Executive Functions (EFs) are higher-order cognitive abilities needed to pursue and achieve goals.
They enable understanding complex concepts, problem-solving, planning, and relationship management.
EFs have been historically difficult to define, unlike specific deficits like short-term memory problems.
EF impairments are hard to predict due to the lack of a single, directly tied behavior.
EFs have been associated with the frontal lobes since the case of Phineas Gage in 1848.
An iron rod through his left frontal lobe dramatically altered his behavior and personality.
EFs include working memory, inhibitory control, cognitive flexibility, planning, reasoning, and problem-solving.
The executive system manages other cognitive abilities like attention and memory.
It allows individuals to change overlearned behaviors and adapt to novel situations.
Deficits in EF can cause poor goal-directed behavior and impaired social functioning.
This chapter will cover:
Models explaining EF and its association with frontal lobes.
Lesion and neuroimaging evidence for the neuroanatomy of EF.
Neuropsychological tools for assessing EF, including ecological assessment.
Rehabilitation and training approaches for EF.
How EFs evolve through the lifespan.
The interplay between genetics and EF.
EF Models
Different frameworks have been proposed to conceptualize the executive system.
Luria (1973) was the first to conceptualize an EF model.
He considered problem-solving behavior as depending on skills relying on the frontal lobes.
EF components: anticipation, planning, execution, and self-monitoring.
Lezak (1995) defined EFs as mental capabilities for formulating an objective, planning, implementing actions for that objective and intervening in complex tasks.
These functions depend on shifting between tasks, inhibiting responses, and updating working memory.
Self-regulation of behavior is the ability to regulate behavior according to internal goals (Goldberg and Podell, 2000).
This involves mental representation and inhibiting inappropriate responses.
Stuss and Benson (1986) proposed a model related to Lezak’s, inspired by Luria.
EF processes: initiation of behavior, planning, sequencing, and organization.
Stuss later placed EF in a hierarchical framework, receiving input from lower and higher-level processes.
Sohlberg and Mateer (1987) conceptualized a clinical model of EF involving six components:
Initiation and drive.
Response inhibition.
Task persistence.
Organization.
Generative thinking.
Awareness.
Grafman (2002) introduced the structured event complex (SEC) framework.
The PFC stores hierarchical knowledge that appears as EFs when activated.
An SEC is a goal-oriented set of events structured in sequence, representing knowledge, morals, abstractions, concepts, social rules, event features, event boundaries, and grammars.
Badre and D’Esposito (2007) consider the PFC crucial to flexible and organized action.
They found a posterior-to-anterior gradient within the PFC depending on the manipulated level of representation.
Koechlin and Summerfield (2007) proposed a theory using information theory to describe the architecture of executive control in the lateral PFC.
Action selection is guided by hierarchically ordered control signals.
The models agree that EFs involve several subfunctions, mostly managed by the PFC.
Lesion and neuroimaging studies are valuable for identifying the neural bases of EF.
Neuroanatomy of EF
The PFC is commonly associated with EF, comprising over 30% of cortical cells.
It is the most recently evolved brain region and is highly developed in humans.
The PFC has three subdivisions: medial, dorsolateral, and orbitofrontal.
The PFC receives input from neocortical, hippocampal, cingulate, substantia nigra, and thalamic areas.
It sends projections back to the medial dorsal nuclei, amygdala, septal nuclei, basal ganglia, and hypothalamus.
Much evidence for the anatomic association between PFC and EF comes from studying individuals who suffered a brain injury.
EF Lesion Mapping Studies
Neuropsychologic studies of patients with focal brain lesions provide an opportunity to study specific lesion-deficit associations.
Lesion studies allow causal hypotheses.
PFC and Underlying White Matter Tracts
Due to its position in the skull, the PFC is frequently damaged after traumatic brain injury (TBI).
Studies show that the dorsolateral PFC (dlPFC) and orbitofrontal cortex (OFC) are associated with EF impairments.
Larger PFC lesions may result in dysexecutive syndrome, which impairs cognition, emotion, social behavior, and goal-oriented behavior.
Damage to the OFC and anterior cingulate cortex (ACC) can impair social and motivated behavior.
Lesions to the dlPFC show issues with goal-directed behaviors, cognitive control, inhibition, planning, and working memory.
These deficits impair everyday activities and autonomy, predicting negative outcomes.
Nonfrontal brain areas are also associated with EF, forming larger executive neural networks.
The PFC is linked with the limbic region, crucial for mnemonic interactions and emotional response regulation.
The PFC and hippocampus connect to the nucleus accumbens, which integrates cortical and limbic information into goal-directed behavior.
Damage to the anterior corona radiata and superior longitudinal fasciculus was associated with poorer functional recovery in a verbal fluency task.
The Trail Making and 20 Questions tasks were better predicted by PFC lesions.
White matter damage impairs signal transmission between cortical areas within a functional brain network.
EF relies on PFC neural networks, but other cortical (parietal and temporal cortex) and subcortical brain regions connected to the PFC can influence the correct functioning of EF.
Working Memory
Neural activity in dlPFC stores and maintains working memory representations.
Neuronal activity in monkey dlPFC neurons is selective for the spatial location of a remembered target.
Lesions of the dlPFC impair the monkeys’ ability to remember locations in the contralesional visual field.
Neuroimaging studies usually fail to find neural activity in the human dlPFC during similar working memory tasks.
Neuropsychologic studies sought to resolve this controversy.
An intact lateral PFC is necessary for normal performance in working memory, especially with distraction or increased task difficulty.
The OFC mediates executive control functions underlying the coordination of multiple working memory processes.
The OFC is engaged when problems involve more than one cognitive process.
The n-back task requires monitoring a series of stimuli, maintaining activation of potentially relevant items, discarding irrelevant information, and comparing items to identify a match.
The coordination of information processing and information transfer between multiple processes is regulated by the OFC.
Barbey et al. (2011) reported that:
Medial OFC lesions were associated with deficits in executive control functions underlying the joint maintenance, manipulation, and monitoring of information in working memory, particularly at high levels of cognitive load.
Medial OFC lesions were not associated with pervasive deficits on tests of working memory maintenance or manipulation.
Medial OFC lesions were not associated with reliable deficits in tests of verbal or spatial reasoning.
Lateral OFC lesions did not yield impairment in working memory function.
The neural architecture of the medial OFC is necessary for the coordination of working memory maintenance, manipulation, and monitoring processes.
Critical white matter tracts involved in working memory include the cingulum, parietofrontal pathways, thalamocortical projections, and the right cerebellar white matter.
These tracts connect the parietal lobes with the lateral PFC and ACC, as well as the thalamic radiations.
Inhibitory Control
The PFC serves as a source of inhibitory control over other brain areas.
Certain PFC regions, such as the right inferior frontal gyrus, are specialized for inhibitory control.
Cognitive inhibition/control was associated with lesions to the dlPFC and ACC.
Cognitive Flexibility
Lesions to different prefrontal areas result in different deficits of cognitive flexibility.
Damage to the OFC impairs reversal learning, but not attentional set-shifting.
Damage to the lateral PFC impairs shifting of attentional sets but not reversal learning.
The OFC is crucial for signaling outcome expectancies.
Cognitive flexibility and adaptive aspects of intellectual function remain to be well characterized.
The neural bases of cognitive flexibility were examined by Barbey et al. (2013a).
Latent scores for psychometric intelligence reliably predict latent scores for cognitive flexibility.
These processes depend on a shared network of frontal, temporal, and parietal regions, including white matter association tracts.
Selective damage within the right superior temporal gyrus supports insight and recognition of novel semantic relations.
Core elements of cognitive flexibility emerge from a distributed network of brain regions that support competencies for human intelligence.
Planning
Patients with PFC lesions show difficulties in decision-making and problem-solving in real-world situations.
Tests that reflect real-world task requirements more accurately show lower performance at a global level.
Patients showed difficulty in organizing and structuring their plan’s problem space and allocating adequate effort to each problem-solving phase.
Patients had difficulty with real-world planning problems with no right or wrong answers and in generating their own feedback.
These deficits involve:
Inadequate access to SECs.
Difficulty in generalizing from particulars.
Failure to shift between mental sets.
Poor judgment regarding adequacy and completeness of a plan.
Reasoning
Analogic reasoning is at the core of the generalization and abstraction processes that enable concept formation and creativity.
Damage to the left rostrolateral PFC region impaired the ability to reason by analogy.
Analogic reasoning plays a role in relational matching or integration.
Problem Solving
The PFC plays an important role in problem-solving behavior.
Clinical studies describe patients with impairments in everyday life, including planning, problem-solving, and decision-making.
These impairments include difficulty in generating possible problem solutions and then selecting an appropriate solution.
The anterior group showed reduced ability in generating possible solutions and also impairment in selecting appropriate problem solutions.
Working memory, processing speed, and emotional intelligence predict individual differences in everyday problem-solving.
Social problem-solving, psychometric intelligence, and emotional intelligence are supported by a shared network of frontal, temporal, and parietal regions, including white matter association tracts.
EF Neuroimaging Evidence
Numerous studies used functional neuroimaging techniques (PET and fMRI) to identify cerebral areas involved in specific EF tasks.
Neuroimaging studies have shown an association between EF and the frontal lobes.
The vlPFC controls retrieval of representations from the posterior cortex and the online maintenance of these accessed representations.
The dlPFC region is implicated in manipulating information from these representations.
The dlPFC has been repeatedly shown to be involved in complex functions such as making plans for the future.
Working Memory
Neuroimaging evidence has documented associations between working memory tasks and activation of the dlPFC or vlPFC.
When single-task conditions were compared to dual-task conditions all subjects showed an increase in activation bilaterally in the dlPFC, even though neither task activated PFC when performed alone.
Smith and Jonides (1998) reported different patterns of brain activation between processing verbal information and spatial information.
Recent meta-analysis found increased activation in the lateral premotor cortex, dorsal cingulate and medial premotor cortex, dlPFC and vlPFC, frontal poles and medial and lateral posterior parietal cortex.
Inhibitory Control
Normal human behavior and cognition are tightly associated with the ability to inhibit inappropriate thoughts, impulses, and actions.
Right inferior frontal cortex is a critical region for stop signal response inhibition.
Regions identified in this study were strongly lateralized to the right hemisphere and included the middle and inferior frontal gyri, frontal limbic area, anterior insula, and inferior parietal lobe.
Cognitive Flexibility
Recent reviews have identified a distributed network of frontoparietal regions involved in flexible switching, including vlPFC, dlPFC, anterior cingulate, and right anterior insula, the premotor cortex, the inferior and superior parietal cortices, the inferior temporal cortex, the occipital cortex, and subcortical structures such as the caudate and thalamus.
Left vlPFC cortex resolves the conflict during task switching to facilitate flexible performance.
Activations in the cerebellum were found to be related to set-shifting.
Planning and Problem Solving
The dlPFC has been identified as the critical area for performance on the Tower of London planning task.
The right PFC is involved in constructing the plan for solving the problem, whereas the left PFC is involved in supervising the execution of the plan.
Medial anterior PFC, in association with the ventral striatum, was engaged when participants executed tasks in sequences that were expected.
The polar PFC, in association with the dorsolateral striatum, was involved when participants performed tasks in sequences that were unpredictable.
Cortical and subcortical regions not located in PFC were also activated by the versions of the Tower of London task, including the caudate nucleus, the presupplementary motor area, the anterior premotor cortex, the posterior parietal cortex, and the cerebellum.
The dlPFC plays a critical role in planning through functional interactions with multiple cortical and subcortical regions.
Reasoning
Quantitative meta-analysis identified consistent areas of activations across studies in neural systems located in both cortical (frontal and parietal cortices) and subcortical (left basal ganglia) structures.
Activation in the PFC was specific to when two dimensions of variation needed to be considered simultaneously and integrated.
Neuropsychologic Tools to Assess EF
Many neuropsychologic tests were developed over the years to assess EF.
Ideal assessment involves gathering data from several tests and sources.
Standard Tests
The Wisconsin Card Sorting Test assesses the ability to learn and shift rules.
The Trail Making part B assesses the ability of set-shifting.
The Stroop Color Word Interference Test assesses the ability of inhibition of irrelevant information.
The Tower of Hanoi assesses planning abilities.
The Delis–Kaplan Executive Function System (D-KEF) and the Behavioral Assessment of Dysexecutive Syndrome (BADS) allow a wider assessment of several EF aspects.
Ecologic Tests
Ecologic settings are more useful in evaluating EF demands.
The Frontal Systems Behavior Scale (FrSBe) evaluates apathy, disinhibition, and executive dysfunction.
The Brock Adaptive Functioning Questionnaire (BAFQ) evaluates the frequency of EF deficits.
Ecologic Validity
Even the most ecologically valid tests are unlike most real-life situations.
Significant correlations between informant-reported DEX symptoms and neuropsychologic test performance exist.
EF tests were more specifically associated with patients’ lack of insight, as reflected in differences between their self-report DEX ratings and informant reports.
Clinicians should be aware that a caregiver whose spouse/offspring/parent has a left dACC/dlPFC lesion is at higher risk of feeling burdened.
Training to Improve EF
Patients with dysexecutive syndrome often fail to fully regain normal EF.
Cognitive Rehabilitation of EF
Cognitive domain significantly affects everyday life and social functioning.
Cognitive rehabilitation has aimed at improving specific EF abilities, such as planning, inhibition, or updating.
Some EF training studies focus on awareness of EF deficits during treatment.
From EF Rehabilitation to Social Reintegration
EF deficits reduce individuals’ abilities to regain satisfactory social lives.
This is a novel patient-centered intervention for community reintegration of veterans with mild TBI.
Noninvasive Brain Stimulation: TMS and tDCS
Noninvasive brain stimulation may enhance EF recovery/training through the induction and enhancing of plasticity processes.
TMS uses a small coil held near the scalp to generate a magnetic field that produces an electric field in the underlying brain areas.
High-frequency TMS was delivered twice a week over a 4-week period to rats to see if it could restore neuronal activity and improve neurophysiological and behavioral functions.
tDCS uses low-amplitude direct current to modify neuronal activity.
Anodal tDCS elicits prolonged increases in neural excitability and facilitates regional brain activity, while cathodal tDCS elicits the opposite effects.
Development of EF
EFs gradually develop and are among the last mental functions to reach maturity.
On average, children and older adults both show poorer EF performance relative to young adults.
During childhood and adolescence, EFs are continuously improving.
By the child’s first year the ability to inhibit overlearned behavior is developed, allowing the child increased attentional control over the environment.
Parental socioeconomic status predicted the children’s working memory performance at age 4.5 years and planning by first grade.
Executive control does not explain any age-related variance in complex cognition over and beyond the effects explained by a slower speed of processing.
EF and Genetics
EF may be influenced by genetic polymorphisms.
5-HT
Twin studies have established that genetic influences may account for a significant amount of variation in EF.
One candidate gene associated with EF is serotonin (5-HT).
A recent study found a significant interaction between 5-HTTLPR and parental supervision for cognitive flexibility.
COMT
Dopamine plays a key role in the modulation of activities in the PFC during EF tasks.
Catechol-O-methyltransferase (COMT) is an enzyme that metabolizes catecholamine neurotransmitters, such as dopamine, epinephrine, and norepinephrine.
The COMT gene is a regulator of dopamine levels in the PFC.
BDNF
Brain-derived neurotrophic factor (BDNF).
BDNF promotes survival and synaptic plasticity in the brain.
APOEE4
Apolipoprotein E allele 4 (APOEE4).
APOE transports, recycles, and clears lipids in the brain.
Summary and Outstanding Questions
EF encompasses a broad range of cognitive functions, which include planning, problem-solving, flexibility, and cognitive controls.
Impairments in EF reduce a person’s ability to return to work or school and to resume satisfactory social activities.
Longitudinal studies provide information about whether EF recovery facilitates functional and social outcomes.