3 Project 2 – Spatial cue-priming effects on accuracy in signal detection tasks
3.2 Experiments 6 and 7 - Preconditions for spatial cue-priming in signal detection tasks detection tasks
Two additional attempts to study spatial cue-priming effects on signal detection performance with improved masking of primes failed to show any significant priming effects.
These experiments, which are reported in the following section were conducted before Experiment 5 but are included here as Experiments 6 and 7 here because they are not part of the submitted manuscript. The original purpose of Experiment 6 was to replicate the priming effects found in Experiment 4 under conditions of improved masking while at the same time investigating the role of cue-target SOA in priming effects on signal detection. Priming effects in Project 1 were found to decrease with increasing cue-target SOA but preliminary results suggested that priming effects on signal detection occur only with longer cue-target
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SOAs. Prime recognition was made more difficult by using arrangements of two stimuli as cues and primes in which location of the critical stimulus varied from trial to trial. In addition, primes were masked by a star shaped stimulus which was found to increase masking in preliminary experiments.
3.2.1 Experiment 6
Experiment 6 was designed to replicate the priming effect on signal detection with improved masking and to disentangle the effects of prime-cue SOA and cue-target SOA. To this end, we changed prime and cue stimuli in a way that would reduce prime visibility while still allowing for priming effects and varied cue-target SOA with constant prime-cue SOA. In a letter discrimination task, we found that priming effects seem to be largest at short cue-target SOAs. In contrast, resource allocation to a location in space seems to develop over time (Luck, Hillyard, Mouloua & Hawkins, 1996). By using a wide range of cue-target SOAs we aimed to clarify this discrepancy.
3.2.1.1 Method
Participants. 18 new participants (16 women; age 19-27, M = 21.9) completed 3 sessions of the experiment in exchange for course credit or payment of €42. All had normal or corrected to normal vision. 4 additional participants were excluded after the practice session because of poor performance. One additional participant was excluded because of a large amount of eye movement errors.
Task. Participants had to perform the same task as in Experiment 4. In Experiment 6 the cue and prime symbols could appear above or below fixation and were accompanied by distractors.
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Stimuli. Stimuli were presented using the same setup as in Experiment 4. In this experiment, we introduced two possible positions for prime and cue stimuli. Primes and cues were the same squares and diamonds as in Experiment 4. The cue was presented above fixation half of the time and below fixation on the remaining trials (2° from fixation to centre of the symbols). At the opposing position, a distractor, which was an overlay of both the square and the diamond prime, was presented together with the prime. Then the cue was presented at the opposing position together with another distractor at the prime’s position (this time an overlay of both cue stimuli).
Thus, primes were always masked by the distractor stimulus and prime and cue were
Figure 3.6 Sequence of stimulus events in Experiments 6 and 7.
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presented at different locations. This was done to minimize perceptual interactions between the two stimuli and to increase masking of the prime. Preliminary research suggested that in this particular square or diamond discrimination task, star shaped masks are more effective than square or diamond masks. Furthermore, the added spatial uncertainty should render recognizing the primes more difficult. Targets and target masks were the same as in Experiment 4 and adapted to participants performance in the same way. In Experiment 6 prime-cue SOA was held constant at 72 ms and cue-target SOA varied in four steps of 165 ms from 165 to 659 ms. Other timing parameters were the same as in Experiment 4 and are given in Figure 3.6.
Design and procedure. Design and procedure were essentially the same as in Experiment 4 with the difference that the primes could be presented at two different locations.
Each possible combination of 2 prime positions, 2 primes, 2 cues, 2 targets and 4 SOAs was presented once in each block. Dependent measures were analyzed the same way as in Experiment 4.
Apparatus. We used the same experimental setup as in Experiment 4.
Analyses. Trials with eye movements were excluded using the same procedure as in Experiment 4 with the exception that thresholds for trials exclusion were increased to 1.12°
visual angle deviation from baseline, because long cue-target SOAs made eye movement errors due to random drift occur very frequently.
3.2.1.2 Results
Sensitivity. Target detection performance is shown in Figure 3.7A. We found that sensitivity increased with cue-target-SOA, F(3, 51) = 39.8, MSe = 0.094, p < .001, from d’ = 1.24 at 165 ms to 1.93 at 494 ms, but then levelled with 1.92 at 659 ms. Congruency did not have an
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effect on target detection, F(1, 17) < 0.1, MSe = 0.036, p = .772, nor was there a significant interaction with SOA, F(3, 51) < 1, MSe = 0.037, p = .464.
Figure 3.7 Results in Experiment 6. (A) d’ as a measure of sensitivity for congruent and incongruent trials as a function of cue-target-SOA. (B) c as a measure of response criterion for congruent and incongruent trials as a function of SOA. (C) Response times for congruent and incongruent trials as a function of cue-target-SOA. (D) Prime recognition performance in percent correct as a function of cue-target-cue-target-SOA.
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Criterion. A similar pattern emerged for decision criteria (Figure 3.7B) that were unaffected by congruency, F(1, 17) < 0.1, MSe = 0.012, p = .798, but differed at the different SOA levels, F(3,51 = 10.7, MSe = 0.233, p < .001. Post-hoc t-tests showed significant differences between the 659 ms SOA and all other SOAs (t(17) > 2.7, p < .015 in all cases) and between 494 ms and 329 ms SOA, t(17) = 5.3, p < .001. Again, there was no significant interaction of Congruency and SOA, F(3, 51) < 1, MSe = 0.013, p = .749.
RT. RT (see Figure 3.7C) decreased with increasing cue-target SOA from 845 ms at 165 ms to 686 ms at 659 ms, F(3, 51) = 61.4, MSe = 2500, p < .001, but was unaffected by congruency, F(1, 17) < 0.1, MSe = 634, p = .974. There was no interaction of Congruency and SOA, F(3, 51) < 1, MSe = 478, p = .419.
Prime Recognition. Prime recognition performance (see Figure 3.7D) was unaffected by cue-target-SOA, F(3, 51) < 0.1, MSe = 0.048, p = .925, and did not differ from chance level (mean d’ across all SOAs = 0.046, t(17) = 1.1, p = 0.272).
3.2.1.3 Discussion
In Experiment 6 prime recognition performance was reduced to chance level. At the same time, we found no effects of primes on performance, as both sensitivity and response bias were unaffected by congruency. As target stimuli were the same as in Experiment 4, changes in prime stimuli or in SOAs have to account for these negative results. Variation of cue-target SOA yielded strong effects on target detection performance, bias and RT. RT difference between the shortest and longest SOA is larger than what would be expected from an attentional modulation of target processing. Thus, slow responses at short cue-target SOAs could result from interference from cue processing. In order to detect targets, participants need to know which side to decide on. Thus they need to have processed the cue stimulus
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which might not be the case at target presentation with short SOAs. Decay of target representations until the cue has been processed could then explain poor recognition performance at short SOAs. Effects of SOA on decision criteria could be explained by reduced trial by trial variability of signal strength through attention at long SOAs (Rahnev et al, 2011).
Several possible explanations for the lack of priming effects on target detection in Experiment 6 can be conceived. The most obvious change from Experiment 4 lies in the more complex prime and cue stimuli. These changes led to better masking as was intended.
However, given that prime visibility in Experiment 4 was not associated with priming effects, it seems unlikely that reduced prime visibility is the cause for the absence of priming in Experiment 6. Alternatively, primes might have simply been too weak to affect detection performance, as their critical features were more difficult to discriminate. One reason for this could be spatial uncertainty. As participants did not know at which location the relevant cue symbol would appear, they had to divide their attention across both locations. Naccache, Blandin & Dehaene (2002) found that temporal attention is a prerequisite for priming effects.
If the same is true for spatial attention, then spatial uncertainty could reduce priming effects.
However, given that cues could also appear at either location, attention might have been divided, yet should not have been completely absent. Another aspect is that prime duration was reduced from 24 ms to 12 ms. In addition to reducing prime visibility, shortening prime duration might have reduced the strength of the primes. However, from other studies (Mattler, 2003; Vorberg et al., 2003) it seems that the magnitude of priming effects is largely determined by prime-cue SOA rather than prime duration.
Another possible explanation can be found in the introduction of long cue-target SOAs. In Experiment 6, cue-target-SOA had an effect on target detection performance. This
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suggests that participants were not able to shift their attention completely to the cued location in time for target presentation. Thus, it might be that for priming effects to occur, it is necessary that participants are under stress to quickly shift their attention, either because priming effects are a direct result of accelerated attention shifting in congruent trials compared to incongruent trials or because speed stress is necessary for primes to be sufficiently processed. Either way, the introduction of long cue-target-SOAs might subjectively reduce speed stress in the task and therefore reduce priming effects.
In a second attempt to replicate priming effects on target detection performance we conducted another experiment. As both spatial uncertainty and reduced prime duration might be crucial to reduce prime visibility we kept these changes but otherwise used the same parameters as in Experiment 4.
3.2.2 Experiment 7
In Experiment 7 we tried to clarify which changes from Experiment 4 to Experiment 6 were responsible for the vanishing of the priming effect. The most prominent changes were the introduction of long cue-target SOAs and the more complex prime and cue stimuli. Seeing that we successfully reduced prime visibility with the more complex configuration, we used the same SOAs as in Experiment 4 but with the primes and cue configuration from Experiment 6. If priming effects are again absent in this experiment we would conclude that this configuration is too complex to be processed fast enough to allow priming effects. On the other hand, if priming effects emerge it would mean that variations in cue-target SOA can have more profound effects than previously believed and at the same time replicate the effect from Experiment 4 with improved masking.
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Participants. 12 new participants (10 women; age 19-32, M = 24.9) were tested in 3 sessions. 3 additional participants were excluded after poor performance in the practice session.
Task. Participants had to perform the same task as in Experiment 6.
Stimuli. Stimuli were presented using the same setup as in the previous experiments.
Prime and cue stimuli were the same as in Experiment 6. Targets and target masks were the same as in the previous experiments and adapted to participants performance in the same way.
Prime-cue SOA and cue-target SOA varied in the same way as in Experiment 4. However, prime duration was 12 ms like in Experiment 6. Other timing parameters were the same as in Experiment 4 and are given in Figure 3.6.
Apparatus. The same setup as in the previous experiments was used. Eye movements were measured as in the previous experiments. The same eye movement thresholds as in Experiment 4 were used for trial exclusion.
Design and procedure. Design and procedure were the same as in Experiment 6.
Analyses. Dependent measures were analyzed the same way as in Experiment 6.
3.2.2.2 Results
Sensitivity. Target detection performance (see Figure 3.8A) was unaffected by Congruency, F(1, 11) = 0.2, MSe = 0.042, p = .635, but decreased with SOA, F(3, 33) = 7.4, MSe = 0.041, p = .002. There was a marginal interaction between the two factors, F(3,33) = 2.4, MSe = 0.048, p = .087. Paired t-tests evaluating priming effects for each of the 4 SOAs by
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comparing performance on congruent and incongruent trials showed no significant differences (p > .09 in all cases).
Figure 3.8 Results in Experiment 7. (A) d’ as a measure of sensitivity for congruent and incongruent trials as a function of prime-cue-SOA. (B) c as a measure of response criterion for congruent and incongruent trials as a function of SOA (C) Response times for congruent and incongruent trials as a function of prime-cue-SOA.(D) Prime recognition performance in percent correct as a function of prime-cue-SOA.
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Criterion. Decision criterion values (c, see Figure 3.8B) decreased with SOA, F(3, 33)
= 20, MSe = 0.029, p <.001, which indicates more conservative decision with short prime-cue SOAs than with long cue-target SOAs. The analysis of decision criteria revealed no effect of Congruency, F(3, 33) = 0.6, MSe = 0.004, p = .465 nor an interaction of Congruency and SOA, F(3, 33) = 0.9, MSe = 0.011, p = .002.
RT. RT (see Figure 3.8C) was neither significantly affected by Congruency, F(1, 11) = 0.3, MSe = 385, p = .579, nor SOA, F(3, 33) = 2.1, MSe = 1657, p = .116, nor was there an interaction between the two factors, F(3, 33) = 1.3, MSe = 780, p = .28.
Prime Recognition. Prime recognition performance (see Figure 3.8D) was unaffected by SOA, F(3, 33) = 1.3, MSe = 0.076, p = .03 and was not significantly better than chance, mean d’ = 0.076, t(11) = 1.4, p = .179 (averaged across SOAs).
3.2.2.3 Discussion
Just like in Experiment 6 we failed to replicate any priming effects on target detection performance. This rules out that the absence of priming effects in Experiment 6 was due to the variation of cue-target SOAs in a wider range. Rather, it seems that the prime-cue configuration used in Experiments 6 and 7 is not suited to affect attention in a detection task.
3.2.3 General discussion
Two differences between the primes and cues used in Experiment 4 and those used in Experiment 6 and 7 might account for the discrepancy in priming effects. Firstly, prime recognition performance was better in Experiment 4 than in Experiments 6 and 7 where participants could not recognize primes with better than chance accuracy. To the extent that consciousness is critical for cue-priming effects in signal detection tasks, priming effects
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should covary with prime recognition performance. Accordingly, priming should be absent when primes are not visible, i.e. chance level performance in the recognition task.
Secondly, it might be that priming effects in Experiment 4 arise because primes affect cue processing but do not directly affect attention. According to this notion primes could facilitate or hinder cue processing at early levels of processing. One possible mechanism is priming based on perceptual similarity of prime and cue. Mattler (2006) addressed this problem in a different task by varying perceptual similarity. He found larger priming effects when primes and cues were perceptually similar than with dissimilar stimuli, showing that priming effects are in part based on perceptual similarity. If priming effects in Experiment 4 are entirely based on perceptual similarity the absence of priming in Experiments 6 and 7 could be explained be the relative absence of such similarity. Even though prime and cue symbols were the same in Experiments 6 and 7 as in Experiment 4 primes were never presented at the same location as the cue which might have prevented perceptual priming effects in Experiments 6 and 7. This issue was resolved by results of Experiment 5, where we manipulated similarity of prime and cue symbols and found that this was the critical difference between the former experiments.