Motor accounts of inverse priming

Motor self inhibition. The first hypothesis on inverse priming was the motor self-inhibition account (Schlaghecken, & Eimer, 2002; Schlaghecken, Bowman, & Eimer, 2006). The core assumption was that inverse priming reflects an automatic, inhibitory mechanism located within the motor system serving to counteract the initial prime-evoked response activation.

The account was inspired by the demonstration of the time-course of priming effects in the inverse priming paradigm. At short mask-target SOAs, positive priming effects with benefits for prime-congruent targets were observed which reversed at longer mask-target SOAs such

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that performance on incongruent trials was better (Mattler, 2007; Schlaghecken, & Eimer, 2000; Schlaghecken, & Maylor, 2005; Schlaghecken, Birak, & Maylor, in press; Sumner, &

Brandwood, 2008). This facilitation-followed-by-inhibition time course was also reflected in electrophysiological data (lateralized readiness potentials, LRP, Eimer, & Schlaghecken, 1998, 2003; Praamstra, & Seiss, 2005). The reversal of prime-induced response activation was interpreted as a developing self-inhibition of the primed response and dis-inhibition of the alternative response. A close link between conscious prime perception and the reversal of priming effects (Eimer, & Schlaghecken, 2002; Klapp, & Hinkley, 2002) suggested that automatic self-inhibition is triggered by the removal of perceptual evidence for the prime due to masking. Later on, the additional assumption of a minimum sensory strength of the prime to trigger the inhibitory mechanism was added (Schlaghecken, & Eimer, 2002). This idea was motivated because weakening the perceptual strength of the prime elicited only positive, but not inverse priming. For instance, peripheral primes (Schlaghecken, & Eimer, 2000) or primes whose perceptual salience was reduced by embedding them within a random dot pattern (Schlaghecken, and Eimer, 2002) failed to produce inverse priming. To summarize the account, a prime of sufficient perceptual strength activates its assigned response and due to masking this preliminary activation becomes inhibited leading to inverse priming if this time-consuming process could develop until target presentation with sufficiently long mask-target SOAs.

Mask triggered inhibition. In the following, demonstrations of inverse priming with completely unmasked primes casted serious doubts on this original version of the self-inhibition account. In contrast to previous results (Eimer, & Schlaghecken, 2002) inverse priming also occurred with primes presented about 2° above and below fixation while

“masks” were placed at fixation (Jaśkowski, Biłuńska, Tomanek, & Verleger, 2008). Thus,

“masks” did not reduce prime visibility at all but rather act like a distracter and still inverse priming occured. Inverse priming with such non-masking flankers has been repeatedly demonstrated (Lleras, & Enns, 2006; Jaśkowski, 2007; 2008). Furthermore, Schlaghecken and colleagues (2008) found a dissociation of priming effects from visibility in a perceptual learning experiment. While prime visibility continuously improved across five learning sessions, priming effects remained the same. These results brought them to relax their claim of a close causal link between prime visibility and priming effects (Schlaghecken, et al., 2008). Along the same lines, others advocated a more general inhibitory mechanism triggered by the mask irrespective of prime visibility (Jaśkowski, & Przekoracka-Krawczyk, 2005;

Mattler, 2005; Lleras, & Enns, 2006). Thus, the only difference of these mask-triggered

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inhibition accounts to the self-inhibition account is whether the primed response inhibits itself or the inhibition is stimulus-triggered by the mask (Sumner, 2007; Wilson, Tresilian, &

Schlaghecken, 2010). In other words: Not the removal of the prime evidence itself by the mask is critical, but the occurrence of another potentially relevant visual stimulus causes inhibition of any on-going motor activation. Therefore, inhibition is especially effective when the masking stimulus contains task-relevant features (Jaśkowski, & Verleger, 2007).

Object Updating. The idea of an inhibitory mechanism was challenged by some researchers underscoring the special design of masking stimuli in many early studies on inverse priming (Lleras, & Enns, 2004; Verleger, et al. 2004). Typically, these early studies employed masks simply built by superimposing the prime alternatives upon one another (e.g. Eimer, 1999;

Eimer, & Schlaghecken, 1998; Eimer, Schubö, & Schlaghecken, 2002; Schlaghecken, &

Eimer, 2000; 2004; Schlaghecken, Münchau, Bloem, Rothwell, & Eimer, 2003). Within the framework of their object updating theory, Lleras and Enns (2004) argued that the rapid successive stimulus presentation in the inverse priming paradigm leads to the formation of a common object representation which is iteratively updated each time the visual input changes.

Novel elements become especially salient due to a novelty bias in perceptual processing implying that those mask elements, which are not part of the prime, gain a saliency advantage.

In case of the superposition mask, this perceptual interaction leads to a salience bias in favor of the prime-opposite object representation. This current perceptual state is directly translated to the motor system. Thus, even though priming is inverse with respect to the prime, it is in fact a positive priming effect because new features in the mask activate the prime-incongruent response. Consequently, object updating explains inverse priming without the need to assume an inhibitory mechanism.

The Object Updating account is capable of explaining inverse priming with so-called relevant masks – masks that contain task-relevant features like the superposition mask. It is now widely accepted that with such relevant masks the perceptual interaction of prime and mask at least contributes to inverse priming (Jaśkowski, & Przekoracka-Krawczyk, 2005; Klapp, 2005; Klauer, & Dittrich, 2010; Kiesel, Berner, & Kunde, 2008; Mattler, 2005; Schlaghecken, Rowley, Sembi, Simmons, & Whitcombs, 2007; Sumner, 2007). The role of object updating receives further support from the finding that inverse priming effects with relevant masks are typically much larger compared to inverse priming with irrelevant masks. Such irrelevant masks are completely free of task-relevant features like for instance a grid of vertical and horizontal lines for the case of arrow-shaped prime and target stimuli (Jaśkowski, 2007, 2008;

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Jaśkowski, et al., 2008; Jaśkowski & Przekoracka-Krawczyk, 2005; Kiesel, et al., 2008;

Lleras & Enns, 2004, 2005, 2006; Schlaghecken & Eimer, 2006; Verleger, Görgen, &

Jaśkowski, 2005; Verleger et al., 2004).

Importantly, however, Object Updating is incapable of covering all instances of inverse priming because inverse priming has been repeatedly demonstrated with irrelevant masks completely free of task-relevant features (Eimer & Schlaghecken, 2002; Klapp, 2005; Klapp

& Haas, 2005; Lleras & Enns, 2005; Schlaghecken & Eimer, 2006; Schlaghecken et al., 2007;

Sumner, 2008). Inverse priming with irrelevant masks can be explained by the above mentioned accounts proposing an inhibitory mechanism located in the motor system (Jaśkowski, & Przekoracka-Krawczyk, 2005; Lleras, & Enns, 2006; Schlaghecken, & Eimer, 2002). In opposition to this idea, some more recent accounts proposed different mechanisms operating at a perceptual or a central level.