1. Problems with the Concept of Executive Functioning
The basis for the concept of executive functioning (EF) arose in the 1840s in the initial efforts by scientists to understand the functions of the frontal lobes generally and the prefrontal cortex (PFC) specifically (Harlow, 1848, 1868; Luria, 1966). Indeed, the concept predates the term “EF” by more than 120 years. The concept of EF was at first defined by default as what the prefrontal lobes do (Pribram, 1973, 1976); they are, as Pribram said, the executive brain. The term “EF” came out of these earlier efforts to understand the neuropsychological functions mediated by the prefrontal or premotor regions of the brain. This history has led to a conflating of the term “EF” with the functions of the PFC and vice versa.
Over time, this conflation has led to a circularity of reasoning in that the functions of the PFC are said to be EF while EF is then defined back to the functions of the PFC. It has also led to a slippage in the discourse on EF between two separate levels of analysis (Denckla, 1996). One is the neuropsychological level involving thought (cognition), emotion, and verbal or motor action (behavior); the other is the neuroanatomical level involving the localization of those neuropsychological functions to specific regions of the brain and their physiological activity. But EF is not exclusively a function of the PFC given that the PFC has various networks of connections to other cortical and subcortical regions as well as to the basal ganglia, amygdala and limbic system, and cerebellum (Denckla, 1996; Fuster, 1989, 1997; Luria, 1966; Nigg & Casey, 2005; Stuss & Benson, 1986). The PFC may well engage in certain neuropsychological functions that would not be considered to fall under the umbrella of EF, such as simple or automatic sensory–motor activities, speech, and olfactory identification, to name just a few.
Thus, despite an extensive history concerning the nature of EF and of the functions of the PFC, several significant problems continue to exist in the definition of the term “EF,” its conceptualization, and its measurement. EF is a term describing psychological functions and is therefore a construct at the psychological level of analysis. If our understanding of EF is to advance, the concept of EF and its nature must be defined separately and specifically at the psychological level without reference to the neurological level being an essential part of that definition. Such a cross-referencing of levels is of interest to neuropsychology in determining what specific brain regions engage in what specific functions. But this activity requires that we have such functions properly defined at the psychological level first before we can determine what brain networks give rise to that psychological function. If EF and its larger purposes in human life are not well defined and developed, only confusion can reign at the neurological level as one searches for the neural networks that supposedly underlie a vaguely defined and poorly crafted psychological construct, perhaps in vain.
This book was written to address four related problems that currently exist in the concept of EF. First, there is neither a consensus definition of EF nor an explicit operational definition of the term that can simply, clearly, and efficiently determine which human mental functions can be considered executive in nature and which ones cannot be so classified. Simply put, What is EF? When definitions are too general or vague, as is EF, they leave considerable opportunity for misinterpretation as well as for including within the term’s conceptual realm excessive semantic baggage that would easily have been pared away had the definition had greater clarity and precision.
The first problem leads to the second. How is EF to be assessed? If the term is not defined operationally then anything goes; any measure or test can be declared to be executive in nature by mere assertion alone or through its alignment with any of 33 constructs attributed to it (Eslinger, 1996). As a case in point, my colleagues and I declared the Simon game to be a test of EF in one of our studies on adult attention-deficit/hyperactivity disorder (ADHD) (Murphy, Barkley, & Bush, 2001). This game requires one to replicate increasingly longer sequences of musical notes by depressing keys corresponding to each note. We asserted that this was a test of EF because it assessed nonverbal working memory and that it was similar to that of digit span in the verbal domain; this assertion went unquestioned in reviews of our paper. Granted, working memory has been defined as holding information in mind that is used to guide subsequent behavior, and this game seems to fulfill that definition. But then why is holding information in mind (working memory) itself an executive function? As a consequence of lack of definitional clarity, many tests and measures have been declared to be executive functioning tests without much basis for challenge. And so we cycle back to the first problem—the absence of an operational definition of EF, which then spawns the use of any variation of a test a researcher or clinician wishes to use to measure EF. To make the tautology complete, models of EF are then empirically developed based on tests of EF. Yet if there are problems in the choice of what measures assess EF, then these problems carry forward to create problems in the theories constructed upon them. Not the least of these problems is that of method variance—the theory will largely reflect the types of tests used to construct the model rather than being based on conceptual clarity and operational definition.
This state of affairs is actually related to a third larger problem this book intends to address: the lack of a coherent theory of EF. Theories are not just constructs, but mechanisms, which is to say explanations of relationships among constructs. They address the questions of How does EF work? or What does it do? Without a coherent theory of EF, constructs have multiplied to the extent that 33 or more have been claimed to be involved in this “metaconstruct” or umbrella term (Eslinger, 1996). The relationships among these various constructs have only been vaguely explained. For instance, are response inhibition and planning and problem solving related to each other? If so, in what way are they interactive? If not, why not, given that they have all been considered to be EF? So they must have some relationship to each other or else combining them under the umbrella construct of EF is nonsensical. If our goals are precision of thinking and definition as well as utility of prediction, this situation is patently unacceptable. Prediction requires explanation or understanding, and that requires propositions about how things relate to each other. Listing a set of constructs that are presumed to make up an umbrella, family, or metaconstruct will not suffice. There must be some operational definition as to what makes that list executive in nature and just how the constructs being labeled as EF relate to each other. In the absence of such an explanation, there is no theory of EF.
All three of these problems pertain to a much larger fourth problem. That issue is contained in the question Why EF? (Barkley, 2001). This is a different question from those raised above. Without answering it one is likely to get only partial answers to the other questions. To answer why humans developed EF, one must think about ultimate ends. For what purposes does EF exist? What is it accomplishing? What problem(s) in human daily existence does this mental mechanism, or more likely suite of mental mechanisms, exist to solve? The only explanation for that question comes out of evolution. Thus, this book will take an evolutionary or “adaptationist” stance in addressing the question Why EF?
This book seeks to address all four of these issues. Each will now be discussed in more detail to support the contention that these are serious problems even if 160+ years of history has made it seem as if they had been resolved. If there is to be further advancement of our understanding of the concept of EF, these problems stand in our way.
