The mental lexicon is defined as a mental dictionary that contains information regarding a word's meaning, pronunciation, syntactic characteristics, and so on.  Although this definition has been challenged over the years, this remains the most consistent definition of the term.
The mental lexicon is a construct used in linguistics and psycholinguistics to refer to individual speakers’ lexical, or word, representations. However, not all scientists agree as to the utility of the mental lexicon as a scientific construct.
The mental lexicon differs from the lexicon in that it is not just a general collection of words; instead, it deals with how those words are activated, stored, processed, and retrieved by each speaker. An individual’s mental lexicon changes and grows as new words are learned and is always developing, but there are several theorists that argue 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. Scientists also study the areas of the brain involved in lexical representations. The following 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 amount 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 in a more complex manner, with links between phonologically and semantically related lexical items. This is suggested by evidence of phenomena such as slips of the tongue, which showed that replacing words such as anecdote for antidote.
Also, while dictionaries contain a fixed number of words to be counted, and remain outdated as language is continually changing, the mental lexicon consistently updates itself with new words and word meanings, while getting rid of old, unused words.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.
Lexical Decision Tasks have been used for many years to access how the mental lexicon is structured. Participants in this task 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. Reaction times from this task indicate that certain words are more "active" in participants minds after related words have been presented. An example of this would be to present the word "bread" to the participant and then see an increased reaction time later to the word "butter". Since the word "bread" had activated all related words, including "butter", this increased reaction time demonstrates that related words are stored closely in the mental lexicon. By doing lexical decision tasks, researchers have been able to analyze what words are stored with what related counterparts, and what can activate these words.
Theories and perspectives
Not all linguists and psychologists believe in the mental lexicon’s existence and there is much controversy over the concept. 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 quicker.
- 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 suggesting that 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.” Similarly, we do not observe words “as elements in a data structure” that are “retrieved from memory, but rather as stimuli that alter mental states.” Others work around Chomsky’s theory that “all syntactic and semantic features are included directly in the abstract mental representation of a lexical word.”
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 children learn several words a day throughout development. The figure at left 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.
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.
Brain and the mental lexicon
The following are some hypotheses pertaining to semantic comprehension in the brain:
- Organized Unitary Content Hypothesis (OUCH): this hypothesis posits that lexical items that co-occur with high frequency are stored in the same area in the brain.
- Domain-Specific Hypothesis: this hypothesis uses the theory of evolution to posit that certain categories that have an evolutionary advantage over others (such as useful items like tools) have their specialized and functionally dissociated neural circuits in the brain.
- Sensory/Functional Hypothesis: this hypothesis posits that the ability to identify (i.e. be able to recognize and name) living things depends on visual information, while the ability to identify non-living things depends on functional information. Thus this hypothesis implies that modality-specific subsystems compose an overarching semantic network of lexical items.
Anomic aphasia, aphasia (expressive + receptive aphasia) and Alzheimer's disease can all affect recalling or retrieving words. Anomia renders a person completely unable to name familiar objects, places and people, a involves specific naming difficulties; sufferers of anomia have difficulties recalling words. Anomia is a lesser level of dysfunction, a severe form of the "tip-of-the-tongue" phenomenon where the brain cannot recall the desired word. Stroke, head trauma and brain tumors can cause anomia.
Expressive and receptive aphasia are neurological language disorders. Expressive aphasia limits the ability to convey thoughts through the use of speech, language or writing. Receptive aphasia affects a person's ability to comprehend spoken words, causing disordered sentences that have little or no meaning and which can include addition of nonce words.
Harry Whitaker states that Alzheimer’s disease patients are forgetful of proper names. Patients have difficulty generating names, especially with phonological tasks such as words starting with a certain letter. They also have word-retrieval difficulties in spontaneous speech but still have relatively preserved naming of presented stimuli. Later, loss of naming of low-frequency lexical items occurs. Eventually, the loss of ability to comprehend and name the same lexical item indicates semantic loss of the lexical item.
Shrinking of the Mental Lexicon
The majority of current research focuses on the acquisition and functioning of the mental lexicon, with out much focus on what happens to the mental lexicon over time. Current studies suggest that as individuals age, they become less capable of storing and remembering words indicating that their "mental dictionary" is, in fact, shrinking. One study showed that the size of a Japanese woman's healthy mental lexicon of kanji shrank at a rate of approximately 1% per year between ages 83 and 93 on average. This was tested through a simple naming task once when the subject was 83, and then again at the age of 93.
Storage of Acronyms
As research on the mental lexicon continues to expand into our modern world of abbreviations, researchers have begun to question whether the mental lexicon has the capacity to store acronyms as well as words. Using a lexical decision task with acronyms as priming words, researchers saw that acronyms could in fact prime other related information. This finding suggests that acronyms are stored alongside their related information in the mental lexicon just as a word would be. Research also demonstrated that these acronyms would still prime related information despite accurate capitalization (i.e.- bbc had the same priming effects as BBC).
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