Lexicon is a word of Greek origin meaning vocabulary. It is a list of words together with additional word-specific information (i.e. a dictionary)
In linguistics the definition of lexicon is slightly more specialized – it includes lexemes used to form words
A lexeme is a unit of linguistic analysis that:
belongs to a particular syntactic category
has particular meaning.
Lexemes may be:
The mental lexicon can therefore defined as individual dictionaries of words and lexemes stored in the mind.
The mental lexicon differs radically from a dictionary in that there are so many words and yet they are found so fast.
Native speakers can recognize a word of their language in 200ms or less and can reject a non-word sound sequence in about half a second!
In a 1940 study Seashore & Erickson (http://pages.slc.edu) estimated that an educated adult knows more than 150,000 words and be able to use 90% of these.
Although an enormous vocabulary is available to any speaker of a language not all of these words have equal status, it is a firmly established statistical fact that some words are used far more frequently than others, and that those words used more frequently are recognized faster. This is called the familiarity effect.
Hartvig Dahl (http://pages.slc.edu) counted the frequency of different words in a transcript of 1,058,888 running words of spoken conversation.
He found that the most frequently spoken word was the first person singular; on the average every sixteenth word was “I”.
The familiarity effect illustrates a clear difference between the mental lexicon and a dictionary – in a dictionary it takes no longer to look up a less commonly used word; but in the mental lexicon familiar words are more rapidly accessed.
The familiarity effect is measured using a lexical decision task:
Lexical decision tasks consistently shows faster response times for high-frequency, high-familiarity words.
One speculation about the reason for this effect is that frequently used words are easier and quicker to find because they are stored in many different places in the brain.
Another less intuitive finding that fits with this speculative theory is the finding that words that have more than one sense (homographs – e.g. content, object) are recognized slightly faster) than equally familiar words like neighbour that have only one sense (non-homograph)
This implies that homographs are multiply represented for the variety of meanings.