Neural and behavioral correlates of intentional actions

Abstract

An essential characteristic of human behavior is the ability to act out of intrinsic motivation to achieve desired goals. These kinds of actions have been classed as intentional or voluntary. They contrast with actions that are performed in response to external stimuli. Recent research has begun to explore the functional underpinnings of internally guided actions and how they differ from externally guided actions. From these studies, one can conclude that the two action modes differ with respect to their neural and behavioral correlates. Most importantly, functional neuroimaging and also lesion studies point to a crucial role of the frontomedian cortex in intentional actions. The present paper reviews the neuroanatomical, behavioral, electroencephalographical and functional neuroimaging findings related to intentional actions.The reviewed data suggest two dimensions for the control of action: First, a medial-lateral dimension with regard to intentional vs. stimulus-based action control. Second, an anterior–posterior dimension related to more abstract vs. specific decisions of action parameters.

Introduction

Every human being is familiar with the concept of intentional, willed action. Yet, it turned out to be very difficult to define intentional action scientifically. To cite James, 1890, James, 1950: “Desire, wish, will, are states of mind which everyone knows, and which no definition can make plainer” (p. 486).

After this opening sentence, James, 1890, James, 1950 characterizes intentional actions in much the same way as present research still does (Brass and Haggard, 2008, Frith, 1992, Haggard, 2008, Jahanshahi and Frith, 1998), namely by contrasting them with automatic or reflex actions. James argues that, in contrast to automatic actions, voluntary actions are performed purposefully and with the intention to reach a certain goal. As a result, he reasons, an action can be performed voluntarily only if the subject has first acquired knowledge of its consequences.

In addition to being purposive and goal-directed, current definitions consider intentional actions to be endogenously controlled and stimulus-independent (Brass and Haggard, 2008, Haggard, 2008). Furthermore, they require attention and offer a choice between alternatives (Jahanshahi & Frith, 1998). They can be inhibited and are not necessarily executed once the intention to act has been formed, but can be carried out after a delay. Automatic actions, in contrast, are considered to be triggered by an environmental stimulus. They do not require attention, or only to a lesser degree, and are conducted in an immediate and stereotyped manner. Once the action program has started, automatic actions cannot be stopped and inhibited.

However, it must be noted that contrasting intentional actions with automatic actions, although very helpful for theoretical considerations, does not capture human behavior as it takes place in everyday life. Human actions are rarely totally externally determined, nor are they ever completely internally guided. Rather, they almost always comprise external and internal components. Therefore, it might be more reasonable to assume that human actions exist along a continuum between the two extremes. Considering the predominant component, actions can then be categorized into those that are more internally guided, for example, switching on the TV instead of reading a book, and those that are more externally guided by environmental stimuli, for example stopping at a red traffic light.

Moreover, voluntary action should not be considered to be a unitary concept. Rather, it comprises at least three different components: what, when and whether (Brass & Haggard, 2008). The what component concerns the decision of which action to perform, the when component the decision when to perform the action (Müller et al., 2007, Krieghoff et al., 2009), and the whether component refers to the decision whether or not to perform the action (Brass & Haggard, 2007). In the following, we will refer to the what decision as internal/external action selection and to the when decision as internal/external action timing. The terms internally/externally guided actions will be used whenever no differentiation between the what and when component is made.

The aim of the present review is to integrate research from three different domains, namely functional and structural neuroanatomy, behavioral research and psychophysiological research. While there are overview articles covering one or the other domain (e.g. Brass and Haggard, 2008, Haggard, 2008, Passingham et al., 2010), an integrative overview of all domains is still lacking.