Executive Function and Intellectual Disability: Innovations, Methods, and Treatment

The definition of intellectual disability (ID) has evolved over the past century, with consensus in the scientific and practice communities around a framework that includes significantly lower than average cognitive functioning (intellectual capacity) and challenges in everyday activities in home, school, community, and employment environments (adaptive behaviour). These two critical elements, cognition and adaptation, are understood as distinct constructs—one that focuses on information processing and reasoning skills, and one that represents the navigation of day-to-day life. Yet, it has been understood for quite some time that the link between intellectual capacity and adaptive behaviour is complex, and likely involves important auxiliary constructs that are not currently included in the technical definition of ID.

In this special issue of JIDR, we highlight one such construct, executive function (EF), that plays a key cognitive regulatory role in the link between intellectual capacity and adaptation in everyday life. A great deal of progress has been made in the study of EF in the fields of child development, neuropsychology, neuroscience, and psychopathology over the past several decades. Along the way, leading scholars in the study of EF have postulated that dysregulated EF is so pervasive among clinical populations that it may be an indicator itself of the presence of developmental psychopathology (Zelazo, 2020). Others have suggested that there may be unique EF ‘fingerprints’ associated with specific neurodevelopmental conditions that may guide both basic science and treatment in those populations (Ozonoff & Jensen, 1999).

Despite major advances in EF research, our understanding of executive function and dysfunction among individuals with ID has large gaps. Though a growing number of studies report laboratory-based and proxy-report findings regarding EF outcomes in specific subpopulations (neurogenetic conditions; co-occurring autism spectrum disorder and ID), relatively little discussion has taken place regarding the conceptual importance of EF and the role that it may play as a key auxiliary factor in the lives of individuals with ID. Indeed, with its focus on regulatory aspects of cognition, EF may have a unique influence on the link between overall intellectual challenges and one’s ability to meet the demands of daily routines and community living.

The articles presented in this special issue provide a contemporary, in depth exploration of EF in ID. They address questions regarding the nature of EF in individuals with ID, the emergence of EF during early childhood in ID-related conditions, the influence of EF on other domains of development, and implications of EF for adaptation and well-being throughout the lifespan. Each topic explored in this special issue reflects an important frontier in describing the role of EF for people with ID and can serve as a catalyst for future research that continues to address unanswered questions. In addition, the articles in this special issue collectively identify methodological challenges regarding the valid assessment of EF in ID that our field will continue to face in the future. The insights gained from these new contributions are highlighted in the sections below, which provide a brief overview of the conceptual and methodological themes represented in this work collectively.

EF in people with ID

Perhaps the most foundational contribution in this special issue relates to characterising the nature of EF in people with ID. Taken together, the papers included in this special issue generate a picture of wide-ranging vulnerabilities in EF and provide evidence for the notion that EF may be a common area of challenge in this population. These results are captured in the metanalysis conducted by Spaniol and Danielsson (2022), who report a broad impairment in EF in individuals with ID even when compared to TD children at similar mental ages. Their metanalysis demonstrates that EF challenges seem to be observed in each EF domain considered; however, a great deal of heterogeneity was observed among the studies included in their metanalysis. In a theme that will be explored further below, the heterogeneity that they report is at least partially explained by phenotypic EF profiles that tend to vary among neurogenetic conditions.

A second article in this special issue further characterises the interplay between EF an ID. Erostarbe-Pérez, Reparaz-Abaitua, Martinez-Pérez & Magallon-Recalde (2022), examined the relationship between EF, assessed via both laboratory and proxy-report measures, and intelligence, as assessed with the Wechsler Intelligence Scale for Children – Fourth Edition (Wechsler, 2003) in a sample of students with ID and Borderline Intellectual Functioning (BIF). They report a significant correlation, moderate in magnitude, between intelligence and EF in the sample considered. As in typical development, the correlations were stronger for some aspects of intelligence, such as perceptual reasoning, when compared to other more crystallised aspects of intelligence, such as verbal comprehension. One notable finding from their study was that the EF component process of working memory was most strongly correlated with intelligence. This aligns with the fact that working memory is particularly impacted in many individuals with ID, and raises a critical question: is working memory a link between intelligence and EF in this population? Or, is this correlation confounded by the fact that recent intelligence tests tend to include working memory in their overall IQ composite scores, a concerned raised by Spaniol and Danielsson (2022)? This issue of measurement and confounds will be revisited in the methodological discussion below, and will be important to address in future work on the nature of EF in ID.

Etiology-specific findings.

This special issue also includes several articles that provide new knowledge regarding the notion of unique EF ‘fingerprints’ (Ozonoff & Jensen, 1997) that can be characterized in subgroups of people with ID. Several articles focus on aspects of EF in Down syndrome, the most common neurogenetic condition associated with ID. Carretti, Meneghetti, Doerr, Toffalini and Lanfranchi (2022) take a developmental approach to the critical EF construct of working memory in Down syndrome. Though the majority of working memory research in Down syndrome examines performances relative to mental-age equated typically developing children, this study takes a developmental trajectories approach to describing the important domain of visuo-spatial working memory in this population. Carretti et al. (2022)’s findings emphasize the need to identify complex and nuanced patterns of relative strength and weakness within specific syndrome profiles. In their study, greater difficulties emerged in visuo-spatial working memory for tasks that require the simultaneous recall of positions (pattern) when compared with tasks that require sequential recall (pathways). And yet, Carretti et al. (2022)’s findings also demonstrate that visuo-spatial working memory overall seem to develop in alignment with mental age in this population. Taken together, these results show the complexity of working memory in DS, with heterogeneity within profiles, but mental-age appropriate developmental performances.

Two manuscripts in this special issue examine EF in Down syndrome through proxy-report measures. Onnivello et al. (2022) characterize caregiver-reported data from the Behavior Rating Inventory of Executive Function-Preschool (BRIEF-P, Gioia, Espy & Isquith, 2003) and the Behavior Rating Inventory of Executive Function-Second Edition (BRIEF 2; Gioia, Isquith, Guy & Kenworthy, 2015) in children with Down syndrome and examine its relationship with adaptive behavior. Csumitta, Stephan, LaQuaglia, Miller, and Lee (2022) compare EF proxy-report data from parents and teachers of children with Down syndrome using the BRIEF-2 (Gioia et al., 2015). Both studies report general difficulties with EF in this population, but with a profile of peaks and valleys, including greater difficulties with shifting and working memory and fewer difficulties in emotional control and organization of materials. Moreover, the group-level profile seems to partially vary by factors like age and environmental context. Preschool and school-age children varied somewhat in their EF profiles in the Onnivello et al. (2022) study, and greater EF challenges were reported by teachers relative to parents in the Csumitta et al. (2022) study. These and other similar dimensions are important to consider when describing syndrome-related performances on the group level.

Two other neurogenetic conditions were also examined in the present special issue, Williams syndrome and Prader-Willi syndrome, generating converging findings regarding the link between overall intellectual functioning and the EF component process of cognitive flexibility. Greiner de Magalhães, Pitts, and Mervis (2022) analysed BRIEF-2 (Gioia et al., 2015) parent data for a large sample of 6- to 17-year-old children and adolescents with Williams syndrome. They found that the Cognitive Regulation Index posed the greatest area of challenge at the group level. Within-group variability was also observed, such that challenges with emotion and behaviour regulation dimensions were associated with increases in maladaptive behavior and challenges in adaptation. Interestingly, they also report an association between intellectual functioning and cognitive flexibility, in that those with greater intellectual capacity also showed increased flexibility. These findings shed new light on the established phenotypic profile that is generally associated with childhood and adolescence in Williams syndrome, and the link between flexibility and intellectual capacity may offer critical new insights regarding intervention approaches to support greater capacity in this area.

Along these lines, EF in adults with Prader-Willi syndrome is examined by Chevalère, Camblats, Laurier, Mourre, Estival and Postal (2022), in this special issue, with a particular focus on the connection between cognitive flexibility and emotion processing. Individuals with Prader-Willi syndrome are at elevated risk for emotional lability and difficulties with emotion identification. In this study, cognitive flexibility was assessed through a task switching paradigm with emotionally-valenced pictures (positive, neutral, and negative). The authors report that adults with Prader-Willi syndrome experienced greater switching difficulties when compared to peers with and without ID not only in neutral but also in emotional contexts, both positive and negative. These results provide evidence for a particular impairment in cognitive flexibility in Prader –Willi syndrome above and beyond the intellectual level.

The emergence of EF in children with ID-related conditions

As with many important phenotypic outcomes in neurogenetic conditions, questions about EF have matured beyond the nomothetic characterisation of strengths and challenges cross-sectionally during childhood or adolescence. One more recent advance involves an attempt to identify early precursors of later adaptive (or maladaptive) outcomes. The identification of early foundational skills that support later, more advanced outcomes can lead the anticipatory use of early enrichment experiences, with the potential for preventing the onset of maladaptive cascades. In line with this effort, Schworer Fidler, Kaur, Needham, Prince and Daunhauer (2022b) present a study in this special issue that involves a comparison of EF precursors among infants with and without Down syndrome. The ability to study the phenotypic emergence of EF foundations in infants with ID is somewhat unique, in that Down syndrome is a neurogenetic condition that is diagnosed prenatally or soon after birth. Describing early cognitive regulatory skills in infancy is feasible in this neurogenetic condition, which is often not the case in ID-related conditions that are diagnosed in the toddler or preschool years. Schworer and colleagues (2022b) measured early cognitive regulatory skills, including sustained attention, attention disengagement, and early goal-directed actions on objects in infants with Down syndrome and an MA-equated group of typically developing infants. They report subtle differences on the group level related to looking time during sustained attention tasks, and differences between the two groups of infants on the dimension of goal-directed actions on objects. Most relevant to the identification of precursors, when examining which dimensions were predictive of early EF performance at a 6-month follow up visit, Schworer et al. (2022b) found that latency to shift attention at baseline was strongly associated with EF performance at the follow up visit. Since both attention shifting and the toddler EF task (an A-not-B task) involve some degree of shifting, this suggests that there may be continuity during the infant and toddler years within specific aspects of cognitive regulation. Future work in this area offers the promise of more informed intervention during the earliest stages of development, and EF foundations like attention shifting are likely a worthwhile target area that, if strengthened, can have long term positive downstream effects.

The emergence of EF throughout childhood is also examined in Barton and McIntyre’s (2022) contribution to this special issue. They report on a sample of children who were identified as having developmental delays during early childhood who were subsequently assessed in middle childhood. Though only approximately 15% of their sample was subsequently designated as having ID, there were no significant differences between those with and without an ID diagnosis in the degree of parent-reported EF challenges observed during middle childhood. Both groups demonstrated elevated challenges in EF, and across the overall sample, EF performance was associated with both adaptive and maladaptive behaviour outcomes. Notably, approximately 31% of participants in the no-ID group during middle childhood had a diagnosis of ADHD. These findings provide support for the idea of executive dysfunction as a broader indicator of neurodevelopmental challenge, regardless of the presence or absence of ID.

The influence of EF on other domains of development

Another theme that is addressed in this special issue involves the influence of EF on other domains of development. In a novel empirical study, Kristensen, Lorenz, Zhou, Piro-Gambetti, Hartley, Godar, Diel, Neubauer, and Litovsky (2022), describe the relationship between aspects of EF and language performance among young adults with Down syndrome. Because both of these constructs can pose specific challenges to many people with Down syndrome, this new study addresses noteworthy questions regarding the role that cognitive regulation plays in the language phenotype in DS. Another feature of the Kristensen et al. (2022) study is its focus on early adulthood, a phase of the lifespan when many people with Down syndrome begin to demonstrate cognitive decline. Developmental research in Down syndrome has generally focused on childhood, adolescence, and older adulthood, leaving educators and clinicians with less of an understanding of early vulnerability for skill loss in this population. In their study, Kristensen et al. (2022) report associations between measures of both attention and inhibition with receptive and expressive vocabulary in their sample. In addition, both verbal and visuo-spatial working memory were associated with receptive vocabulary. The EF components were measured through a set of tasks that required verbal and nonverbal responses, providing further evidence for a relationship between cognitive regulation and language that is not necessarily mediated by verbal processing. The findings presented in Krisetensen et al. (2022)’s study, therefore, contribute not only to our knowledge base regarding two critical constructs, but also to the importance of EF in the transition to adulthood in Down syndrome more broadly.

In addition, the Onnivello et al. (2022) study described above focuses on the early phases of the lifespan in DS and examines the connection between EF and adaptive behavior in preschool versus school age children with DS. Though few connections were observed between these two domains in the preschoolers, a significant association between working memory and adaptive communication skills was reported. The lack of early association could be attributed to the more basic nature of early activities of daily living, which may require only a modest degree of cognitive regulation. In contrast, many associations were observed between EF and adaptive behaviour in the school-age sample. This could be explained by the greater degree of complexity required to respond to the demands of home and school daily life during middle childhood in Down syndrome.

In another special issue article, Santegoeds, Van der Schoot Roording-Ragetlie, Klip and Rommelse (2022), explored the connection between EF and behaviour problems in a group of children with mild to borderline ID and co-occurring psychiatric disorders. Their results suggest the presence of particular challenges in working memory and processing speed in their sample, although both domains were not clearly predictive of internalising or externalising problems. Internalising problems were primarily predicted by cognitive and metacognitive factors, such as visuo-spatial information processing and verbal skills, while no predictors were identified for externalising problems. It may be that case that in the population studied, other neurodevelopmental and/or environmental factors better explain the presence of maladaptive behaviours.

Implications for treatment and adaptation throughout the lifespan

Treatment to support the development of EF component processes in people with ID is an important and under-researched area. Existing EF interventions are generally not designed to account for the needs of individuals with ID. However, it is a worthwhile endeavour to examine whether evidence-based approaches that have been successfully implemented in the general pediatric population may also be impactful for people with ID. In the current special issue, Roording-Ragetlie et al. (2022) conducted a triple-blind, randomised control trial of the CogMed Working Memory Training (CogMed; www.cogmed.com) intervention to determine its potential efficacy with children who have BIF and co-occurring neuropsychiatric diagnoses. Their noteworthy findings demonstrate that participation in the CogMed activities for both treatment and control groups had effects for both near and far transfer working memory tasks. These findings are somewhat surprising, in that the treatment condition was designed to adapt to the performance of each participant, increasing in difficulty across trials. In contrast, the control condition involved repeated practice of similar working memory tasks without any adaptive escalation in task difficulty. Regardless of whether the tasks adapted to the child’s skill level or remained at a baseline level of difficulty, both treatment groups demonstrated significant improvements in their working memory performance from baseline to post-treatment. As the authors note, this study demonstrates that there may be utility to simply exercising working memory skills on a frequent basis, and that increasing the level of challenge over time may be of little utility, at least in this particular subpopulation of children with BIF and neuropsychiatric diagnoses.

Methodological issues

Measuring EF.

This special issue presents novel contributions related the nature of EF in ID, but the manuscripts in this special issue also highlight current challenges in our ability to pose research questions about EF in ID and answer them using sound research tools. From a methodological perspective, one ongoing measurement concern pertains to the valid assessment of EF for individuals with ID. Most standardised cognitive assessment batteries are designed for the general population, and are not tailored to meet the needs of subgroups of people with ID. For this reason, there has been a recent effort to validate measures of cognition and communication for various neurogenetic syndrome conditions, especially for use in upcoming treatment trials. In this special issue, Schworer, Esbensen, Fidler, Beebe, Carle and Wiley (2022a), present a comprehensive account of the psychometric properties of some of the most commonly measures to assess working memory in people with Down syndrome. They found that several of these measures met their a priori criteria for reliability, validity, and feasibility for their entire sample of children and adolescents with Down syndrome. They also found that other measures had greater utility for subgroups of individuals with Down syndrome in their sample, but weaker psychometric properties when examined at the group level. These nuanced findings regarding the appropriate and valid measurement of EF in individuals with ID can help improve study design for both basic and treatment research, and can minimize confounding factors like floor effects that can pose a challenge to the interpretability of findings. Additional studies of this nature will strengthen both developmental and treatment science for people with ID.

The collection of articles in this special issue raises questions regarding the most beneficial approach to EF assessment in individuals with ID. Some papers in this special issue relied on laboratory measures that are used in the general population, but difficulties were encountered with participant understanding of instructions, or difficulties with tasks due to challenges in other developmental domains, such as motor or language. Moreover, using tasks for children without disabilities often necessitates the use of measures normed on younger children, which prohibits the use of standard scores. With the development and use of measures that are designed to eliminate these confounds, our field has the potential to develop a richer understanding of EF in ID.

While some articles in this special issue approached EF measurement with laboratory assessment, others relied on proxy-report of behaviors in daily life contexts. One advantage of this approach is that proxy-report measures that were developed for the general population have been demonstrated to be appropriate for use in individuals with ID and retain their psychometric properties. Also, several papers in this special issue reach the conclusion that proxy-report measures used to assess EF in the general population, such as the BRIEF and its different forms, might be as well a sensitive tool to identify difficulties and profiles at least for some specific ID etiologies such as Down and Williams syndrome (Csumitta et al., 2022, Onnivello et al., 2022, Greiner de Magalhaes et al., 2022). Using proxy-report measures has the advantage of eliminating confounding factors of laboratory test and increased ecological validity, but important issues arise when interpreting parent- or teacher-report data. What is considered normative to one parent or teacher may be appraised differently by others. Moreover, there are ongoing questions regarding whether laboratory and proxy report measures assess the same constructs. With these considerations in mind, both from a clinical and a research point of view, it is likely that a more comprehensive picture of EF in ID will require a multi-informant approach, combining both laboratory tests and proxy-report measures.

EF components.

In addition to the need for valid assessment measures of EF in ID, several larger methodological themes are present in the collection of articles in this special issue. The first relates to the continued scientific discussion regarding the overall ‘architecture’ of EF, and which components ought to be included in models of EF. Though questions regarding the dissociability of EF are still debated in the broader EF literature, most research on EF in clinical populations has worked with the assumption that EF can be divided into related, but separable component processes (Ozonoff & Jensen, 1999). The consensus regarding the dissociability of EF in ID is evident in that none of the articles in this special issue approach EF as a unitary construct. However, there is considerable variability in terms of determining which component processes ought to be included as key components of EF. There are articles in this special issue that exclusively focus on specific EF processes, such as working memory or cognitive flexibility, that are key constructs often included in many models of EF. However, Spaniol et al.’s meta-analysis of the association between EF and ID included the constructs of inhibition and shifting, but working memory was only partially considered. The rationale for this decision was that working memory dual request tasks are often present in intelligence tests, and as such, the association between working memory and intelligence is confounded. For this reason, the authors decided to consider ‘updating’ as the only aspect of working memory included in their metanalysis. However, the decision to narrow the definition of working memory led to the exclusion of a majority of studies on working memory in ID. Establishing greater consensus regarding key constructs and common data elements for study design is an important challenge for future EF research in this subfield.

Concluding Thoughts

Taken together, the papers in this special issue offer new insights that help us better understand EF in individuals with ID, suggesting that EF in this population, as in typical development, may play a critical role in determining how intelligence influences everyday life. The studies included in this special issue can prove to be a starting point for identifying and addressing new challenges in the study of EF in individuals with ID. A primary challenge that researchers should address is the need for greater consensus regarding EF architecture in individuals with ID. Building a shared set of definitions and establishing consensus regarding component processes will likely reduce the heterogeneity in study design and findings. As discussed above, challenges for our field involve continuing to identify valid and reliable measurement tools to assess EF specific for people with ID, factoring in areas of strength and challenge that might confound performances on EF assessments. But perhaps the most significant challenge ahead concerns the translational implications of this area of study. The results of the studies reported in this special issue highlight the crucial role of EF in everyday life, particularly from the school age onwards. For these reasons, it is of vital importance to develop intervention programs that will foster EF development in this population. In particular, considering the connection between executive function and many aspects of everyday life, such as adaptive behaviour, language, spatial orientation, academic achievement, and job skills, as well as the fact that this connection become stronger with age, we believe that EF will be an important area to target through education and intervention at various stages of development in people with ID. Overall, the work presented in this special issue suggests that cognitive regulatory foundations in individuals with ID will continue to play an important role in fostering independence and function in academic environments, employment, and community engagement throughout the lifespan.