The mental lexicon is a component of the human language faculty that contains information regarding the composition of words, such as their meanings, pronunciations, and syntactic characteristics. The mental lexicon is used in linguistics and psycholinguistics to refer to individual speakers' lexical, or word, representations. However, there is some disagreement as to the utility of the mental lexicon as a scientific construct.
The mental lexicon differs from the lexicon more generally in that it is not just a collection of words; instead, it deals with how those words are activated, stored, processed, and retrieved by each speaker/hearer. Furthermore, entries in the mental lexicon are interconnected with each other on various levels. An individual's mental lexicon changes and grows as new words are learned and is always developing, but there are several competing theories seeking to explain exactly how this occurs. Some theories about the mental lexicon include the spectrum theory, the dual-coding theory, Chomsky's nativist theory, as well as the semantic network theory. Neurologists and neurolinguists also study the areas of the brain involved in lexical representations. The following article addresses some of the physiological, social, and linguistic aspects of the mental lexicon.
Recent studies have also shown the possibility that the mental lexicon can shrink as an individual ages, limiting the number of words they can remember and learn. The development of a second mental lexicon (L2) in bilingual speakers has also emerged as a topic of interest, suggesting that a speaker's multiple languages are not stored together, but as separate entities that are actively chosen from in each linguistic situation.
Methods of inquiry Although the mental lexicon is often called a mental "dictionary", in actuality, research suggests that it differs greatly from a dictionary. For example, the mental lexicon is not organized alphabetically like a dictionary; rather, it seems to be organized by links between phonologically and semantically related lexical items. This is suggested by evidence of phenomena such as slips of the tongue, for instance replacing anecdote with antidote.
While dictionaries contain a fixed number of words to be counted and become outdated as language is continually changing, the mental lexicon consistently updates itself with new words and word meanings, while getting rid of old, unused words. The active nature of the mental lexicon makes any dictionary comparison unhelpful. Research is continuing to identify the exact way that words are linked and accessed. A common method to analyze these connections is through a lexical decision task, in which participants are required to respond as quickly and accurately as possible to a string of letters presented on a screen to say if the string is a non-word or a real word.
Model of the mental lexicon
In the sample model of the mental lexicon pictured to the right, the mental lexicon is split into three parts under a hierarchical structure: the concept network (semantics), which is ranked above the lemma network (morphosyntax), which in turn is ranked above the phonological network. Working in tandem with the mental lexicon, in particular with the phonological network, is the mental syllabary, which is responsible for activating articulatory gestures in response to the phonological network. According to the theory which this diagram illustrates, different components both within and outside of the mental lexicon are linked together by neural activations called S-pointers, which form pathways together with large clusters of neurons called buffers (e.g. "concept production" and "word audio" in the diagram).
Theories and perspectives
One theory about the mental lexicon states that it organizes our knowledge about words "in some sort of dictionary." Another states that the mental lexicon is "a collection of highly complex neural circuits". The latter, semantic network theory, proposes the idea of spreading activation, which is a hypothetical mental process that takes place when one of the nodes in the semantic network is activated, and proposes three ways this is done: priming effects, neighborhood effects, and frequency effects, which have all been studied in depth over the years.
Priming is a term used in lexical decision tasks that accounts for decreased reaction times of related words. Interchangeable with the word "activation" in many cases, priming refers to the ability to have related words assist in the reaction times of others. In the example above, the word bread "primed" butter to be retrieved faster. Neighborhood effects refer to the activation of all similar "neighbors" of a target word. Neighbors are defined as items that are highly confusable with the target word due to overlapping features of other words. An example of this would be that the word "game" has the neighbors "came, dame, fame, lame, name, same, tame, gale, gape, gate, and gave," giving it a neighborhood size of 11 because 11 new words can be constructed by only changing 1 letter of "game". The neighborhood effect claims that words with larger neighborhood sizes will have quicker reaction times in a lexical decision task because neighbors facilitate the activation of other neighborhood words. Frequency effects suggest that words that are frequent in an individual's language are recognized faster than words that are infrequent. Forster and Chambers, 1973, found that high frequency words were named faster than low frequency ones, and Whaley, 1978 found that high frequency words were responded to faster than low frequency ones in a lexical decision task.
In the spectrum theory, at one end "each phonological form is connected to one complex semantic representation", at the opposite end, homonyms and polysemes have their "own semantic representation[s]". The middle of the spectrum contains the theories that "suggest that related senses share a general or core semantic representation". The "dual coding theory (DCT)" contrasts multiple and common coding theories. DCT is "an internalized nonverbal system that directly represents the perceptual properties and affordances of nonverbal objects and events, and an internalized verbal system that deals directly with linguistic stimuli and responses". Others work around Chomsky's theory that "all syntactic and semantic features are included directly in the abstract mental representation of a lexical word".
Recent models of the mental lexicon focus more on how a single cognitive function is processed across multiple neural systems instead as a single process under the influence of multiple neural systems. Connectionism sees lexical entries as the emergent properties of patterns of neural activation created through experience, rather than as discrete representations stored in memory; this leads to a better understanding of how neighbourhood size and frequency sensitivity are related. It also provides an alternative explanation for the way people use rules to access words compared to looking them up in a dictionary format. Hybrid models incorporate the distributed approach to learning along with more localised representations for common (high-frequency) or irregular forms of language. Thus, hybrid models offer a single explanation for both rule-based language behaviours as well as item-based language behaviours.
Alternative theories Not all linguists and psychologists support the mental lexicon's existence and there is much controversy over the concept. In a 2009 article, Jeffrey Elman proposes that the mental lexicon does not exist at all. Elman suggests that because context, both linguistic and nonlinguistic, is fundamentally inseparable from language, the human mind should be viewed more holistically when discussing the storage of lexical information. In Elman's view, this is a more realistic approach than assuming that the mental lexicon stores every minute contextual detail about every single lexical item. Elman states that words are not observed "as elements in a data structure" that are "retrieved from memory, but rather as stimuli that alter mental states".
Development
First language development
One aspect of research on the development of the mental lexicon has focused on vocabulary growth. Converging research suggests that at least English-speaking children learn several words a day throughout development. The figure on the right illustrates the growth curve of a typical English-speaking child's vocabulary size.
The words acquired in the early stages of language development tend to be nouns or nounlike, and there are some similarities in first words across children (e.g., mama, daddy, dog). Fast mapping is the idea that children may be able to gain at least partial information about the meaning of a word from how it is used in a sentence, what words it is contrasted with, as well as other factors. This allows the child to quickly hypothesize about the meaning of a word.
Research suggests that, despite the fast mapping hypothesis, words are not just learned as soon as we are exposed to them, each word needs some type of activation and/or acknowledgement before it is permanently and effectively stored. For young children, the word may be accurately stored in their mental lexicon, and they can recognize when an adult produces the incorrect version of the word, but they may not be able to produce the word accurately.
As a child acquires their vocabulary, two separate aspects of the mental lexicon develop, named the lexeme and the lemma. The lexeme is defined as the part of the mental lexicon that stores morphological and formal information about a word, such as the different versions of spelling and pronunciation of the word. The lemma is defined as the structure within the mental lexicon that stores semantic and syntactic information about a word, such as part of speech and the meaning of the word. Research has shown that the lemma develops first when a word is acquired into a child's vocabulary, and then with repeated exposure the lexeme develops.
Retrieval efficiency also changes during lexical growth. Eye-tracking studies and reaction time studies indicate that children with faster word processing (at 18 months of age) have larger vocabularies when they reach preschool age. Therefore, it appears that lexical growth is influenced by both the ability to learn new words and the improved connection between lexical representations within a given amount of time. Repeated exposure has resulted in the consolidation of both semantic and phonological aspects of words over a prolonged period resulting in an increase in efficiency of retrieving words.
Bilingual development
The development of the mental lexicon in bilingual children has increased in research over recent years, and has shown many complexities including the notion that bilingual speakers contain additional and separate mental lexicons for their other languages. Selecting between two or more different lexicons has shown to have benefits extending past language processes. Bilinguals significantly outperform their monolingual counterparts on executive control tasks. Researchers suggest that this enhanced cognitive ability comes from continually choosing between L1 and L2 mental lexicons. Bilinguals have also shown resilience against the onset of Alzheimer's disease, monolinguals being an average of 71.4 years old and the bilinguals 75.5 years old when symptoms of dementia were detected, a difference of 4.1 years.