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Words are the backbone of language activity. An average 20-year-old native speaker of English will have a vocabulary of about 42,000 words. These words are connected with one another within the larger network of lexical knowledge that is termed the mental lexicon. The metaphor of a mental lexicon has played a central role in the development of theories of language and mind and has provided an intellectual meeting ground for psychologists, neurolinguists, and psycholinguists. Research on the mental lexicon has shown that lexical knowledge is not static. New words are acquired throughout the life span, creating very large increases in the richness of connectivity within the lexical system and changing the system as a whole. Because most people in the world speak more than one language, the default mental lexicon may be a multilingual one. Such a mental lexicon differs substantially from a lexicon of an individual language and would lead to the creation of new integrated lexical systems due to the pressure on the system to organize and access lexical knowledge in a homogenous manner. The mental lexicon contains both word knowledge and morphological knowledge. There is also evidence that it contains multiword strings such as idioms and lexical bundles. This speaks in support of a nonrestrictive “big tent” view of units of representation within the mental lexicon. Changes in research on lexical representations in language processing have emphasized lexical action and the role of learning. Although the metaphor of words as distinct representations within a lexical store has served to advance knowledge, it is more likely that words are best seen as networks of activity that are formed and affected by experience and learning throughout the life span.

Over the past decades, psycholinguistic aspects of word processing have made a considerable impact on views of language theory and language architecture. In the quest for the principles governing the ways human speakers perceive, store, access, and produce words, inflection issues have provided a challenging realm of scientific inquiry, and a battlefield for radically opposing views. It is somewhat ironic that some of the most influential cognitive models of inflection have long been based on evidence from an inflectionally impoverished language like English, where the notions of inflectional regularity, (de)composability, predictability, phonological complexity, and default productivity appear to be mutually implied. An analysis of more “complex” inflection systems such as those of Romance languages shows that this mutual implication is not a universal property of inflection, but a contingency of poorly contrastive, nearly isolating inflection systems. Far from presenting minor faults in a solid, theoretical edifice, Romance evidence appears to call into question the subdivision of labor between rules and exceptions, the on-line processing vs. long-term memory dichotomy, and the distinction between morphological processes and lexical representations. A dynamic, learning-based view of inflection is more compatible with this data, whereby morphological structure is an emergent property of the ways inflected forms are processed and stored, grounded in universal principles of lexical self-organization and their neuro-functional correlates.

Neurolinguistic approaches to morphology include the main theories of morphological representation and processing in the human mind, such as full-listing, full-parsing, and hybrid dual-route models, and how the experimental evidence that has been acquired to support these theories uses different neurolinguistic paradigms (visual and auditory priming, violation, long-lag priming, picture-word interference, etc.) and methods (electroencephalography [EEG]/event-related brain potential [ERP], functional magnetic resonance imaging [fMRI], neuropsychology, and so forth).

We summarize findings from across a range of methods, including behavioral measures of overall processing speed and accuracy, electrophysiological indices that tap into the early time course of language processing, and neural measures using structural and functional imaging. We argue that traditional claims about rigid constraints on the ability of late bilinguals to exploit the meaning and form of the morphology and morphosyntax in a second language should be revised so as to move away from all or none command of structures motivated from strict dichotomies among linguistic categories of morphology. We describe how the dynamics of morphological processing in neither monolingual or bilingual speakers is easily characterized in terms of the potential to decompose words into their constituent morphemes and that morphosyntactic processing is not easily characterized in terms of categories of structures that are learnable and those that are unlearnable by bilingual and nonnative speakers. Instead, we emphasize the high degree of variability across individuals and plasticity within individuals in their ability to successfully learn and use even subtle aspects of a second language. Further, both of the bilingual’s two languages become active when even one language is engaged, and parallel activation has consequences that shape both languages, thus their influence is not in the unidirectional manner that was traditionally assumed. We briefly discuss the nature of possible constraints and directions for future research.

Research in neurolinguistics examines how language is organized and processed in the human brain. The findings from neurolinguistic studies on language can inform our understanding of the basic ingredients of language and the operations they undergo. In the domain of the lexicon, a major debate concerns whether and to what extent the morpheme serves as a basic unit of linguistic representation, and in turn whether and under what circumstances the processing of morphologically complex words involves operations that identify, activate, and combine morpheme-level representations during lexical processing. Alternative models positing some role for morphemes argue that complex words are processed via morphological decomposition and composition in the general case (full-decomposition models), or only under certain circumstances (dual-route models), while other models do not posit a role for morphemes (non-morphological models), instead arguing that complex words are related to their constituents not via morphological identity, but either via associations among whole-word representations or via similarity in formal and/or semantic features. Two main approaches to investigating the role of morphemes from a neurolinguistic perspective are neuropsychology, in which complex word processing is typically investigated in cases of brain insult or neurodegenerative disease, and brain imaging, which makes it possible to examine the temporal dynamics and neuroanatomy of complex word processing as it occurs in the brain. Neurolinguistic studies on morphology have examined whether the processing of complex words involves brain mechanisms that rapidly segment the input into potential morpheme constituents, how and under what circumstances morpheme representations are accessed from the lexicon, and how morphemes are combined to form complex morphosyntactic and morpho-semantic representations. Findings from this literature broadly converge in suggesting a role for morphemes in complex word processing, although questions remain regarding the precise time course by which morphemes are activated, the extent to which morpheme access is constrained by semantic or form properties, as well as regarding the brain mechanisms by which morphemes are ultimately combined into complex representations.