Project V: Imaging control operations in inverse priming

In Project V, we applied functional magnetic resonance imaging (fMRI) to study the anatomical location of the mechanism underlying inverse priming with response-compatible stimuli for both types of masks (Krüger, Klapötke, Bode, & Mattler, submitted, see Appendix V). With such response-compatible stimuli, the results obtained so far suggest that primes can directly activate motor preparation processes which are subject to motor inhibition. The regulation of these motor preparation processes involves cognitive control operations

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(Ridderinkoff, Wildenberg, Segalowitz, & Carter, 2004). Consequently, these control processes would also be expected to be located in the motor system. In contrast, it would be also possible that control operations in inverse priming intervene with processing on ealier levels such that activation of competing perceptual representations is altered by cognitive control. We applied fMRI to investigate whether control operations in inverse priming with response-compatible stimuli are indeed located within the motor system or whether they are also detectable at perceptual levels of processing.

Imaging inverse priming requires a concrete model of the processes underlying the effect from which predictions can be derived concerning the expected neural effects. Based on the evidence gathered so far and in line with current motor accounts on inverse priming (Boy, Husain & et al., 2010; Jaśkowski, & Verleger, 2007; Lleras, & Enns, 2006), we hypothesized that inverse priming reflects a series of control processes which become apparent in dynamic changes of motor preparation processes. These motor activations can be illustrated by electrophysiological recordings over the motor cortices. Then, the LRP can be calculated which is suitable to infer the underlying motor preparation processes (Colebatch, 2007). In inverse priming, a three-phasic pattern of motor activation processes has been repeatedly demonstrated (Eimer, & Schlaghecken, 1998, 2003; Praamstra, & Seiss, 2005). An initial activation phase of the response associated with the prime is followed by a reversal probably due to mask presentation. While incongruent targets simply continue this reversed tendency until a response threshold is reached, congruent targets induce a further reversal and redirect activation accumulation in the initial prime-induced direction (see Figure 5, right column).

Consequently, on congruent trials a second, independent compensation of the misleading mask-induced reversal is required which is unnecessary on incongruent trials. The abortion of such premature response tendencies is supposed to be accomplished by a central cognitive control instance moderating response competition (Ridderinkoff, et al., 2004). The aim of the present MRI study was to localize the anatomical source of this control instance in inverse priming. We expected to find either evidence that cognitive control affects processing at perceptual or post-perceptual levels.

Previous physiological evidence suggests that the control instance is directly involved in the regulation of motor activation (Boy, Evans, et al., 2010; Boy, & Husain, et al., 2010; Sumner, et al., 2007). Then, the neural correlate of cognitive control in inverse priming would be expected to be part of the motor system. However, it is also plausible that an early perceptual mechanism controls activation of perceptual representation of the two stimulus alternatives

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(instead of controlling response preparation processes). From this perspective, motor processes as reflected in the LRP are only secondary consequences of the early competition.

Supposedly, then the neural site of control would also be located in the visual system.

To obtain an optimal measure of control processes in inverse priming we systematically varied regulatory demands by implementing four different experimental conditions for each mask type. These conditions resulted from factorial combination of a short (33 ms) and long mask-target SOA condition (150 ms) with congruent and incongruent primes and targets. At short SOAs positive priming is expected while effects should reverse at long SOA (Boy, &

Sumner, 2010; Mattler, 2007; Schlaghecken, & Eimer, 2000). As a neural measure of control demands we contrasted parameter estimates (of the general linear model fitted to the MRI-data) for the long and short SOA in the congruent condition. On congruent trials, increasing SOA enlarges regulatory demands because the misleading tendency towards the wrong response is lasting longer. Thus, conflict between ongoing task processing and target processing grows and consequently correction requires more cognitive effort. However, increasing SOA does not only manipulate control demands but is of course accompanied by corresponding physical differences in the temporal structure of the stimulus sequence which might also affect neural activity. To control for such confounded control-unrelated SOA effects, we considered the main effect of SOA on incongruent trials which involves the same perceptual effect whereas control operations are supposed to be unaffected by SOA changes on incongruent trials. This is because on incongruent trials only an initial mask-induced reversal occurs and target presentation simply confirms the reversed tendency irrespective of when the target is presented. Target presentation does not necessitate any change in response preparation processes and consequently, additional compensatory control operations are not required at all. This model is illustrated in Figure 5.

Three predictions can be derived from this model of the role of cognitive control in inverse priming. 1) Because changing SOA on incongruent trials does only reflect physical changes in the stimulus sequence a corresponding neural affect should be restricted to the visual system.

2) Because the same visual effect should also occur on congruent trials, the activity in the same visual areas should be susceptible to SOA variation on congruent trials too. 3) While SOA effects on incongruent trials should be limited to the visual system, further SOA effects are expected to be found in the congruent condition reflecting varying control demands.

Assessing the anatomical locus of these congruency-specific SOA effects allows us to infer the origin of control operations in inverse priming.

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Figure 5. Response activation model for inverse priming. The model is based on electrophysiological research demonstrating the time course of LRPs in inverse priming (Eimer, & Schlaghecken, 1998; 2003; Seiss, &

Praamstra, 2005). The figure illustrates the time course of the activation difference of left- and right-hand responses for congruent (upper row) and incongruent trials (lower row) as well as short (left column) and long SOA (right column). Three consecutive phases can be distinguished: first, the prime activates the associated response which is the correct one on congruent trials and otherwise the incorrect one. Second, the presentation of the mask reverses prime-related activation and drives it towards the unprimed response alternative until target presentation. Third, targets contribute to differential response preparation. Congruent targets induce a further reversal while incongruent targets simply lead to a continuation of mask-induced activation. The model assumes that reversals of activation processes are triggered by control operations. The regulatory demands related to these control operations depend on target SOA on congruent trials. With increasing target SOA, the mask-induced misleading bias also increases and compensation by target processing requires more and more resources such that regulatory demands are larger. Note that, regulatory demands are independent of SOA on incongruent trials because only the mask-induced reversal occurs. Therefore, neural activation which increases with SOA on congruent trials but not on incongruent trials reflects the increase of regulatory demands and associated control processes in inverse priming.

The MRI results obtained nicely fitted with these predictions. SOA effects on both, congruent and incongruent trials, were observed for both types of masks only in higher areas of the visual system at the transition from occipital to temporal lobe (V5/MT+, Malikovich, et al., 2007, cf. Figure 4 from Krüger, et al., submitted; Appendix V). These areas are involved in motion perception (Wilms, et al., 2005) and were previously related to the processing of

c ongru e nt

SOA 33 ms

Correct response

Incorrect response

Correct response

Incorrect response

SOA 150 ms

inc ongr ue nt

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apparent motion (Goebel, Khorram-Sefat, Muckli, Hacker, & Singer, 1998). Probably, the employed stimulus sequence (cf. Figure 1 from Krüger, et al., submitted; Appendix V) also induced motion signals in our participants which are supposed to be more pronounced at long SOA. The importance of the temporal delay between the stimuli for the perception of apparent motion has been demonstrated before (Shepard, & Zare, 1983).

In addition to these SOA effects common to both Congruency conditions, several brain regions showed SOA effects on congruent but importantly not on incongruent trials even at a rather liberal threshold (cf. Figure 5 and Table 3 from Krüger, et al., submitted, Appendix V).

Such control specific neural effects were most pronounced in the supplementary motor area (SMA) and common to both types of masks. In large parts of the SMA, the individual increase in activity from short to long SOA in the congruent condition was correlated to the priming effects on RTs (for both masks, see Figure 6 from Krüger, et al., submitted, Appendix V). Moreover, several areas of the parietal and frontal cortex were specifically involved in control operations with relevant masks but not with irrelevant masks. These areas included the rostral cingulate zone (Picard, & Strick, 1996), bilateral insula, the left postcentral gyrus and left supramarginal gyrus potentially reflecting a control network specifically involved with relevant masks.

Thus, application of fMRI to the inverse priming paradigm provided further support for a motor locus when response-compatible arrow stimuli are used. The foci were located in the SMA which has previously been found to be crucial for inverse priming (Boy, Evans, et al., 2010; Boy, & Husain, et al., 2010; Sumner, et al., 2007). The moderating function of the SMA with respect to conflicting response tendencies has also been demonstrated in the Eriksen Flanker Task (Hazeltine, 1990). Moreover, suppression of inappropriate response tendencies (Swick, et al., 2011), sudden adaptation of instantiated motor plans (Matsuzaka, &

Tanji, 1996), response inhibition in reaction to sudden task changes (Chen, Scangos, &

Stuphorn, 2010) or in general the instantiation and control of relevant movements (Nachev, et al., 2008) have also been related to the SMA. Thus, the SMA seems to be the central motor control component regulating response preparation processes in inverse priming irrespective of which type of mask is used. With relevant masks, further components come into play augmenting the effect.

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