Project IV: Inverse non-motor priming

Non-motor priming is a straightforward extension of the traditional motor priming paradigm which was introduced by Mattler (2003). Instead of three stimuli, the stimulus sequence comprises an additional target stimulus in inverse non-motor priming. The other stimuli are identical in both paradigms: primes are followed by a masking stimulus which is followed by third stimulus (the target in motor priming). Instead of requiring a specific motor response, this third stimulus serves as a cue announcing which one of two alternative tasks has to be performed with respect to the upcoming target stimulus. Just like in motor priming, primes and cues can either announce the same task (congruent condition) or different tasks

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(incongruent condition). The inverse non-motor priming effect consists in better performance in the incongruent condition as opposed to the congruent condition (Mattler, 2006; 2007).

Due to the astonishing analogies between motor and non-motor priming with respect to the role of prime visibility and the time course of the effects (Mattler, 2003; 2007), it seems reasonable to assume a common underlying mechanism producing both effects. Therefore, the application of inverse non-motor priming can provide important insights in the study of this common mechanism. Because inverse non-motor priming cannot originate from the motor system, the existence of the effect with irrelevant masks would conflict with the idea of a crucial involvement of a direct translation route from perceptual analysis to motor output in inverse priming with irrelevant masks. To test this, Project IV comprised three experiments applying a new variant of the non-motor priming paradigm developed by Mattler (2006; 2007;

Krüger, & Mattler, in prep., see Appendix IV). In Experiment 1, the new paradigm was first introduced using a relevant mask while other aspects were very similar to the experiments carried out by Mattler (2007, Exp. 2 and 3). However, using a relevant mask leaves open the possibility that effects are due to perceptual interactions of primes and masks rendering incongruent parts of the mask more salient which facilitates preparation of the prime-incongruent task. Therefore, we conducted Experiment 2 and 3 which studied inverse non-motor priming with irrelevant masks. We used the orthogonal mask also employed in Project II and III and combined it with either double arrow primes and cues (Experiment 2) or the arrow pattern stimuli (Experiment 3). The stimuli are depicted in Figure 4.

Figure 4. Stimuli used in Project IV. Experiment 1 used a relevant mask, Experiment 2 and 3 irrelevant masks.

Target stimuli were the same throughout the entire series of experiments and consisted of two superimposed photographs. One (out of 16) instance is depicted here. Note also that primes and cues are non-compatible to the tasks since there is no obvious common dimension between the two alternative tasks and the stimuli (see General Discussion).

Prime

Exp. 1

Exp. 2

Exp. 3

Mask Cue Target

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Subjects were required to perform one out of two alternative tasks on a complex target stimulus which was composed of two overlaid photographs, one depicting a human face, the other one a visual scene. The preceding cue indicated on which aspect of the target stimulus subjects had to focus. One cue required the subject to attend to the face and report whether it is a female (left response) or male face (right response). The alternative cue required the subject to attend to the scene reporting whether the depicted setting is located indoor (left response) or outdoor (right response). Previous to cue onset, a prime followed by a mask were shown. Apart from prime-cue congruency, we also manipulated the mask-cue SOA which proved to be a crucial determinant of the size of inverse non-motor priming effects (Mattler, 2007). All in all, subjects performed four sessions, the first one serving to practice the task, the last one was a prime recognition session.

In Experiment 1, we replicated inverse non-motor priming with relevant masks using the new paradigm. Corroborating previous findings (Mattler, 2007), inverse priming effects were independent of the individual ability to recognize the prime stimuli. Evidence for inverse non-motor priming with irrelevant masks in Experiment 2 and 3 was more ambiguous. Even though we included data from two one-hour sessions of 24 subjects in each experiment, we almost completely failed to observe a stable inverse non-motor priming effect across the entire sample (an inverse effect on error rates became manifest in Experiment 3). However, we further found positive correlations across subjects between individual priming effects and prime visibility data, especially in Experiment 2, i.e. inverse effects were more pronounced among those subjects being unable to consciously perceive the primes. This result led us to divide the entire sample according to prime visibility. A subgroup of 8 participants per experiment, showed prime recognition performance indistinguishable from chance-level.

Among these subjects we observed a stable inverse priming effect across the entire range of mask-cue SOAs tested in Experiment 2. In Experiment 3, the size of inverse priming effects depended on the mask-cue SOA and we only found an inverse effect for the 150-ms-SOA.

To sum up, in contrast to relevant masks, inverse non-motor priming with irrelevant masks seems to be rather instable and was only observed among those subjects showing subliminal prime perception. The modulation of inverse priming effects with irrelevant masks by masking efficiency is in line with analogous findings from Project II (all but the superposition mask) and Project III (Experiment 1; see Krüger, & Mattler, in prep., Appendix III). The existence of inverse non-motor priming with irrelevant masks casts doubts on the idea that a direct link to the motor system is essential for inverse priming to occur. On the other hand, the

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small effect might result from a different mechanism as inverse motor priming. This issue will be further addressed in the General Discussion.

Project I to IV provided evidence for discernible mechanisms involved in the generation of inverse priming with response-compatible and non-compatible stimuli. Disregarding the small but stable non-motor priming effect in Project IV, response non-compatible stimuli seem to require the use of a relevant mask to achieve inverse priming (Project I and III, similar results were also reported by Jaśkowski, & Ślósarek, 2007; Kiesel, et al., 2008). Under these conditions, the effect seems to arise completely from perceptual levels of processing (Project I, Mattler, 2006) supposedly because perceptual processing of incongruent targets is facilitated due to enhanced processing of prime-incongruent mask features. On the other hand, inverse priming with response-compatible stimuli like arrows was demonstrated for both types of masks (Project II and III, e.g. Eimer, & Schlaghecken, 1998; Schlaghecken, & Eimer, 2006; Schlaghecken, et al., 2007). In opposition to the mask-induced effect with relevant masks, this effect seems to be completely due to motor processes irrespective of whether a relevant (Eimer, 1999; Klapp, & Haas, 2005; Klapp, & Hinkley, 2002) or an irrelevant mask is used (Project III, Boy, Evans, et al., 2010; Boy, Husain, et al., 2010; Klapp, 2005;

Schlaghecken, et al., 2009; Sumner, et al., 2007). Thus, the perceptual effect which generates the effect with relevant masks when non-compatible stimuli are used is not detectable anymore with compatible stimuli. Such compatible stimuli might directly activate their corresponding response (Georgopoulos, et al., 1989; Kornblum, et al., 1990; Eimer, 1993, 1995; Eimer, et al., 1995; Procter, et al., 1995) and the inhibition-based reversal due to the mask might only be reinforced when a relevant mask is used. To test whether inverse priming with response-compatible stimuli truly originates from such a mechanism located at motor levels of processing we applied functional magnetic resonance imaging in order to reveal the neural substrates underlying this effect.