Abstract: The word frequency effect refers to the phenomenon of processing high-frequency words faster and more accurately than processing low-frequency words in language production. As the age increases in adulthood, the word frequency effect will also change. The Transmission Deficit Hypothesis suggests that ageing weakens the connection between the stored language information nodes, and the word frequency effect increases further. In contrast, according to the Rank Frequency Account, the relative word frequency and the word frequency effect remain unchanged; thus, it can be deduced that ageing does not change the word frequency effect. Furthermore, the Logogen Model assumes that the word frequency decreases with an increase in experience and contact, thereby predicting a reduction of the word frequency effect as the age increases.
There are differences in the age-related word frequency effect in language production. Numerous studies have demonstrated that the word frequency effect among the elder is greater than that among the young. However, some studies have shown that elderly people have less or no difference in the word frequency effect in comparison with young people, suggesting that the differences in the word frequency effect depending on age may be caused by differences in tasks, stimuli, and individual cognitive abilities. However, the word frequency effect is relatively stable throughout the life cycle. More precisely, it is more difficult to name low-frequency words than high-frequency words for both the young and the elder. There is an age difference between the young and the elder in processing low-frequency words. Compared with the elder, the younger people have higher accuracy and higher activation levels in language-related brain areas (e.g., insula and middle temporal gyrus) and cognitive control-related brain areas (e.g., cingulate cortex). Moreover, there are differences in the word frequency effect between the verb naming task and the noun naming task. In the action picture naming task, high-frequency words activate specific brain regions, whereas in the object naming task, only low-frequency words activate of specific brain regions.
The word frequency effect may occur in different stages of language production. Previous studies have demonstrated that the word frequency effect may occur in the lemma selection, the connection stage between lemma selection and phonological code retrieval or in the phonological code retrieval in spoken language production. The word frequency effect in writing production appears later than in spoken language production and may occur in the orthographic word form. In addition, there are differences in the time course of the word frequency effect between the younger and older individuals. For the elder, with an increase in age a decline in general cognitive ability, the time course of the word frequency effect is slightly delayed when compared with the young.
Some studies also explored the change in the word frequency effect in patients with degenerative diseases, suggesting that it can be used as a sensitive indicator for early detection and diagnosis of related diseases. The word frequency effect and the acquisition age effect both impact on the word processing, however, during different stages. The acquisition age affects the visual and semantic processing of the vocabulary, whereas the word frequency effect merely acts on vocabulary retrieval. In the future, we can further distinguish the influence of word frequency effect from the age of acquisition effect on ageing effect of language production, and extend the studies to patients with neurodegenerative diseases.

Key words: language production, word frequency effect, aging, time course, neural mechanism