Humans interact with their environment in at least two different ways, according to whether the pattern of behaviour is selected by the agents themselves, or specified by external information.
One type of action is primarily performed in response to environmental demands. Here, the selection of the action is already implied by external information. For example we usually stop at a crossing when the traffic light turns red. Such actions are considered to be under stimulus control. Arbitrary stimulus–action mappings can be established through learning (e.g., Logan, 1988; Thorndike, 1911), or by instructions which specify an intentional set (e.g., Brass, Wenke, Spengler, & Waszak, 2009; De Houwer, Beckers, Vandorpe, & Custers, 2005; Waszak, Wenke, & Brass, 2008; Wenke, Gaschler, & Nattkemper, 2007; Wenke, Gaschler, Nattkemper, & Frensch, 2009). Once stimulus–response (S–R) associations are established, external stimuli trigger their associated re-actions more or less automatically (Hommel, 2000; Woodworth, 1938).
In another type of action, agents themselves select an appropriate action. Their selection normally aims to produce specific environmental effects (Hommel, Müsseler, Aschersleben, & Prinz, 2001; Lotze, 1852; Prinz, 1997), or to satisfy particular needs (Skinner, 1953). For example, someone who remembers a friend’s birthday may select a specific present with the intention of making the friend happy. This class of actions have two distinguishing psychological characteristics: they involve internal generation of the action, and they involve mental representation of the effects of the action.
The brain regions involved in these two types of actions are dissociable at least in part (Goldberg, 1985; also see Waszak, Wascher, Keller, Koch, Aschersleben, & Rosenbaum, 2005). Responding to external stimuli predominantly recruits a circuit involving the parietal lobes and the lateral premotor areas (PMA). In contrast, internal action generation more heavily relies on fronto-median brain areas including the pre-supplementary motor area (pre-SMA) and cingulate motor areas (CMA). Interestingly, different brain systems are not only involved in internal versus external decisions on what to do (Cunnington, Windischberger, Robinson, & Moser, 2006; Dirnberger, Fickel, Lindinger, Lang, & Jahanshahi, 1998; Lau, Rogers, & Passingham, 2006; Müller, Brass, Waszak, & Prinz, 2007), but also in self-initiated versus externally triggered timing of actions (i.e., in the decision regarding when to act; Cunnington, Windischberger, Deecke, & Moser, 2002; Deiber, Manabu, Ibanez, Sadato, & Hallet, 1999; Jahanshahi, Jenkins, Brown, Marsden, Passingham & Brooks, 1995; Jenkins, Jahanshahi, Jueptner, Passingham & Brooks, 2000; Krieghoff, Brass, Prinz, & Waszak, 2009).
The intentional and reactive routes to action necessarily converge on a single motor execution system, which Sherrington (1906) termed the ‘final common path’. Thus, both pre-SMA and premotor cortex initiate movements by their projections to the primary motor cortex. Intentionally selected and externally specified actions therefore have the same kinematics (Jahanshahi et al., 1995). However, surprisingly little is known about how the mode of selection affects the subjective experience of action. Specifically, only a few studies (Haggard, Aschersleben, Gehrke, & Prinz, 2002a; Haggard, Clark, & Kalogeras, 2002b; Haggard & Clark, 2003; Repp & Knoblich, 2007; Sebanz & Lackner, 2007) directly investigated how the mode of action selection affects the experience of being in control of one’s actions and their effects: the so-called sense of agency. These studies appear to be generally consistent with the intuitive prediction that intentional selection of an action produces a more pronounced sense of agency than external specification of the same action.
We measured the experience of action using a temporal binding effect. Temporal binding refers to the finding that actions are perceived to occur later, and their effects (tones) earlier, when voluntary actions are performed in an operant context than in control conditions where the time of actions or tones are judged in isolation (Haggard et al., 2002b). That is, operant actions and their effects are attracted towards each other in perceived time. This temporal attraction effect cannot be explained by mere contiguity-based association of events, because it was absent when two sensory events (i.e., two successive tones) or two motor events (i.e., two successive keypresses) occurred separated by a similar interval (Haggard et al., 2002a). Further, these temporal shifts were reversed when tones followed TMS-induced passive movements rather than voluntary actions (Haggard et al., 2002b). Instead temporal binding seems specific to intentional actions, and has therefore also been termed “intentional binding” (Haggard et al., 2002b).
Aims of the study
So far, most of the studies on action experience (e.g., Haggard et al., 2002b; Haggard & Clark, 2003) manipulated “voluntariness” by comparing active self-initiated movements and passive involuntary movements. Therefore, they do not allow any conclusions regarding how the mode of action selection affects action experience. The current experiment addresses this issue directly by studying active movements under various conditions of action selection within the same task context.
Most previous studies investigating the functional and neuronal basis of selection mode compared internally generated and stimulus-driven action with respect to either action selection (what-dimension; e.g., Herwig, Prinz, & Waszak, 2007; Müller et al., 2007) or action timing (when-dimension; e.g., Jahanshahi et al., 1995). In our study, by contrast, both dimensions varied simultaneously and independently. Participants performed either a left or a right key press (action choice) in either the first or the second of two designated intervals (action timing). Participants made left or right keypresses, either in response to a specific cue, or as they freely chose. Moreover, the time of each keypress could either be explicitly cued to occur in one of two intervals, or subjects freely chose in which interval to act. The task is based on the paradigm introduced by Krieghoff et al. (2009), and is summarized in Fig. 1.
Limiting the manipulation of timing selection to the choice between two alternative intervals maximizes comparability of the what- and the when-dimensions. However, in our task participants always internally determined the time point for initiating action within the designated interval, independent of whether they internally choose the specific interval, or whether the interval is specified by the cue. Therefore, this task does not compare completely free with completely fixed selections, but instead systematically varies the degree of internal generation required across the two conditions.
Each keypress was followed by a tone. We used four different tones that were contingently mapped to the specific combination of the what and when parameters of an action (i.e., a left keypress during the first interval evoked tone-1, a left keypress during the second interval tone-2, etc.). Note that these action–effect mappings remained the same whether the selection of action was internally generated or externally triggered. That is, each combination of what and when-selection produced a specific outcome, allowing us to directly compare how mode of selection influences effect prediction. Temporal binding was assessed as in previous studies (Haggard et al., 2002b). Participants monitored a rotating clock hand (Libet, Gleason, Wright, & Pearl 1983) while making a movement and/or hearing a tone. They judged the time of movement or tone onset in separate blocks. Comparing the perceived time of movements in blocks where they evoked tones with blocks without tones provides a measure of binding of actions to effects. Similarly, the perceived time of tones in blocks where the participant evoked tones through their actions was compared with blocks where tones occurred without action to measure the binding of effects to actions (see “Methods”; Fig. 1).
This design allowed us to compare several hypotheses regarding how selection mode might affect temporal binding. These hypotheses are summarized in Fig. 2. On one hypothesis (Hypothesis 1), binding should be more pronounced for internally selected than for stimulus-based actions. This view reflects the folk notion of “free will”, which links the sense of control to free selection between alternative actions. If the overall degree of internality of selection determines binding independent of dimension, then most binding should be observed for completely internal actions, least binding for completely stimulus-cued actions, and intermediate levels of binding in mixed selection conditions where either action identity or action timing are specified by the cue, but not both (Fig. 2, 1a). Our design also allowed us to compare whether selecting what action to perform, or when to perform it had the greater effect on action experience. If binding depends on internal selection of what action to perform (e.g., Herwig et al., 2007; Herwig & Waszak, 2009; but see Hommel et al., 2001, for a review of findings that demonstrated action–effect binding for externally chosen actions), then we should observe more binding when participants can freely choose between action alternatives than when the cue specifies the action (Fig. 2, 1b). Conversely, if temporal binding primarily depends on internal timing of actions (e.g., Haggard et al., 2002b), then binding should be more pronounced in the internally than the externally timed conditions (Fig. 2, 1c).
An alternative “selection compatibility” hypothesis (see Fig. 2, 2) predicts that temporal binding might depend on a common mode of selection for what- and when-information. That is, pronounced binding should occur when the same route to action (i.e., either the internally generated route or the stimulus-driven route) controls both action selection and action timing. By the same token, one should observe less temporal binding when the output of the two systems has to be combined in the mixed mode conditions. This possibility is supported by previous reports of interference between internally timed and stimulus-triggered action (Astor-Jack & Haggard, 2005; Obhi & Haggard, 2004). For example, the reaction time to make a simple response to an external trigger stimulus is increased when the internal action system has prepared to make the same action. Such interference does not occur when the same system specifies all parameters of an action.