عملکرد اجرایی در افراد مبتلا به معلولیت ذهنی

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. In the Betula database, a group of adults with intellectual disability (ID, n = 46) was defined from measures of verbal and non-verbal IQ. A control group, with two people for every person with intellectual disability (n = 92), was chosen by matching on the following criterion in order of priority: IQ higher than 85, age, sex, sample, level of education, and years of education. Three types of tasks of executive functions were included on two occasions, with 5 years between testing sessions: The Tower of Hanoi, executively loaded dual task versions of word recall, and verbal fluency. Adults with ID showed significant impairments on verbal fluency and on the executively loaded dual task word recall task (at encoding but not at recall). There were no group differences on the Tower of Hanoi. No significant differences between the two test occasions were found. The results are interpreted in terms of individuals with ID having problems with speed of accessing lexical items and difficulties with working memory-related executive control at encoding, which includes shifting between tasks. There are, however, not necessarily problems with inhibition. The dual task results additionally imply that the adults with intellectual disability were more sensitive to strategy interruptions at encoding, but that dividing attention at recall did not have such detrimental effects.

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 and 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).

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 performance was found on fluency tasks and word recall with executive load (dual task) at encoding; but not on word recall with executive load at test or Tower of Hanoi problem solving. Compared to previous results, the lower performance results are a replication, but the lack of group difference on some EFs tasks is a novel finding. The results were interpreted such that individuals with ID have problems with speed of accessing lexical items and working memory-related executive control at encoding, which includes shifting between tasks. There were not, however, necessarily problems with inhibition. There was only one significant decline (with a small effect size) in EFs between the two testing occasions, implying that significant changes in EF did not occur in adults with ID (or controls) over the 5-year period of this study.