Acting while perceiving: assimilation precedes contrast

Marc Grosjean 0 1 2 Jan Zwickel 0 1 2 Wolfgang Prinz 0 1 2 0 Present Address: J. Zwickel Neuro-Cognitive Psychology Unit, Department of Psychology, Ludwig Maximilian University Munich , Leopoldstrasse 13, 80802 Munich, Germany 1 M. Grosjean (&) Leibniz Research Centre for Working Environment and Human Factors , Ardeystrasse 67, 44139 Dortmund, Germany 2 M. Grosjean J. Zwickel W. Prinz Department of Psychology, Max Planck Institute for Human Cognitive and Brain Sciences , Munich, Germany To explore the nature of speciWc interactions between concurrent perception and action, participants were asked to move one of their hands in a certain direction while simultaneously observing an independent stimulus motion of a (dis)similar direction. The kinematics of the hand trajectories revealed a form of contrast eVect (CE) in that the produced directions were biased away from the perceived directions (Experiment 1). SpeciWcally, the endpoints of horizontal movements were lower when having watched an upward as opposed to a downward motion. However, when participants moved under higher speed constraints and were not presented with the stimulus motion prior to initiating their movements, the CE was preceded by an assimilation eVect, i.e., movements were biased toward the stimulus motion directions (Experiment 2). These Wndings extend those of related studies by showing that CEs of this type actually correspond to the second phase of a bi-phasic pattern of speciWc perception-action interference. - Many daily activities involve performing an action while simultaneously encoding one or more perceptual events. In contrast to most experimental tasks aimed at studying speciWc interactions between perception and action (for overviews, see Prinz & Hommel, 2002; Proctor & Reeve, 1990), such situations are characterized by the fact that what is perceived temporally overlaps with ongoing action for relatively long periods of time and does not (necessarily) specify what movements are to be concurrently produced. As common as these situations may seem, they have only recently gained interest. An illustrative example comes from a study by Jacobs and ShiVrar (2005), in which they asked people to judge the speed of point-light walkers while either standing, bicycling, or walking themselves. They found that speed judgments were less accurate during walking than during bicycling or standing. This Wnding not only demonstrates that action can interfere with perception, but that the nature of interference depends on whether what is produced (e.g., walking) shares representational features with what is perceived (e.g., seeing someone else walk; for related eVects, see Hamilton, Wolpert, & Frith, 2004; Schub, Prinz, & Aschersleben, 2004; Wohlschlger, 2000; Zwickel, Grosjean, & Prinz, 2007). Interference eVects of this type are referred to as speciWc, as opposed to unspeciWc, because they relate to changes in performance that are determined by the relationship or degree of feature overlap (at a representational level) between the contents of perception and action (Msseler, 1999). The purpose of the present study was to further this line of investigation by exploring how perception speciWcally interacts with action under such concurrent conditions. To do so, we relied on a task introduced by Schub, Aschersleben, and Prinz (2001) in which they showed that the simultaneous perception and production of movements leads to a repulsion between perception and action: movements were biased away from the stimulus motions that were simultaneously perceived. In the current study, we sought to explore the time course of this eVect and, in particular, establish whether it is actually preceded by an attraction between what is perceived and produced. That is, a movement bias toward the concurrently observed motions. Unlike conventional stimulusresponse (SR) compatibility-type tasks (e.g., Proctor & Reeve, 1990), Schub et al.s (2001) paradigm was developed to investigate speciWc interactions that arise when what is perceived is functionally unrelated to what is simultaneously produced. These terms are used to refer to any situation in which the movement required by the participant is not speciWed nor consists of a reaction to the stimulus that is concurrently presented (see also Zwickel et al., 2007). Their paradigm also involved more dynamic stimuli and movements than those usually encountered in the SR compatibility literature (for similar eVorts see, e.g., Brass, Bekkering, Wohlschlger, & Prinz, 2000; Chua & Weeks, 1997; Grosjean & MordkoV, 2001). On a given trial n, the currently presented stimulus motion (Sn) speciWed the required movement (Mn+1) for the subsequent trial, whereas the currently required movement (Mn) was speciWed by the stimulus (Sn1) presented on the previous trial. The stimulus consisted of a dot that moved along a sinusoidal trajectory of either a small, medium, or large amplitude, and the required movement consisted of drawing on a graphics tablet sinusoidal trajectories of one of the same (i.e., small, medium, or large) amplitudes without visual feedback. Thus, participants were required to perform a previously speciWed action while simultaneously encoding a functionally unrelated, but feature-overlapping, stimulus motion. The question of interest in this task was whether the perception of the amplitude of Sn would speciWcally interact with the produced Mn amplitude. This was the case. When participants were required to produce a medium-amplitude trajectory, watching a small-amplitude trajectory led to an increase in movement amplitude, whereas watching a large-amplitude trajectory led to a decrease in movement amplitude. Although Schub et al. (2001) did not perform any Wne-grained analyses of the time course of their eVect, they did report that it was already present at the Wrst extreme value of the sinusoidal movement trajectories, which corresponded to 600700 ms after movement onset (as inferred from their Fig. 2). These results were taken as evidence of a new type of speciWc perceptionaction interaction that can be characterized as a contrast eVect (CE). The eVect is contrast-like in that it is typiWed by a repulsion (as opposed to an attraction) between perception and action. Schub et al. (2001) accounted for their eVect by drawing inspiration from models of reaching in the presence of stationary distractor objects (e.g., Tipper, Howard, & Houghton, 1998). In particular, they proposed that people try to minimize the amount of interference between perception and action by engaging in the mutual partial inhibition of the representations or features that overlap for what is perceived and produced (i.e., the codes responsible for representing the amplitudes of the motions and movements, respectively). This, in turn, causes the representations underlying the perceived motion and produced movement to shift away from each other, thereby leading to a CE. Schub et al.s (2001) study demons (...truncated)