The executive function theory and ADHD - Willcutt - Article


Executive functions (EF) are neurocognitive processes that maintain an appropriate problem solving set to attain a future goal. The executive functions represent cognitive inputs that facilitate decision making. They do that by maintaining information about possible choices in the working memory, while integrating this knowledge with information about the current context to identify the optimal action for the situation.

Some scientists say that the symptoms of attention-deficit/hyperactivity disorder (ADHD) are the result of a primary deficit in a specific EF domain. They base this hypothesis on the observation that prefrontal lesions sometimes produce behavioral hyperactivity, distractibility, or impulsivity, as well as deficits on EF tasks.

For EF weaknesses to be considered the primary deficit in ADHD at least four criteria must be met:

  • Groups with ADHD must consistently exhibit weaknesses on EF measures.

  • EF weaknesses must account for a substantial proportion of the variance in ADHD symptoms in the population.

  • EF weaknesses must be present in most individuals with ADHD.

  • EF weaknesses and ADHD symptoms must be attributable to common etiologic influences.

Executive functions and ADHD measures

EF tasks comprise at least four factors, namely response inhibition and execution, working memory and updating, set-shifting and task-switching, and interference control. Also, most models distinguish between verbal and spatial working memory. Many also include additional processes, such as planning and organization, vigilance, and visuospatial orienting. Concerning ADHD they identified thirteen measures that were frequently used in previous research as important measures of ADHD.

Results

The results show significant differences between groups with and without ADHD. ADHD seems to be associated with weaknesses in several key EF domains. However, the results need to be interpreted carefully. The results may be influenced by differences in sampling procedures, the diagnostic criteria that are being used, and which control variables are being used. With regards to these concerns, the following conclusions can be drawn:

  • Sampling methods and diagnostic algorithms do not affect the results.

  • With regards to ADHD-subtypes, the results suggest that the hyperactive-impulsive type is associated with minimal executive impairment. This means that the hyperactive-impulsive type possibly doesn’t have the same cause as the combined type. Also, the EF weaknesses seem to be primarily associated with DSM-IV inattention symptoms rather than hyperactivity-impulsivity symptoms.

  • Some say that results should always be controlled for intelligence, academic achievement, and comorbidity. However, suffering from ADHD may cause impairments in those variables, making it difficult to determine causality. It is therefore difficult to come to a conclusion, although several studies have shown that even with the use of control variables, the results are significant.

Conclusion

The results indicate that executive function weaknesses are significantly associated with ADHD. However, they do not mean that weaknesses in executive functions are the only cause of ADHD. Instead it seems that executive function weaknesses are one of several causes. Models are appearing that try to integrate several possible causes and try to account for the neuropsychological heterogeneity of ADHD.

Future research

Future research may clarify the relationship between executive functions and ADHD and may lead to a more comprehensive model of ADHD. Three directions are recommended. First of all the authors recommend an evaluation of the psychometric characteristics of executive function tasks. The future studies should use tasks with existing reliability data, or carefully assess the reliability of each task. Secondly, the structure of executive functions should be clarified. Many of the executive tasks involve the use of multiple neurocognitive processes. Future studies should attempt to develop tasks that are better able to isolate specific structures of executive functions. Lastly, competing neuropsychologic theories should be directly tested to integrate different theoretical perspectives.

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