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Definition
Verbal fluency is a cognitive function that facilitates information retrieval from memory. Successful retrieval requires executive control over cognitive processes such as selective attention, selective inhibition, mental set shifting, internal response generation, and self-monitoring. Tests of verbal fluency evaluate an individual’s ability to retrieve specific information within restricted search parameters (Lezak, Howieson, Loring, Hannay, & Fischer, 2004). The two most common parameters are (1) semantic fluency, tested by asking the examinee to generate semantic category exemplars (most commonly names of animals); and (2) phonemic fluency, assessed by asking the examinee to generate words beginning with a single letter, most commonly F, A, and S.
Historical Background
Verbal fluency has been studied in healthy and clinical populations since at least the 1940s (e.g., Bousefield & Sedgewick, 1944). Standardized tests of verbal fluency date back to the written fluency test of Thurstone and Thurstone (1962), clearly a method of limited value in assessment of persons with dominant upper extremity limitations such as hemiparesis or psychomotor slowing. This problem was circumvented by the oral verbal fluency measures developed by Borkowski, Benton, and Spreen (1967), early proponents of systematically examining word fluency in persons with brain damage. These authors identified a series of “easy” versus “moderately difficult” letters based on word frequency in English, including the “easy” letters F, A, and S, that became a subtest of the Neurosensory Center Comprehensive Examination for Aphasia (Spreen & Benton, 1969) and continue to be used today (e.g., Heaton, Miller, Taylor, & Grant, 2004). They also presented word fluency data for adults with versus without brain damage, supporting their hypotheses of the utility of word fluency assessment.
Since the initial publication, word fluency tasks have been investigated in detail for normative and descriptive data, and included in assessment tasks. For example, the Controlled Oral Word Association Test (COWAT) appears as part of the Multilingual Aphasia Examination (Benton & Hamsher, 1978; Benton, Hamsher, Rey & Sivan, 1994). In addition to literally hundreds of worldwide experimental studies using a broad variety of cues (e.g., animals, first names, colors, fruits, towns, modes of transportation, and letters (C, F, L, S, P, S, N, and F)), there currently are many standardized versions using both phonemic cues (e.g., the Delis-Kaplan Executive Function System includes B, H, and R as well as F, A, and S; Delis, Kaplan, & Kramer, 2001) and semantic cues (e.g., Western Aphasia Battery [Kertesz, 2006] includes animal naming), and versions exist in multiple languages, including French, Spanish, Chinese, Norwegian, and Flemish (e.g., P, R, and V in French; Cardebat, Doyon, Puel, Goulet, & Joanette, 1990).
Current Knowledge
Correlates of Verbal Fluency
Although studies have produced differing results, in general verbal fluency is correlated with age, increasing through childhood and decreasing in older age. Within-group variability in test scores also increases in older age. As noted by Barry, Bates, and Labouvie (2008), while vocabulary is considered a crystallized ability that may improve throughout the lifespan as knowledge is acquired, verbal fluency requires executive functions such as “the ability to initiate and maintain effort and organize information for retrieval”, which is thought to decline with age beginning in midlife. Age effects may be confounded with the impact of motor speed, however, as well as the possibility that individuals with early-stage degenerative diseases or comorbid conditions such as depression were included in the study sample. Test scores also correlate with years of education. Women typically have slightly higher scores than men, although there is some evidence that these differences are related to hormone levels rather than gender per se.
Overall performance with a given stimulus cue is thought to be related to a combination of the number of items available to that person (e.g., via years of education or idiosyncratic specialized knowledge) and the number of words meeting that criterion in a given language (Cardebat et al., 1990). Thus, for example, the category of fruits yields fewer items than animals in English, and the letter Z yields fewer items than the letter F. As time progresses during the task, adults have been observed to generate less typical exemplars on semantic tasks and lower-frequency words on phonemic tasks (Crowe, 1998).
Neuropsychology of Verbal Fluency
Both semantic and phonemic verbal fluency tasks clearly require the complex interplay of a variety of cognitive functions, including selective and sustained verbal attention, vocabulary knowledge, storage and retrieval of long-term semantic and lexical knowledge, and aspects of executive function such as strategic search and switching. Imaging studies suggest that the two types of fluency may engage different cognitive processes, as they are differentially sensitive to certain experimental manipulations (e.g., phonemic fluency is more disrupted by concurrent repetition of digits in reverse order than by object identification tasks, where the reverse is true for semantic fluency), and are differentially affected in clinical populations (Gierski & Ergis, 2004). Neuroimaging studies suggest that the two types of tasks recruit different brain regions. In general, phonemic fluency tasks appear to be more dependent on frontal systems related to strategic search, such as left dorsolateral prefrontal cortex, whereas verbal fluency impairments are more commonly linked to temporal lobe systems related to semantic knowledge. This might reflect the different strategies used to perform the task (Stuss et al., 1998).
Verbal Fluency in Clinical Populations
Impaired verbal fluency has been associated with virtually every disease and disorder affecting the brain, including dementia, traumatic brain injury, Parkinson's Disease, Huntington's Disease, depression, and schizophrenia, as well as in individuals with psychiatric and developmental disorders (see chap. 11 in Mitrushina, Boone, Razani, & D'Elia, 2005 for a summary). Attempts to find consistent patterns in patients with unilateral versus bilateral lesions or left- versus right-hemisphere lesions have yielded inconsistent results, although as expected, individuals with left-hemisphere lesions associated with aphasia have difficulty on both types of tasks and those with right-hemisphere disorders have particular difficulty with stimuli that focus on visual attributes.
Performance on verbal fluency tests has been widely studied in individuals with Alzheimer-type dementia (e.g. Jones, Laukka & Backman, 2006). Verbal fluency tasks – particularly semantic fluency tasks such as animal naming – are becoming increasingly popular in the early or even preclinical detection of Alzheimer-type dementia, which is based on a growing body of literature documenting both impairments in semantic fluency early in the disease and the finding that rate of decline in scores mirrors disease progression. The sensitivity of semantic fluency to Alzheimer-type dementia is consistent with the notion that this type of fluency is highly dependent on temporal lobe improve clarity. As might be expected, there also is evidence that phonemic fluency is affected more than semantic fluency in frontotemporal dementia, although to date it has not been used in early detection of the disease.
Psychometric Properties of Verbal Fluency Tests
In general, verbal fluency measures have demonstrated strong inter-rater reliability, with more modest test-retest reliability. Validity and reliability data should be considered for individual tests, however, and in the context of the intended purpose of administering the test (e.g., screening for dementia vs. measuring change over time in recovery from an acquired neurological disorder). Readers are referred to entries for specific tests, such as the Controlled Oral Word Association Test and F-A-S Test.
Future Directions
Much remains to be learned about the cognitive processes underlying semantic and phonemic fluency. The frontal–temporal dichotomy is useful for heuristic reasons, but does not account for the considerable variability within and among clinical groups and is underspecified in the context of current models of dynamic cortical networks and intracortical connectivity (e.g., Kennedy et al., 2009). The finding of impaired verbal fluency is so ubiquitous in clinical populations that it may have limited diagnostic utility without consideration of characteristics such as error types and patterns of recall over time. Advances in this area may inform differential diagnosis, as well as early detection of disease processes.
Cross References
References and Readings
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Patterson, J. (2011). Verbal Fluency. In: Kreutzer, J.S., DeLuca, J., Caplan, B. (eds) Encyclopedia of Clinical Neuropsychology. Springer, New York, NY. https://doi.org/10.1007/978-0-387-79948-3_1423
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