HANDBOOK OF CLINICAL NEUROLOGY Series Editors

First, executive functions are defined. Then the development of executive functions in children, from infancy to 10–11 years of age, is briefly described. The relation between the speed of processing and the development of executive functions is addressed. Finally, tools and pointers for evaluating executive functioning in younger and older children are discussed. A cautionary note is sounded, in that almost no executive function measure requires only one executive function. A child might fail a working memory task because of problems with inhibitory control (not working memory), fail an inhibitory control task because of working memory problems, or fail a cognitive flexibility, planning, or reasoning task because of problems with inhibitory control or working memory. DEFINITION OF EXECUTIVE FUNCTIONS Executive functions (EFs) are a family of top-downmental processes that make it possible for us to pay attention and stay focused; reason and problem solve; exercise choice, discipline, and the self-control to avoid being impulsive, rash, or reacting without thinking; see things from different perspectives; mentally consider alternatives, see how different ideas or facts relate to one another, and reflect on the past or consider an imagined future; and flexibly adjust to change or new information (Jacques and Marcovitch, 2010; Diamond, 2013; Zelazo et al., 2016). EFs are recruited when it would be illadvised, insufficient, or impossible to go on autopilot or rely on instinct or intuition, such as when presented with novel, unanticipated challenges. Using EFs is effortful. It is not easy to manipulate numbers, facts, or ideas in your head. It is easier to give into temptations than to resist them. It is easier to continue doing what you have been doing than to change or to put thought into what to do next. It only makes sense, therefore, to reduce the demands on one’s EFs as much as possible. It makes sense to write down notes rather than try to hold everything in your head. It makes sense to avoid situations where you will be strongly tempted rather than taxing your willpower. There are three core EFs—working memory (WM), inhibitory control, and cognitive flexibility—and each is composed of two subparts.WM involves actively holding information in mind and mentally working with that information, i.e., mentally working with information that is not perceptually present (Baddeley and Hitch, 1994; Smith and Jonides, 1999; Kent, 2016). WM is critical, e.g., for making sense of anything you read or hear spoken that is longer than a word or two—for you have to hold in mind what you read or heard earlier and relate that to what you are reading or hearing now. WM is critical for doing any mental calculations or mentally playing with ideas or possibilities. Those “aha” moments when you suddenly see how one thing relates to another happen are made possible by your working memory ability. The subparts of WM refer to content area. Thus, there is verbal WM and visuospatial WM. Some people define WM as holding information in mind while mentally working on either that material *Correspondence to: Adele Diamond, Department of Psychiatry, UBC, 2255 Wesbrook Mall, Vancouver, BC, Canada V6T 2A1. Tel: +1-604-822-7220, Fax: +1-604-822-7232, E-mail: adele.diamond@ubc.ca Handbook of Clinical Neurology, Vol. 173 (3rd series) Neurocognitive Development: Normative Development A. Gallagher, C. Bulteau, D. Cohen and J.L. Michaud, Editors https://doi.org/10.1016/B978-0-444-64150-2.00020-4 Copyright © 2020 Elsevier B.V. All rights reserved or any other, or the ability to actively hold information in mind in the face of interference or distraction (Engle and Kane, 2004; Unsworth and Engle, 2007; D’Esposito and Postle, 2015). WM conceived of in this way includes holding in mind a question or comment you want to raise while following what is being said. Here you are not mentally manipulating that question or comment (you are just holding it mind) but while doing that you are actively processing other information. There is agreement that holding information in mind without actively processing that or other information or inhibiting distraction, is short-term memory, not WM. Inhibitory control involves being able to control one’s attention, behavior, thoughts, or emotions to override a strong internal predisposition or external lure, and instead do what you intend to do (Simpson et al., 2012; Wiebe et al., 2012; Diamond, 2013; Watson and Bell, 2013). Thus, it involves resisting a strong inclination to do one thing and, instead, doing what is most needed or appropriate (Diamond, 2011). The two subcomponents of inhibitory control are (a) self-control or response inhibition and (b) interference control. Selfcontrol involves control over one’s behavior and control over one’s emotions in the service of controlling one’s behavior. Self-control is about suppressing a dominant response, or one’s first impulse, and giving amore appropriate response instead. The strong inclination might be, for example, to cut in line, grab what you want without asking or paying, reflexively striking back at someone who has hurt your feelings, or blurting out the first thing that comes to mind. Self-control is the opposite of acting impulsively, thinking instead before you speak or act so you do not do something you might regret, waiting before rushing to judgment. It also includes the discipline to stay on task and complete what you started, resisting all the temptations to quit, even if the reward might be a long time in coming. The subcomponent of interference control involves controlling one’s attention and thoughts. At the level of attention it is selective attention, resisting distractions in the environment and sustaining one’s focus, as one might need to do in a noisy restaurant or when singing in a round (Driver, 2001). At the level of cognitive inhibition it is resisting internal distraction, such as extraneous or unwanted thoughts, resisting mind-wandering (Anderson and Levy, 2009; Swallwood and Schooler, 2015; Keulers and Jonkman, 2019). When WM is defined as staying focused on what one is holding mind and resisting distractions, it immediately becomes apparent how it is the internal counterpart to selective attention, which involves staying focused on something in the external world and resisting distractions. The close connections and interactions between WM and selective attention, both at the behavioral and neural levels, have been widely documented (Awh et al., 2005; Nobre and Stokes, 2011; Gazzaley and Nobre, 2012; de Fockert, 2013). Inhibitory control is critical for avoiding social faux pas and for a civil society where people abide by rules and norms (Diamond and Ling, 2016). Choice would not be possible were we not able to resist, at least partially, the pull of external stimuli, our emotions, or old habits of thought or behavior. Without inhibitory control, we would be at the mercy of external stimuli, internal urges, and old habits of thought or action. Inhibitory control makes it possible for us to choose (i.e., exercise voluntary control over) how we react and to change how we behave rather than being “unthinking” creatures of habit or impulse (Diamond, 2013). It does not make it easy, but it makes it possible. The third core EF, cognitive flexibility, is also referred to as set shifting or mental flexibility. One subcomponent of cognitive flexibility is switching between different tasks or mindsets, seeing something from different perspectives, e.g., when you shift from thinking about the economic consequences of an event to thinking about the human consequences or when you shift from thinking about someone’s flaws to thinking about that person’s virtues or hardships. The other aspect of cognitive flexibility is quickly and flexibly adjusting to change, such as accommodating to a sudden change of topic or finding an alternative route to your goal when the path you intended to take is blocked. The opposite of cognitive flexibility is cognitive rigidity, not being able to see another way of looking at things or being unable to accommodate to change. Alexander Graham Bell gave us an example of poor cognitive flexibility when he said, “When one door closes, another door opens, but we often look so long and so regretfully upon the closed door, that we do not see the ones which open for us. ” From these three core EFs, higher-order EFs are built such as reasoning, problem solving, and planning (Collins and Koechlin, 2012; Lunt et al., 2012). Reasoning and problem solving are essentially what fluid intelligence is, and the correlation between WM and fluid intelligence is extremely high (Fry and Hale, 2000; Kane and Engle, 2002; Chen and Li, 2007; Fukuda et al., 2010; Duncan et al., 2012). Thus, fluid intelligence could be considered a higher-order EF. A distinction is often made between hot and cool EFs (Zelazo and Carlson, 2012). Hot refers to situations where EFs are needed in a situation where emotions are high, where you really care about the outcome. Cool refers to situations that are more affectively neutral. Cool EFs seem to be more predictive of academic achievement (Brock et al., 2009) whereas hot EFs are more predictive of behavior in socially charged situations (Conner et al., 2008; Kim et al., 2013), although this 226 A. DIAMOND