingual participant groups, thereby not just failing to find a cognitive control advantage in bidialectal speakers over monodialectals/monolinguals, but also in bilinguals; adding to a growing body of evidence which challenges this bilingual advantage in non-linguistic inhibitory control. Study 2 investigated the cognitive representation of dialects using an adaptation of a Language Switching Paradigm to determine if the effort required to switch between dialects is similar to the effort required to switch between languages. The results closely replicated what is typically shown for bilinguals: Bidialectal speakers exhibited a symmetrical switch cost like balanced bilinguals while monodialectal speakers, who were taught to use the dialect words before the experiment, showed the asymmetrical switch cost typically displayed by second language learners. These findings augment Green’s (1998) model by suggesting that words from different dialects are also tagged in the mental lexicon, just like words from different languages, and as a consequence, it takes cognitive effort to switch between these mental settings. Study 3 explored an additional explanation for language switching costs by investigating whether changes in articulatory settings when switching between different linguistic varieties could - at least in part – be responsible for these previously reported switching costs. Using a paradigm which required participants to switch between using different articulatory settings, e.g. glottal stops/aspirated /t/ and whispers/normal phonation, the results also demonstrated the presence of switch costs, suggesting that switching between linguistic varieties has a motor task-switching component which is independent of representations in the mental lexicon. Finally, Study 4 investigated how much exposure is needed to be able to distinguish between different varieties using two novel language categorisation tasks which compared German vs Russian cognates, and Standard Scottish English vs Dundonian Scots cognates. The results showed that even a small amount of exposure (i.e. a couple of days’ worth) is required to enable listeners to distinguish between different languages, dialects or accents based on general phonetic and phonological characteristics, suggesting that the general sound template of a language variety can be represented before exact lexical representations have been formed. Overall, these results show that bidialectal use of typologically closely related linguistic varieties employs similar cognitive mechanisms as bilingual language use. This thesis is the first to explore the cognitive representations and mechanisms that underpin the use of typologically closely related varieties. It offers a few novel insights and serves as the starting point for a research agenda that can yield a more fine-grained understanding of the cognitive mechanisms that may operate when speakers use closely related varieties. In doing so, it urges caution when making assumptions about differences in the mechanisms used by individuals commonly categorised as monolinguals, to avoid potentially confounding any comparisons made with bilinguals.

This thesis describes basic research into visual word recognition and decision making. Determining the best matching lexical representation for a given stimulus involves interactions between representations. The standard task for studying these processes is the lexical decision task (LDT), but there is still debate regarding the factors that affect how individuals make lexical decisions. The nature of lexical interactions and the processes underlying lexical decision-making were addressed here by testing response congruency effects in the masked priming variant of the LDT. The results of seven masked priming experiments showed a robust response congruency effect that depends on the difficulty of the word-nonword discrimination. This finding resolved apparent inconsistencies in previous research. The experiments were simulated using the Bayesian Reader and the Spatial Coding Model (SCM). The probability based Bayesian Reader model failed to accommodate the findings. However, a good fit to the data was provided by a modified version of the SCM in which the assumptions regarding the nature of lexical interactions were changed such that word nodes inhibit only (closely) related competitors. The model also assumes that the difficulty of the word-nonword discrimination affects the degree to which stimulus typicality informs lexical decisions. A critical issue for these experiments involved the definition of orthographic typicality. An algorithm for measuring orthographic typicality and for generating nonwords with a specific level of orthographic typicality (OT3) was developed. An unprimed LDT experiment showed that OT3 affected decision latency even when other standard measures of orthographic typicality were controlled. Two additional masked priming experiments showed that highly typical primes lead to faster word responses and slower nonword responses than less typical primes. Overall, the results of this research enhance our understanding of the processes underlying visual word recognition and lexical decision making, and also have important methodological implications for the field.