General Discussion of Project 3

Overall, results show that spatial cue-priming effects are accompanied by a modulation of early visual potentials, as well as later post-perceptual frontal and parietal components. In Experiment 8, weak but consistent behavioral priming effects were associated with a modulation of target N1 under conditions with very low prime visibility. This was replicated to some degree in the short SOA – long SOA difference in Experiment 10. In Experiment 9, early visual processing of task irrelevant probe stimuli was not affected by primes, even though we found clear behavioral priming effects on discrimination of letter targets. However,

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this does not necessarily mean that early target processing was unaffected by primes. Instead, it might be that there was a lack of priming effects on probe processing because the probes were not task relevant. With short SOAs in Experiments 9 and 10, we found a different pattern of priming effects on target-locked potentials. There was an early modulation of potentials over occipital areas, but instead of an increased N1, we found increased positivity in the period corresponding to the P1. The design of these experiments makes an attribution of these effects to target processing problematic because of a confoundation with cue processing.

Priming effects on later potentials show a more diverse pattern. Modulation of later frontal and parietal components by primes could be interpreted as priming of cue processing and implementation of stimulus-response mapping or task set. Thus, it seems that priming effects in Experiments 9 and 10 are not entirely based on the same processes as in Experiment 8. The critical difference might be found in the presence of a distractor in Experiment 9 and half of the trials of Experiment 10, which introduces the requirement to select between one of two potential target stimuli. Without such a distractor, participants cannot use the cue to decide whether to react to the right or to the left target stimulus, but only to enhance processing of the single target stimulus. Nevertheless, the present results do not exclude the possibility that there were priming effects on early target processing in Experiments 9 and 10. Thus, it might be that in these experiments behavioral priming effects resulted from effects on both early and late selection processes whereas in Experiment 8 primes only acted on early target processing.

This could explain why behavioral priming effects were smaller in Experiment 8 than in the other two experiments. In Experiments 9 and 10, several congruency effects appeared to be stronger or only apparent when a specific side was cued. This could mean that cues and primes were mainly used to shift attention to one side, perhaps because participants focused attention on the other side by default. However, it seems more likely that these priming

Project 3 - Spatial cue-priming effects on physiological measures of target processsing

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effects result from differences in cue processing between square and diamond stimuli, because cue symbols were with cued side.

A somewhat puzzling result was obtained from the long SOA condition in Experiment 10 where priming effects were reversed. At present, we do not have a satisfying explanation for this reversal and future research is needed to clarify its cause. Possible candidates include inhibition of return (Posner & Cohen, 1984) and the negative compatibility effect (Eimer &

Schlaghecken, 1998).

Mechanisms of priming of spatial attention

Overall, it seems that there is considerable variability of priming effects in different tasks. In Experiment 8, small congruency effects were apparent in early target processing. In Experiment 9, larger behavioral effects were found along with physiological priming effects on central and parietal sites. However, we found no evidence that primes modulate processing of stimuli at the non-cued location in Experiment 9. This indicates that primes cannot initiate endogenous shifts of attention by themselves. In Experiment 10, priming effects seemed to be based on both early and late visual processing of targets. These results can be explained by assuming that the locus of attentional selection in a given task depends on where conflict occurs (Luck & Hillyard, 2000). Assuming that participants adapt to use cues in a way that benefits their performance, it seems reasonable to assume that primes activate the same processes as cues. Therefore, priming effects at early levels of processing are found when the task is perceptually difficult (Experiments 8 and 10) but not when the difficulty of the task lies in selecting the correct response. This similarity of prime processing to cue processing suggests that masked symbolic cues can affect the allocation of spatial attention at multiple levels of target processing. However, it is unclear to which extent priming effects in Experiments 9 and 10 are based on perceptual priming of cue processing. This difference

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between the Experiments was also reflected in the modulation of early visual cue related potentials by congruency which was prominent in Experiments 9 and 10, but not in Experiment 8. However, priming effects on later components, that were proposed to reflect cue processing in Experiment 9, were not found in Experiment 10 where the same prime and cue stimuli were used. This suggests either that cue processing differs between the two tasks or that these priming effects are related to target processing.

One serious shortcoming in our experiments is that with short cue-target SOAs electrophysiological effects of target processing cannot be separated from effects of prime and cue processing. Future studies should include conditions without target stimuli which could be used to isolate target induced shifts by subtracting correlates of prime and cue processing.

4.4.1.1 Conclusion

In summary it seems as though the mechanisms of spatial cue-priming effects depend in similar way on task demands as spatial cueing effects. With high perceptual load (bar targets), primes modulate target processing as early as N1, whereas with letter targets primes seem to exert influence at later levels. Priming effects might be larger with letter targets because later processes are more readily affected by primes than selection at early levels.

However, it seems that spatial cue-priming effects are more sensitive to specific task parameters and cue-target SOA than previously thought. Thus, in order to draw conclusions about unconscious processing from cue-priming, careful interpretation of cue-priming effects is required because there can be several possible underlying mechanisms.