Abstract
Abstract: Actions carried out in response to exogenous stimuli and actions selected endogenously on the basis of intentions were compared in terms of their behavioral (movement timing) and electrophysiological (EEG) profiles. Participants performed a temporal bisection task that involved making left or right key presses at the midpoint between isochronous pacing signals (a sequence of centrally-presented letters). In separate conditions, the identity of each letter either (1) prescribed the location of the subsequent key press response (stimulus-based) or (2) was determined by the location of the preceding key press, in which case participants were instructed to generate a random sequence of letters (intention-based). The behavioral results indicated that stimulus-based movements occurred earlier in time than intention-based movements. The EEG results revealed that activity reflecting stimulus evaluation and response selection was most pronounced in the stimulus-based condition, whereas activity associated with the general readiness to act was strongest in the intention-based condition. Together, the behavioral and electrophysiological findings provide evidence for two modes of action planning, one mediated by stimulus-response bindings and the other by action-effect bindings. The comparison of our results to those of an earlier study ( Waszak et al., 2005 ) that employed spatially congruent visuo-motor mappings rather than symbolic visuo-motor mappings suggests that intention-based actions are controlled by similar neural pathways in both cases, but stimulus-based actions are not.
Highlights
Human actions exist on a continuum with regard to whether they are driven endogenously or exogenously
The second aim of the current study was to clarify whether the differences in stimulus-locked and responselocked ERPs observed by Waszak et al (2005) for intention-based and stimulus-based actions were mainly attributable to differences in stimulus evaluation or to differences in movement planning for the two types of action
It can be seen that movements generally preceded the true bisection point. (For a discussion of such “negative asynchronies” in sensorimotor synchronization tasks, see Aschersleben, 2002.) the analysis of variance (ANOVA) for these asynchronies yielded no main effects of the action variable or the intersignal interval (ISI) variable or the block-half variable, Fs(1, 9) < 1, there was a significant interaction between the action variable and the block half variable, F(1, 9) = 14.67, p
Summary
Human actions exist on a continuum with regard to whether they are driven endogenously (internally by the agent) or exogenously (externally by the environment). In the middle ground reside endogenously-driven actions that are (more or less) directed toward producing desired effects in the environment (e.g., singing a nursery rhyme to entertain a child) as well as exogenously-driven actions that are voluntary (e.g., clapping along once the child takes up the tune). We refer to these actions as intention-based and stimulus-based, respectively. Other research on the intentionality of actions is motivated by philosophical concerns related to the subjective experience of free will (e.g., Haggard, Clark, & Kalogeras, 2002; Libet, Curtis, Gleeson, Wright, & Pearl, 1983)
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