The results of the current study demonstrate that fluency performance declines significantly with age. The age-associated decline for the category fluency task was greater than that for letter fluency, though both declined linearly. In general, individuals with higher education performed better across fluency tasks. However, the decline in fluency performance did not vary as a function of education. There were no sex differences in overall performance or in differential performance across the lifespan. Finally, while theta power was negatively associated with age and positively associated with Animal Naming performance, it did not account for the relationship between the two.
Our finding of a greater age-associated decline in category fluency performance replicates other studies that have found a similar effect (e.g., Bolla et al., 1998
). Age-associated patterns on these two tasks may point to underlying brain regions that are differentially affected in normal aging. Evidence from functional neuroimaging studies suggests that inferior and mid-frontal regions mediate letter fluency tasks (Abrahams et al., 2003
; Gaillard et al., 2003
; Phelps et al., 1997
), whereas both frontal and temporal structures underlie tasks of category fluency (Pihlajamaki et al., 2000
). Similarly, Moskovitch (1995)
has proposed a neuropsychological model in which letter fluency is mediated by the frontal lobes and category fluency is mediated by temporal lobe structures. In AD, a disorder that impacts semantic networks, category fluency is more severely affected than letter fluency (Monsch et al., 1992
). Thus, like in AD, frontal–temporal semantic networks may be vulnerable to the effects of the normal aging process. Neuroimaging studies examining category and letter fluency tasks in the same subjects across the lifespan would help clarify this issue. Even in the absence of correlative neuroimaging data, however, the results from this study suggest that the two fluency tasks are mediated by different neuroanatomical substrates.
Although the behavioral findings of varying category and letter fluency performance across the lifespan suggest differential neurobiologic substrates, this was not captured by EEG theta band power. Our finding that theta power decreased as a function of age was inconsistent with what has been reported in the literature, which has generally suggested an increase
in slow wave activity (e.g., theta) with advancing age (Dustman et al., 1993
). Discrepancies with the extant literature could be due to several factors. The participants in the current study were very well screened and the elderly participants did not evidence any symptoms of a neurodegenerative or medical condition that might impact cognitive or brain functioning. Thus, a subject-selection bias could have been operative; that is, older subjects in the study could have been “supranormal.” The findings of age-associated decline in verbal fluency speak against this possibility. Furthermore, the relationships between theta power and fluency were in the expected positive direction, which is consistent with studies that have demonstrated depressed theta frequency bands in patient groups with poorer performance on fluency tasks (Le Roc’h et al., 1993
). It is important to note, however, that EEG recordings in the current study were not conducted during task performance and, therefore, may have been vulnerable to many factors (e.g., drowsiness). The sample size and controlled experimental methodology in the current study would argue against the possibility of Type I errors. As there have been limited studies that have examined the relationship between resting EEG measures and cognitive task performance, discrepant findings are most likely attributable to methodological differences among experiments.
Resting theta power was moderately related to category but not letter fluency task performance. The finding is similar to that of Hoptman and Davidson (1998)
who did not find a relationship between theta power and performance on a written test of letter fluency. While theta has been implicated in a number cognitive processes (Kahana et al., 2001
), our findings suggest some specificity in cognitive processes associated with it. Theta may be particularly linked to both memory encoding and retrieval (Ward, 2003
). The significant association between theta power and category, but not letter, fluency performance in our study is consistent with this idea, given the greater load of associative information retrieval required to complete the task successfully.
Like in other investigations (e.g., Crossley et al., 1997
), a significant association between fluency tasks and education level was found. Education is a common surrogate measure of cognitive reserve (Stern, 2002
; Stern, 2003
). Consistent with this notion, we hypothesized that greater education would be somewhat protective against the effect of age. However, education level did not appear to impact rate of fluency decline across the lifespan. Despite the fact that there was a range of educational achievement, the sample was generally well educated, and this may have impacted the findings. That is, inclusion of individuals with much lower education may have yielded significant interactions involving education. The findings, nonetheless, suggest that education may be less protective against normal aging effects on tasks of fluency.
There was a significant decline in education level across the age groups. This finding most likely represents a cohort bias secondary to random sampling from the population; older individuals, in general, may be less likely to have pursued advanced levels of education. It is important to note that despite the fact that education level was related to performance on both tasks of verbal fluency, the pattern of age-related decline was not due to this relationship. That is, education level was similarly related to FAS performance as it was to Animal Naming performance. That elderly individuals tend to have lower levels of education has important clinical implications. Although education and age do not interact with fluency performance, both demographic variables contribute independently to performance. Thus, both factors should be taken into consideration during clinical evaluation.
Whether there are sex differences in fluency performance remains somewhat elusive. We did not find the female advantage that has been reported by other investigators (Bolla et al., 1990
; Capitani et al., 1998
), though it is important to note that the sex distribution was not consistent across the age groups in our sample. Given the size of the sample in the current study, it is unlikely that our finding was due to Type II statistical error, even if the reported sex effect is of small magnitude, as suggested by Capitani and colleagues (1998)
. Furthermore, sex did not modify rate of age-related decline for either the category or letter task, as has been reported by some authors (Capitani et al., 1998
). Differential performance across the lifespan as a function of sex may be detectable with more qualitative analyses of verbal fluency, as suggested by Tabert and colleagues (2001)
, who demonstrated sex differences in the accuracy of verbal output.
A particular strength of this study is the large sample size and the representation of subjects from several locales. Test administration was computer-based, which may limit the generalizability of our findings. However, performance on the two tasks of verbal fluency was similar to what has been reported previously. Furthermore, computer administered protocols ensure reliable test administration across all subjects and laboratory sites. Future work in this area should focus on functional neuroanatomical correlates of performance on such tasks across the lifespan. Ultimately, the sensitivity of clinical instruments to detect pathology can be refined by better understanding specific changes in normal aging, as well as factors that impact these changes.