Summary of Project 3

In Project 3, electrophysiological correlates of early visual processing were obtained in order to provide evidence that masked priming of spatial attention modulates early target

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

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processing. Spatial attention has been found to modulate the visual P1 and N1 components of target-locked event related potentials. Consequently, if masked primes affect spatial attention at early levels of processing these potentials should be enhanced on congruent trials compared to incongruent trials. In Experiment 8, we applied spatial cue-priming in a bar discrimination task without distractors. In a similar task, Mangun & Hillyard (1991) found effects of cue validity on early visual potentials. Compared to the experiments in Project 1, we found surprisingly small priming effects on RT which were significant only with a long prime-cue SOA of 94 ms. These behavioral priming effects were accompanied by a modulation of central and occipital target N1 which was enhanced on congruent compared to incongruent trials. This finding suggests that masked primes in Experiment 8 modulated a perceptual limited capacity discriminatory process. Given that primes and cues in Experiment 8 were perceptually dissimilar, this provides evidence that masked stimuli can affect attention at early levels of target processing.

Experiment 9 was aimed at using electrophysiological measures to investigate attentional effects at the non-cued side. Interpretation of cue-priming effects suffers from the fact that it is not clear whether primes affect attention directly or affect only processing of cue stimuli. This problem might be solved by showing priming effects on stimuli presented on the non-cued side. If primes can directly affect spatial attention, incongruent primes should direct attention to the non-cued side whereas congruent primes direct attention to the cued side. This should lead to enhanced potentials for stimuli on the non-cued side. In order to provide a measure for processing of stimuli on the non-cued side, we used a design with task irrelevant probe stimuli. Spatial attention has been shown to affect visual potentials elicited by task irrelevant stimuli in a visual search task (Luck & Hillyard, 1995). Therefore, we expected a similar modulation of probe related P1 and N1 components with spatial cue-priming. We used

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a letter discrimination task with distractor and a short cue-target SOA because these conditions produced the largest priming effects in Project 1. Probe stimuli where white square outlines around target locations which were presented before target onset. Probe stimuli were presented on the cued side and on the non-cued side on one third of trials, respectively. On the remaining third of trials, no probe stimulus was presented. These no probe trials were used to isolate probe evoked potentials by calculating difference waveforms for probe and no probe trials. RT in Experiment 9 was affected by prime-cue congruency as well as probe location.

As expected we found larger priming effects (about 30 ms) than in Experiment 8 and participants responded faster when the probe was presented at the cued location, and slower when it was presented at the opposite location. Priming effects, however, were unaffected by probes. Visual probe-evoked potentials were mostly unaffected by attention as there was no difference between probes presented at the cue location and probes presented at the non-cued location. Probe N1 for cued probes was increased on incongruent compared to congruent trials. Apart from this effect, probe-evoked potentials were unaffected by primes. These findings might be explained by assuming that there was no allocation of spatial attention at the time of probe presentation because probes were presented too shortly after targets.

Analysis of target-locked ERPs on no probe trials revealed congruency effects in the period of early visual components. However, interpretation of these effects suffers from an overlap with potentials induced by primes and cues, due to the short cue-target SOA. In addition, later frontal and parietal components were modulated by congruency. Therefore, it is unclear whether priming effects on RT are based on attentional effects of primes at early stages or selection operates at later stages of processing. The finding that primes did affect frontal and parietal potentials might suggest the latter. Comparing Experiments 8 and 9 yields a complex pattern of results. Whereas in Experiment 8 small priming effects with long cue-target SOAs were accompanied by a modulation of target N1, larger priming effects in Experiment 9 were

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found with short cue-target SOAs, but there was no effect on early visual probe related potentials. This is reminiscent of the finding that priming effects on signal detection in Project 2 were found only with longer cue-target SOAs whereas priming effects on letter discrimination were largest with short cue-target SOAs. We speculated that this difference might be due to the presence of a distractor stimulus that introduced conflict in selection processes as opposed to perceptual processes and thereby led to a different focus of attentional processes. It might be that time courses differ for priming effects on selection processes and priming effects on perceptual target processing.

Experiment 10 was conducted to clarify these apparent discrepancies by comparing priming with short cue-target-SOA to priming with long cue-target SOA and also comparing priming without distractor as in Experiment 8 to a condition with distractor similar to Experiment 9. To this end, we varied cue-target SOAs in two steps corresponding to the SOAs used in Experiments 8 and 9, respectively. We employed a bar discrimination task as in Experiment 8. On half of the trials, however, the cued bar was accompanied by a distractor bar at the non-cued side which was short when the target was long and long when the target was short. Combination of SOAs and distractor presence led to four conditions which were presented in random order. Unfortunately, Experiment 10 did not help revealing the important differences between Experiments 8 and 9. Instead the overall picture was made more complex by the finding that there were reversed priming effects with the long 400 ms SOA, i.e.

responses were faster on incongruent trials than on congruent trials. This reversal is surprising especially since the long SOA condition without distractor is very similar to the 94 ms prime-cue SOA in from Experiment 8. The reversal of priming effects thus seems to be associated either to changes in prime and cue symbols which were easier to identify in Experiment 10 than in Experiment 8 or to intermixture with trials with distractor or trials with short

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target SOA. The short cue-target SOA and the easier cues might have led to faster shifts of attention on congruent trials than in Experiment 8 which were then followed by inhibition of return.

In addition to these attempts at showing priming effects on early levels of target processing, two other projects were conducted to study the general preconditions of spatial cue-priming. In Project 4 the possibility of cue-priming effects on participants’ free choices for one side or the other was examined. In Project 5, we tried to study cue-priming effects under conditions in which the cue did not always predict the correct target location.

Project 4 - Free choice cue-priming

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