Executive functions in individuals with intellectual disability
Introduction
The aim of the present study was to investigate executive functions in adults with intellectual disability, and compare them to a closely matched control group longitudinally for 5 years.
Executive functions (EFs) are processes that control and regulate thought and action. There is increasing evidence that EFs can be divided, or “fractionated”, into different subcomponents. Miyake et al. (2000) found evidence supporting the existence of three EF subcomponents: inhibition; updating; and shifting. These subcomponents were separable but still partially correlated constructs. Other tasks that include EFs are planning, decision making, problem solving, fluency and working memory-related dual tasks (e.g. Pennington & Ozonoff, 1996).
Several studies have found evidence that EFs are related to performance on tasks closely associated with intelligence (e.g. Carpenter et al., 1990, Miyake et al., 2001, Salthouse et al., 1998). However, with the division of EFs into subcomponents this picture has become more nuanced. Friedman et al. (2006) found that updating was highly correlated with the intelligence measures, but inhibiting and shifting were not. Arffa (2007) also found that IQ was related to EF tests of sorting, fluency and inhibition, but not to trail making.
Maehler and Schuchardt (2009) made a distinction between learning and intelligence by including both a group with learning difficulty and normal IQ and a group with learning difficulty and low IQ. No differences were found between the two groups on EF measures, but both performed more poorly than a control group without learning problems and normal IQ. This was interpreted as evidence for the fact that EFs are not necessarily related to intelligence, but rather to learning ability. Avila et al. (2009) also found differences between groups with high and low levels of education on a range of executive function tests (digit span backwards, trail making, stroop, and verbal fluency).
There are relatively few studies of EFs in individuals with intellectual disability. However, existing evidence supports the view that such individuals may perform at their mental age (MA) level. For example, Van der Molen, Van Luit, Jongmans, and Van der Molen (2007) carried out a comprehensive assessment of EFs children with ID. They included measures of category fluency, letter fluency, dual task performance, mazes and random number generation. Children with ID performed on par with MA peers on all tasks, which for the dual task performance also was on par with chronologically aged (CA) matched peers. Similarly, in a study on the problem-solving task, Tower of Hanoi, adults with ID performed on par with MA comparison groups (Numminen, Lehto, & Ruoppila, 2001). Performance at the same level as MA comparison groups has also been found on executive-loaded working memory task for both adults (Numminen, Service, & Ruoppila, 2002) and children (Brown, 1974, Conners et al., 1998, Henry and MacLean, 2002, Henry and Winfield, 2010, Maehler and Schuchardt, 2009) with ID. Many studies have also shown below CA comparison group performance for persons with ID (e.g. Conners et al., 1998, Levén et al., 2008). There is however one study that has found contradictory results. Russell, Jarrold, and Henry (1996) found that children with ID performed worse than MA peers on three executive-loaded working memory measures. Nevertheless, there is rather more evidence favouring the conclusion that both children and adults with ID perform at levels commensurate with their mental age.
Like many other cognitive measures, EFs have been shown to be age-related (Bucur & Madden, 2010). One weakness with many of the studies above is that there is almost no evidence concerning adults with ID across the lifespan. Most of the research in this area concerns children or teenagers and is cross-sectional. Hence, the purpose of the current study was to examine EFs in an adult sample of individuals with ID and investigate possible developmental changes in EFs across a relatively long period.
Comparison groups matched for MA were not feasible given that the database we were examining only included people aged 35 years and older. Therefore, a chronological age comparison group was selected, but we were extremely careful to match individuals in the comparison group to the adults with ID on several important measures including sex, age (chronological), years of education and level of education. All matching procedures were carried out at an individual level, as previous studies have generally only used group-matching procedures. It has been reported that level of education correlates with executive functions (Avila et al., 2009) and people with ID seldom continue to higher education such as university level. Therefore, controlling for education in the current study represents a considerable methodological improvement compared to previous work.
The present study uses the longitudinal Betula database (Nilsson et al., 1997, Nilsson et al., 2004) with background and cognitive measures for more than 2000 people to: (1) operationally define a group with IQ below 70; and to (2) define a control group matched on age, sex, level of education and years of education. Individual matching was successful on most measures and participants (see Section 2 for details) and successful matching at the group level was also obtained.
Thus, the research questions were: (1) do adults with ID perform more poorly on executive function tasks compared to a control group matched on age, sex, level of education, and years of education? (2) are there developmental effects on executive functions for persons with ID over a period of 5 years?
Section snippets
Participants
The participants in the present study were a subsample of those in the Betula study. The Betula study is a prospective cohort study where the participants take part in extensive health and memory examinations, and interviews about social factors (Nilsson et al., 1997, Nilsson et al., 2004), the main purpose being to study the development of health and memory functions in adulthood and old age, risk factors of dementia, and premorbid memory functions.
One sample in Betula was tested the first
Results and discussion
The α-level was set to 0.05 and then corrected for the total number of analyses carried out (dividing by 9, αcorrected = 0.006). Means, standard deviation and t-values on group differences for all measures relating to test wave 1 can be seen in Table 2 and relating to test wave 2 in Table 3.
In the nine ANOVAs, there were significant main effects of test wave only for word recall with distracter at both encoding and test, F(1, 133) = 6.94, p < 0.001, r = 0.22, and no significant interaction effects
General discussion
The current study examined EFs over a period of 5 years in adults with ID, comparing them to a very closely matched comparison group. This comparison group was matched not only for age, but for gender, level of education and number of years of education. Adults with ID had a selective impairment in some areas of EFs, including verbal fluency and executive-loaded word recall. However, there were no deficits with respect to performance on the Tower of Hanoi, suggesting that adults with ID do not
Conclusions
The present study has investigated executive functions in adults with intellectual disability, and compared them to a closely matched control group longitudinally for 5 years in the Betula database. The individual matching was based on sex, age (chronological), years of education and level of education. This is a substantial methodological improvement compared to previous studies in this area. A selective impairment on EFs was found for individuals with ID compared to the controls. Lower
Acknowledgement
This research was financed by a visiting fellowship to the first author from The Leverhulme Trust. The Betula Study is funded by the Swedish Research Council (345-2003-3883, 315-2004-6977).
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