We compared the classical motorvisual dual-task priming paradigm with three different stimulus sets as target stimuli in the visual discrimination task: hand pictures, arrows and words. According to previous findings with motorvisual priming, we expected negative priming for each stimulus set: that is, we expect better performance when response in the motor task and stimulus in the visual discrimination task are incongruent on the element-level than when they are congruent. The left/right representation should be occupied by motor response processing, and should, consequently, be difficult to access for perceptual processing of a congruent visual discrimination target.
Importantly, we hypothesized that the negative priming effect should be stronger for hand pictures than for arrows, than for words. Words overlap with button presses only on the verbal semantic ‘left’/‘right’ dimension. Arrows overlap with button presses additionally on the non-verbal symbolic level. Finally, pictures of hand movements overlap with hand movements above the verbal and semantic dimensions also on a variety of low-level physical anatomical dimensions.
We hypothesized that all stimulus sets would show negative priming effects, and that the priming effect would get stronger with higher set-level congruency between stimulus and response. That is, the priming effect should have been stronger for hands than for arrows than for words.
Our predictions have been confirmed by the results for hand and word stimuli. First, for both stimulus set, element-incongruent trials lead to better performance than element-congruent trials, though the effect was not significant for words. Second, the magnitude of the priming effect was stronger for hands than for words. However, for arrow stimuli, the results were surprising and not predicted by our hypotheses. Indeed, a significant motorvisual priming effect was observed, but contrary to our expectancy, it was positive. This result stands in stark contrast to previous motorvisual priming studies with arrowheads (see Thomaschke, Hopkins, & Miall, 2012a
, for a review).
Yet, there is a testable explanation for this unexpected result, based on the planning and control model (PCM) of motorvisual priming (Thomaschke, 2012
; Thomaschke et al., 2012a
). According to the PCM, there is a fundamental difference between the processing of scalar and categorical representations in motor cognition. Categorical representations code action features like the identity of a graspable object, the identity of the acting effector, the valence of the action, etc. These representations classify actions into rather coarse-grained classifications. They convey, among others, also symbolic and semantic information about actions. Scalar representations, on the contrary, code the action’s current position as coordinates in a feature space with metric properties, on dimensions like location, orientation, size, and weight. Scalar representations allow, for instance, computing the future path of actions, or its exact spatial relation to objects.
Categorical representations of action features are known to be involved in action planning and selection, whereas scalar representations are primarily involved in action control (Glover, 2004
; Glover, Wall, & Smith, 2012
). The PCM claims that action planning is primarily responsible for negative motorvisual priming. Selection of an action binds all representations of categorical action features into a compound representation of that action, and shields them against other cognitive processes. Thus, perception of such features is impaired during action (Hommel et al., 2001
; Müsseler, Steininger, & Wühr, 2001
). As action selection (not action control) is the primary explanatory domain of ideomotor theory, our literature review was focused on studies, where stimuli and responses overlapped on categorical dimensions. Accordingly, we have chosen the stimulus sets in for the present study so that they overlapped with the response on a categorical stimulus dimension (i.e., the binary categories ‘left’/‘right’). In line with all previous motorvisual priming studies (James & Gauthier, 2009
; Kunde & Kiesel, 2006
; Kunde & Wühr, 2004
; Müsseler, Wühr, & Prinz, 2000
; see Thomaschke et al., 2012a
, for a review), we hypothesized that the priming effect would be negative.
However, PCM also claims that the processing of scalar representation in action control leads to positive motorvisual priming effects. Scalar representations play an important role in fast online action feedback processing during control; consequently, congruent scalar representations are facilitated. Accordingly positive motorvisual priming has been observed for response–stimulus overlap on various scalar dimensions, like size (Fagioli, Ferlazzo, & Hommel, 2007
; Fagioli, Hommel, & Schubotz, 2007
; Symes, Tucker, Ellis, Vainio, & Ottoboni, 2008
; Wykowska, Hommel, & Schubö, 2011
; Wykowska, Schubö, & Hommel, 2009
), location (Collins, Schicke, & Röder, 2008
; Deubel, Schneider, & Paprotta, 1998
; Fischer & Hoellen, 2004
; Hommel & Schneider, 2002
; Koch, Metin, & Schuch, 2003
; Linnell, Humphreys, McIntyre, Laitinen, & Wing, 2005
; Müsseler, Koch, & Wühr, 2005
), weight (Hamilton, Wolpert, & Frith, 2004
), or orientation (Lindemann & Bekkering, 2009
How does the PCM relate to the present results? Although it is well established in previous literature that arrows are typically processed categorically as symbols denoting the categories ‘left’ and ‘right’ (e.g., Müsseler & Hommel, 1997a
), the arrows might have been processed via scalar representations in our study. Instead of representing and processing the arrows as conveying categorical symbolic information, participants might have encoded and processed locational information of the arrows. They might have attended only to the location of the arrows apex, instead of processing its symbolic meaning. Evaluating whether the arrow’s apex appeared on the left or right side of the decision relevant area, would have also allowed to classify its direction correctly. Thus, the left/right information of the arrows was represented scalar in the form of location information. As response–stimulus overlap on scalar dimensions leads to a positive priming effect, this assumption would be in line with the observed results.
We assume that the scalar processing of arrows was caused by the way we constructed the stimuli. Previous studies with arrows usually described the stimuli by the symbols ‘>’ and ‘<’ appearing in the methods sections. Further information about the thickness of the lines, the angle between these lines and so on is not given. Instead of using the standard font symbols, we constructed the stimuli from scratch as geometric triangles, with relatively broad arrowheads. This might have biased participants to scalar locational encoding of the left/right information.
This interpretation is strongly supported by the temporal dynamics of the priming effect. The influence of action planning typically declines over the course of an action, while the influence of action control increases. If the priming effect for hands and words was due to categorical processing in planning, while the priming effect for arrows was due to scalar processing in control, one would expect over the course of the action a decrease in the former two priming effects, but an increase in the latter one. These were exactly the dynamics observed in the present study as we changed the stimulus onset asynchronies.
Furthermore, the scalar processing of our arrowhead stimuli can be independently tested in Experiment 2, because also the Simon effect has been shown to differ in dynamics for scalar and categorical stimulus–response overlap (see below).
To conclude, for hand and for word stimuli, we confirmed our hypothesis: higher set-level congruency leads to a larger motorvisual priming effect. Yet, the arrow stimuli seem to have been processed as conveying scalar locational information. Processing of scalar information is, however, not within the scope of the ideomotor theory. Thus, our initial hypotheses do not apply to the arrow stimuli. We have modified the hypothesis for Experiment 2 accordingly (see below).