Article summary with A Developmental Perspective on Executive Function by Best & Miller - 2010

Why is there currently no developmental account of executive functions across childhood and adolescence?

A lot of research has been done on the development of executive functions, but unfortunately a few limitations can be identified. Most research has examined narrow age ranges and has focused on preschoolers, when actually more complex tasks do not mature until adolescence or early adulthood. Also, the foundations of executive functions emerge before early childhood, likely within the first year of life. There is also little information about the processes by which children move from one level to another.  

What does the unity and diversity of executive function framework by Miyake entail?

There is disagreement concerning whether executive function is a unitary construct or a set of independent components. Miyake provides a theoretical framework that integrates the opposing perspectives and suggests that the executive function construct consists of interrelated, but distinct, components. There is sufficient evidence to support this theory, however other studies also suggest that the degree of unity and independence of the components may change developmentally. 

What methodological challenges arise when studying executive functions in older children and adolescents?

Firstly, to avoid ceiling effects, researchers often use complex executive function tasks that tap into different executive functions, causing a problem of task impurity. Secondly, the tasks used across an age range are often not uniform. This makes it difficult to make comparisons across age groups. 

What is inhibition as an executive function?

Inhibition is considered foundational for executive functions. It permits an individual to inhibit their impulses and behavioral responses to stimuli in order to select a more appropriate behavior that is consistent with completing their goals. It is an executive function that is difficult to measure, because most inhibition tasks are not pure measures of inhibition and require a minimal amount of working memory. 

How does inhibition develop?

It is in the preschool years that inhibition seems to develop. Children show signs of successful performance on both simple and complex inhibition tasks by age four. Inhibition continues to improve, especially from age five to eight, and especially for tasks that combine inhibition and working memory. At later ages, it also especially seems to develop on computerized tasks. These later refinements seem to involve quantitative improvements in accuracy, possibly due to an increasing efficiency to override prepotent responses. It is possible that inhibition tasks have varying sensitivities, where some are sensitive to the conceptual gains in early childhood whereas others are sensitive to the refinements in strength of the relevant cognitive skills. Mechanisms of development may include brain maturation, increased ability to handle task complexity, increased ability to use rules, and emerging metacognition. 

What does neuroscience say about the development of inhibition? 

The neural response underlying response inhibition was examined by measuring the brain's electrical activity via EEG. A longitudinal study was done that reported qualitative changes in brain activity underlying complex response inhibition, measured by working memory and inhibitory control tasks. A shift was seen from global to localized activity during task completion, which may indicate a growing efficiency of the brain and the growing functionality of the prefrontal cortex for complex response inhibition. Looking at the brain activity from middle childhood to adulthood a continued localization can be observed. There seems to be a decrease in the brain activity that indicates conflict monitoring, which may mean that compensatory mechanisms are present in younger children and that the tasks induce less conflict with increasing age. Research has also indicated that increasing myelination of frontostriatal connections across childhood and adolescence seem to underlie maturation of response inhibition. 

What is working memory as an executive function? 

Working memory involves the ability to maintain and manipulate information over brief periods of time without reliance on external aids or cues. Working memory tasks vary by how much they elicit prefrontal cortex activity. More complicated tasks that require more executive control rely on more executive involvement and more prefrontal cortex activity. The rate and form of working memory development depends on the degree to which a task requires executive processes. 

How does working memory develop?

Working memory improves during the preschool years and by age six, the executive component of working memory is sufficiently developed to be used during complex tasks that require coordination of working memory subcomponents. Research indicates a linear increase from ages four to fourteen and a leveling off between ages fourteen and fifteen. It must be kept in mind that many task place demands on working memory as well as on inhibition, and that for some tests, eliminating the inhibitory component causes the difference in working memory between different age groups to disappear. Unlike the trajectory of inhibition development, which shows large improvements during the preschool years, the trajectory of working memory development seems to be more linear from preschool through adolescence. 

What does neuroscience say about the development of working memory?

The development of working memory occurs gradually and continues to refine through adolescence, especially for tasks that require the maintenance and manipulation of multiple items. fMRI evidence shows a protracted developmental course that leads to localized activity within the prefrontal cortex during working memory functioning. There seem to be qualitative changes as well as quantitative changes in the neural response underlying visuospatial working memory from childhood through adolescence. Over that time period changes are seen in the amount and the location of neural activity during working memory exercises. Increased working memory also correlates with the maturation of myelination of neuronal axons. This indicates that during late childhood and adolescence, the maturation of specific cognitive functions is connected to the maturation of specific neural circuits, rather than to global brain maturation. 

What is shifting as an executive function?

Shifting refers to the ability to shift between mental states, rule sets, or tasks. To be able to perform shifting there appears to be a need for inhibition and working memory processes. The ability to inhibit previously activated mental sets is important for successful shifting. The difference between inhibition tasks and shifting tasks is that shifting tasks rely on switching between two or more mental sets, rather than the inhibition of a single response. Shifting tasks also place demands on working memory by requiring the maintenance and updating of that mental set based on feedback. 

How does shifting develop? 

Preschoolers (ages three and four) can successfully shift between two simple response sets in which the rules are placed in a story context or when there are reduced demands on inhibition. Shifting seems to be unrelated to inhibition and working memory in preschoolers, probably because inhibition and working memory processes are prerequisites for successful shifting. More complex tasks show development in older children and adolescents. Shifting ability continues to improve over many years, however, with different developmental trajectories for the switch cost for accuracy versus reaction time. The shift cost to accuracy diminishes through early adolescence, but the switch cost to reaction time increases until adulthood. With age, participants are more likely to slow down their responses to unsure accuracy. 

What does neuroscience say about the development of shifting?

Neuroscience supports the role of metacognition. Improved metacognition, knowing that slowing down helps performance and being able to detect when it is advantageous to do so, is one mechanism of developing accurate set shifting. fMRI analyses show neural activity in several regions of the prefrontal cortex and other areas as shifting develops. Because cognitive shifting requires a switch between multiple response sets based on feedback, neural networks involving the anterior cingulate cortex and regions of the prefrontal cortex that are responsible for monitoring and dealing with mistakes seem to be critical for successful shifting. 

Do the neuroscience results support Miyake´s model?

The results indicate that the three components are somewhat diverse, however they also suggest that the degree of unity or diversity of executive functions varies from age to age. There seems to be both quantitative and qualitative change in the development of executive functions. Much of the change seems to be quantitative and gradual with more rapid change in the early years. In many relevant brain regions, activity decreases with age, possibly reflecting a growing efficiency of neural responses. Some changes do also appear qualitative, suggesting changes in brain organization. Different regions of the prefrontal cortex support different executive functions. Regional differences in the course of neural development may be responsible for different developmental trajectories of the executive functions. 

Which task-related factors influence the observed developmental trajectories of executive functions? 

Researchers need to be aware of how certain task factors may shape the developmental trajectories of executive functions. They need to be careful in drawing strong conclusions about the development of executive functions, as the following factors may influence the results of their research: task complexity (better success expected on simpler tasks), performance measures, and response modalities. 

What is recommended for future research about the development of executive functions?

Currently research is still very descriptive and it is recommended to look more at the mechanisms of development and see how children move from one level to the next. A few designs seem especially promising, namely:

• Meta-analysis to examine the effects of moderating variables at different ages by including a larger age range. 
• Comparing developmental trajectories for two or more aspects of performance and clues as to whether one aspect influences another. 
• Looking for correlations between a measure of neural activity and executive function performance. Brain maturation could be a developmental mechanism for more advanced executive functioning, or perhaps more executive functioning causes changes in neural activity. 
• The microgenetic method is very useful for looking at the transition phase from one developmental level to the next. 
• Training studies can look at short-term change by training one executive function component and then assessing any immediate changes in other components, or by providing particular experiences and observing any facilitation of executive functioning. 

What role can developmental sequences play in the research for executive functioning?

Examining developmental sequences that underlie the emergence of each executive function can help with the search for possible mechanisms of development. For example, does the development of a more advanced ability supplement an earlier ability, or does it replace it? It can also show if some executive functioning components facilitate the development of other executive functioning components. Different components may be mutually facilitative and there may be a constant back-and-forth of support in the development. Being able to detect possible developmental sequences will allow for a clearer and more detailed understanding of the developmental trajectories of executive functioning components, as well as the developmental relations between those components. 

Which developmental sequences have so far been identified?

So far, likely developmental mechanisms can be distinguished based on biology and environment:

• One model proposes that the development of the anterior attention system during preschool is important for regulating other brain networks. Large neural and behavioral changes in the components during the preschool years are followed by more gradual improvements later. Specific links between brain changes and behavioral changes have yet to be proven. Two aspects need to be kept in mind with this theory. First, that any change in brain function could either be a cause, an effect, or both. Second, it would be better if cognitive neuroscience were tied to theoretical developmental issues such as the degree of generalization of new executive function skills and domain-specific versus domain-general executive function skills. 
• The experiential-based mechanism of executive functioning development looks at the role of metacognition. An awareness of inhibition failures and the following adjustments in response to avoid errors, as well as slowing down to improve accuracy, may stimulate the development of cognitive functioning. Important factors may be the detection of success or failure of one's decisions, latent representation, and focus of attention.