Lexical decision task (LDT)

Lexical decision task (LDT) is a procedure used to measure how quickly people recognize visual stimuli as words or nonwords. It is used in many psychology and psycholinguistics studies. The procedure was devised to understand how words are stored in long-term memory. Lexical refers to the words used in a language.

Subjects are presented with two strings of letters at once. They are to respond positively if both strings are words, and negatively if either string is not a word. Subject response times are faster when the two strings of letters form related words—water and ocean, for example, or bread and wheat.

Background

A number of studies of lexical decisions were conducted during the 1960s and early 1970s. Early studies used single strings of letters and measured the reaction time of the lexical decision, which was the subject’s recognition of the string as a word or nonword. One study found that the reaction time for homographs—words that appear the same but have different meanings, such as bass, bow, and wind—was faster.

David E. Meyer and Roger W. Schvaneveldt separately began further study of lexical decision-making. Meyer was researching at Bell Telephone Laboratories in Murray Hill, New Jersey, while Schvaneveldt was at the State University of New York at Stony Brook. They used paired letter strings rather than single strings. Subjects were high school students who were paid for their participation. One cent was deducted for each second of response time, and three cents were deducted for incorrect answers.

The study included two experiments. Experiment 1 used forty-eight pairs of associated words, such as bread and butter or nurse and doctor; forty-eight pairs of unassociated words, such as butter and nurse or doctor and bread; forty-eight pairs of nonwords; and ninety-six pairs of a nonword and a word. The strings of letters ranged from three to seven in length, with a median length of five letters. Nonwords included both vowels and consonants and were constructed by replacing at least one letter with another; vowels were replaced by vowels, and consonants were replaced by consonants. For example, shovel could be changed to shobel, shavel, or any number of other nonwords through letter substitution. The subjects were run individually and had two practice sessions, after which they received feedback on response time. Subjects were instructed to press a key labeled yes if both strings were words; if one or both were nonwords, the subject pressed a no key.

Meyer and Schvaneveldt found that subject response times were faster when the words were related in meaning. They also found that response times were faster when a nonword was displayed above a word, and error rates were greater when a word was displayed above a nonword. In all other cases, error rates were low. The researchers concluded that the degree of association between words had a significant effect on lexical decisions.

Experiment 2 was largely the same. However, subjects were to press a same key if the screen displayed either two words or two nonwords. If a pair of letter strings included both a word and a nonword, subjects were to press a different key. The study found that same responses were again faster for pairs of associated words. Different responses were faster when the word was displayed above the nonword. Pairs of associated words generated fewer errors. Error rates for word/nonword pairs were not significantly different based on which was displayed above the other.

Overview

Meyer and Schvaneveldt wrote that several conclusions could be made from the study results. One possibility is that the mind processes visual and/or acoustic features of a string of letters and uses this information to find a location in the memory, an address where the word is located. If so, a subject would look for an address for both words and nonwords. The location of words in memory would be significantly different from the location of nonwords because the subject would have been exposed to the words multiple times and probably never to the nonwords. Related words are likely stored more closely together than unrelated words, which would explain the faster response time with related words; the mind finds one memory location and does not have to go far to find the other. If unrelated words are stored at addresses in completely different memory neighborhoods, the mind has to look a greater distance to find the second word.

According to Meyer and Schvaneveldt, another possibility is that locating one word’s address could cause a spread of excitement in the vicinity of that address. This could enable the mind to find it more quickly.

A subject’s speed at identifying two related words is a demonstration of the phenomenon called priming. Meyer and Schvaneveldt’s study demonstrated that human general knowledge, including reading, math, and definitions of words, is held in semantic memory. This is organized in semantic networks, which link related words and ideas.

Twenty-first-century researchers have found that lexical knowledge is stored in many parts of the brain. University of California, Berkeley scanned subjects’ brains with functional magnetic resonance imaging (MRI) while they listened to a radio program. The MRI showed oxygen levels in the blood in various parts of the brain, indicating how active the areas were. Researchers then compared the types of words the subjects were hearing with the brain activity levels and created twelve word categories. The 2016 study found that words were stored in more than one hundred areas of the brain in both the left and right hemispheres. In addition, multiple spots were active for individual words, indicating that language processing may be tied to understanding. Further research into the mis-2020s has found that lexical processes are influenced by myriad factors, including word concreteness, context, task demands, and bilingual language control. Further, each process engages several, and sometimes distinct, brain regions.

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