There have been several reports in the literature of faster visual lexical decisions to words that are semantically ambiguous. All current models of this ambiguity advantage assume that it is the presence of multiple unrelated meanings that produce this benefit. A set of three lexical decision experiments reported here challenge this assumption. We contrast the ambiguity seen in words like bark, which have multiple unrelated meanings, with words that have multiple related word senses (e.g., twist). In all three experiments we find that while multiple word senses do produce faster responses, ambiguity between multiple meanings delays recognition. These results suggest that, while competition between the multiple meanings of ambiguous words delays their recognition, the rich semantic representations associated with words with many senses facilitate their recognition. © 2002 Elsevier Science (USA) Key Words: lexical ambiguity; polysemy; distributed semantic representations.Many words are semantically ambiguous, and can refer to more than one concept. For example, bark can refer either to a part of a tree or to the sound made by a dog. To understand such words, we must select one of these different interpretations, normally on the basis of the context in which the word occurs.Words can be ambiguous in different ways; a word like bark has two semantically unrelated meanings, which seem to share the same written and spoken form purely by chance. More common than this type of accidental ambiguity is the systematic ambiguity between related word senses. For example, the word twist has a range of dictionary definitions including to make into a coil or spiral, to operate by turning, to alter the shape of, to misconstrue the meaning of, to wrench or sprain, and to squirm or writhe. The meaning of this word varies systematically according to the context in which the word is used; for example, there are important differences between what it means to twist an ankle compared with to twist the truth. However, although the meaning of the word is ambiguous between these different interpretations, the interpretations are closely related to each other both etymologically and semantically; this is quite unlike the ambiguity for a word like bark.

The integration of a newly learned spoken word form with existing knowledge in the mental lexicon is characterized by the word form's ability to compete with similar-sounding entries during auditory word recognition. Here we show that although the mere acquisition of a spoken form is swift, its engagement in lexical competition requires an incubation-like period that is crucially associated with sleep. Words learned at 8 p.m. do not induce (inhibitory) competition effects immediately, but do so after a 12-hr interval including a night's sleep, and continue to induce such effects after 24 hr. In contrast, words learned at 8 a.m. do not show such effects immediately or after 12 hr of wakefulness, but show the effects only after 24 hr, after sleep has occurred. This time-course dissociation is best accommodated by connectionist and neural models of learning in which sleep provides an opportunity for hippocampal information to be fed into long-term neocortical memory.

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In this paper we present a novel theory of the cognitive and neural processes by which adults learn new spoken words. This proposal builds on neurocomputational accounts of lexical processing and spoken word recognition and complementary learning systems (CLS) models of memory. We review evidence from behavioural studies of word learning that, consistent with the CLS account, show two stages of lexical acquisition: rapid initial familiarization followed by slow lexical consolidation. These stages map broadly onto two systems involved in different aspects of word learning: (i) rapid, initial acquisition supported by medial temporal and hippocampal learning, (ii) slower neocortical learning achieved by offline consolidation of previously acquired information. We review behavioural and neuroscientific evidence consistent with this account, including a meta-analysis of PET and functional Magnetic Resonance Imaging (fMRI) studies that contrast responses to spoken words and pseudowords. From this meta-analysis we derive predictions for the location and direction of cortical response changes following familiarization with pseudowords. This allows us to assess evidence for learning-induced changes that convert pseudoword responses into real word responses. Results provide unique support for the CLS account since hippocampal responses change during initial learning, whereas cortical responses to pseudowords only become word-like if overnight consolidation follows initial learning.

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