EXECUTIVE FUNCTIONS AND NORMAL AGEING

The finding o f differential frontal lobe involvement in the normal ageing process has led to predictions that older adults may show a pattern o f performance similar to that seen in patients with focal frontal lobe lesions, with habitual actions being relatively preserved, but problem-solving in novel situations involving executive skills being affected.

In terms o f intellectual functioning, scales such as the Wechsler Adult Intelligence scales are adjusted for age (Wechsler, 1981, 1997). This suggests that the average scores o f older people are lower than those for younger people, which could be interpreted as evidence that there is a general decline in cognitive function with age. However, adjustments for increasing age are more marked on performance tasks, which generally have processing speed as an important component, than they are for verbal tasks such as Vocabulary and Information. Other work has shown that scores on crystallised intelligence measures remain relatively stable with age, while fluid intelligence scores decrease (Rabbitt, Donlan, Watson, Mclnnes & Bent,

1995); this is consistent with the frontal lobes being differentially affected by the ageing process.

Impairment on standardised measures o f executive function has been reported with normal ageing in a wide range o f studies. For instance, studies have shown age- related decrements on tests o f set-shifting, such as the Trail Making Test (e.g. Wahlin, Bâckman, Wahlin and Winblad, 1996), Wisconsin Card Sorting Test (Axelrod & Henry, 1992; Daigneault, Braun & Whitaker, 1992) and the set-shifting subtest from the CANTAB battery (Robbins, James, Owen, Sahakian, Lawrence, Mclnnes et al 1998). Poorer performance relative to younger people has also been reported on tests of working memory (e.g. Wingfield, Stine, Lahar & Aberdeen, 1988), tasks involving inhibition o f a prepotent responses, such as the Stroop (Daigneault et al, 1992) and Hayling tests (Andrés & Van der Linden, 2000), and tests o f planning ability such as the Tower o f London (Andrés & Van der Linden, 2000). Poorer performance with increasing age has also been found on tasks of initiation, such as Letter Fluency (Whelihan & Lesher, 1985), Category Fluency (Tomer & Levin, 1993) and Design Fluency (Mittenberg, Seidenberg, O ’Leary & DiGiulio, 1989), although the findings with regard to fluency tests are less consistent (Bryan & Luszcz, 2000). There are few measures o f ‘Intentionality’ in the ageing literature, although one study examined performance on a task based on the Six Elements test, which was designed to investigate participants’ use of strategies (Levine, Stuss, Milberg, Alexander, Schwartz & Macdonald, 1998). They found that older adults performed less well than younger people, and that the qualitative nature o f their performance was similar, albeit less marked, to that of participants with focal frontal lesions. The findings o f age-related deficits on tests associated with frontal lobe functioning have been reported to be more marked than any age-related impairment on tests o f other cognitive functions (e.g. Mittenberg et al, 1989). In addition, it has been proposed that normal age-related deficits on other types o f tasks, such as measures o f memory, may also be attributable to executive impairments (Parkin, 1997).

Some studies have questioned whether age-related impairments are consistent across all executive measures, or whether they are more marked on some tasks than

others. For example, a study looking at the relative performance o f adults in their fifties, sixties and seventies found age-related decrements on some variables, such as the Stroop and number o f categories completed on the WCST, but not on other measures, such as number o f perseverative responses on the WCST (Boone, Miller, Lesser, Hill & E ’Elia, 1990). However, other studies have found fairly consistent age-related change on executive measures compared with non-executive measures, and have concluded that there is general decline in ‘frontal’ abilities with ageing (Whelihan & Lesher, 1985). Factors such as educational level, intellectual level, sample size, and the cut-offs for the age-bands used may account for different findings.

Overall, the evidence indicates that older adults perform more poorly than younger adults on many standardised measures o f executive function. Recent work has attempted to elucidate the skills underlying performance on executive tests, and has questioned whether impairment on such tasks necessarily indicates deficits in executive skills. As discussed previously, executive measures are not pure, and performance is dependent on a range o f non-executive, as well as executive, processes. An investigation into the component processes in modified versions of the Stroop and Trail Making tests concluded that, when these are factored out of performance, there remains an age-related executive impairment on the Stroop, but not the Trail Making test (Wecker, Kramer, Wisniewski, Delis & Kaplan, 2000). As psychomotor speed was one o f the component skills that they factored out, they argued that analysis of older adults’ performance on executive tests should concentrate on errors rather than speed. However, this ignores any effects o f a speed- accuracy trade-off, in that people may deliberately perform slowly in order to minimise their errors. Similarly, discussions on the factors involved in letter fluency tasks have concluded that age-related deficits may be due to factors such as handwriting speed rather than executive processes (Phillips, 1997, 1999). However, when the demands o f the task were manipulated, Phillips (1999) found that older adults generated fewer and less broad strategies than younger people, consistent with the theory that ageing is associated with some decrements in executive processes. It has also been suggested that experience can have a protective effect on performance of letter fluency tasks. This is because greater practice with activities such as

crossword puzzles may have reduced the novelty o f the task, and, following the models o f frontal lobe functioning described earlier, this may make performance less dependent on executive processes (Phillips, 1997; Bryan & Luszcz, 2000).