In order to achieve comparability of results of the post-cue experiment and our earlier studies, we make two preparatory experiments that help to successively add complexity to the original voicekey experiment, see section 2.2. We begin with two baseline blocks, one of which follows the paradigm introduced in section 2.2, the other requiring attention to the red object. The comparison of the two blocks helps us to understand the difference of trials with a red target and those with a green one that will be naturally present in the task switch experiments.
The next step is the introduction of task switching in the form of a color cue indicating the target color in the following trial. Despite the task switch, the paradigm stays identical. Thus, any changes in priming effects are due to the cue and its demand to attend to its color. Besides the impact of updating a task set, physical alterations are also made to a trial. In particular, the response stimulus interval is longer in the pre-cue experiment, as the subject’s reckoning of the cue adds to it. Also, the additional percept of a color patch before the actual trial may change processing compared to the baseline experiment.
In the current section we will present the two experiments together. One reason is their nature of providing prerequisites for the experiment of interest, the post-cue paradigm. Another is that both experiments are performed by the same subjects in one session. The baseline experiment also provides a familiarization with the stimuli and the task for the pre-cue experiment.
6.4.1 Design
We first only slightly altered our traditional negative priming experiment. Two blocks of the simple voicekey task are run, one with green as target color, while the other required a response to the red item. The sequence of these blocks was balanced over subjects. The two baseline parts included five priming conditions, CO, DT, TT, TD and DD.
After the two baseline blocks followed a task switch experiment of 420 trials with breaks every 42 trials paced by the subjects themselves. We implemented a standard way of unpredictable task switches by pseudo randomly presenting a color cue indicating the target color each trial anew. The color patch was removed by a button press by the subject which then started the actual trial. The switch dimension is reflected in a doubling of experimental conditions, as a task switch destroys the confounding of whether an object is repeated in the same color or not and the object to respond to.
6.4.2 Participants
Thirty undergraduate students (19 female, 11 male) from the University of Göttingen took part in the study. Their ages ranged from 20 years to 35 years (M = 24.5 years, SD = 1.6 years).
The participants were rewarded by course credits or are paid 15e. All subjects had normal or corrected-to-normal vision and no color discrimination disabilities. They were not informed about the specific purpose of the experiment and had not taken part in a previous study employing similar stimulus material.
6.4 Preparatory Task Switch Experiments 6.4.3 Procedure
The experiments took place in two special chambers optimized for low noise and standardized lighting conditions. Participants were tested individually in sessions that lasted no longer than 90 minutes. Before the start of the priming tasks, the line drawings of the experimental stimuli along with their names printed in black were shown to the subjects. The subjects were told that they will see these objects overlapping one another, one drawn in green and the other in red. Realizing a classical picture naming task, participants were instructed to name the target objects as quickly and correctly as possible while ignoring the superimposed distractor object. To familiarize participants with the experimental procedure, a 30-trials practice session preceded the main session.
The first two experimental tasks consisted of 210 trials each, which were divided into 5 blocks of 42 trials. After each block, subjects were allowed to take a short break. In a single trial, subjects encountered the following series of events: a fixation cross, centered on the screen for 500 ms; a display containing two superimposed objects until subject responded, but no longer than 2 seconds and then a blank screen for a randomized duration between 0 and 1000 ms.
In the pre-cue experiment, an additional visual cue indicated if the target color of the trial was green or red. The number of trials was doubled compared to the first tasks to 420 trials, so that the participants had to complete 10 blocks of 42 trials. A single trial consisted of the following series of events: a fixation cross for 500 ms, the color cue until the subject pressed a button, the stimulus display and then a randomized RSI of 0-1000 ms. Behavioral errors were noted when subjects failed to give the correct answer. After the participants completed all trials, they were asked to comment on the experimental procedure.
6.4.4 Data Analysis
First, we want to make sure that the baseline experiments show strong priming effects as antici-pated. The two different target colors are then compared. One aspect is a main effect of red/green, which indicates a direct influence of target color to processing speed. Another aspect is an inter-action between target color and priming effects. If such an interinter-action exists, we have to group the task switch trials into conditions with homogeneous target color.
A comparison between overall mean reaction time in the baseline experiment and the pre-cue paradigm yields a measure for global switch costs, i.e. the influence of the presence of task switch-ing on processswitch-ing speed. Comparswitch-ing switch and non-switch trials within the pre-cue experiment leads to specific switch costs. We then check for priming effects in the respective condition, espe-cially in the task switch trials, as no data exists for similar settings.
To clean the data, some procedures to reject outliers from the behavioral data are employed.
In a first step, all reaction times from trials in which a behavioral error occurred, are excluded from the analysis as well as the directly following trial. Reaction times below 250 ms and above 3000 ms were removed from the analysis as they presumably are based on other processes than the ones under investigation due to their unusual duration. In a next step, reaction times where the difference to the mean of the experimental condition exceeded two times the standard deviation were also excluded. Overall, for each participant not more than 10% of the trials per condition were excluded from the analysis.
6.4.5 Results, Baseline Experiment
The overall 2×5×2 (target color × priming × color block order) ANOVA treating reaction times as dependent variable showed a main effect of target color F(1,116) =16.11,p<0.001, a main effect of priming F(4,29) =23.21,p<0.001, but no interaction of target color and priming
6 Perception or Selection Effect
F(4,116) =0.77,p=0.54 and no effects at all of the order in which the two color blocks are tested, see table A.8. Especially the absence of effects of the order in which red or green indicate the target allows a pooling of all subjects, regardless of which target color the subjects encounter first.
CO DT TT TD DD
Reaction Time [ms] 500550600650700750800 ⋆ ⋆
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Figure 6.6: Priming effects of the baseline experiment. Significances of comparisons to the con-trol condition are given. Expectedly, trials with a repeated object that changes its role between prime and probe are slower, identical repetitions of objects lead to an acceleration.
The absence of an interaction between target color and priming allows the determination of priming effects in the entire baseline experiment, i.e. the mean reaction time differences over both target color blocks, see figure 6.6 and table A.9 in appendix A.4. Trials with a green target, 776.10 (95.80) ms, are on average 47 ms slower than trials with a red target, 729.20 (104.59) ms. Even in the pooled situation all priming effects are significant, CO−DT=−18.28 ms, t(29) =−3.09,p<
0.01, CO−TT=45.83 ms, t(29) =4.56,p<0.001, CO−TD=−13.43 ms, t(29) =−2.27,p<
0.05 and CO−DD=20.45 ms, t(29) =3.55,p<0.01.
6.4.6 Results, Pre-Cue Experiment
The overall 2×5 (switch ×priming) ANOVA on reaction times shows main effects of switch F(1,29) =19.94,p<0.001 and priming F(4,116) =15.12,p<0.001 as well as an interaction of both F(4,116) =8.21,p<0.001. Specific switch costs, 27 ms, are determined as the difference between overall mean reaction times in switch trials (783.28 (108.52) ms) and no-switch trials (810.25 (123.80) ms) in the pre-cue experiment. We find global switch costs in the two-sided comparison of the mean reaction time in the baseline experiment (752.65 (95.05) ms), and the pre-cue experiment (796.76 (115.23) ms), which differ by 44.11 ms significantly, t(29) =−3.37,p<
0.01.
For priming effects we compared the appropriate control with the priming conditions and found effects in CO−DT=−24.93 ms, t(29) =−2.83,p<0.01; CO−TT=54.76 ms, t(29) =5.09,p<
6.4 Preparatory Task Switch Experiments
CO DT TT TD DD CO_SW DT_SW TT_SW TD_SW DD_SW
Reaction Time [ms] 650700750800850
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Figure 6.7: Priming effects of the pre-cue experiment. Significances of comparisons to the ap-propriate control condition (indicated by the horizontal lines) are given. Disregarding the control condition the pattern of the priming conditions looks similar in the switch and non-switch stimuli. For illustrative purposes we indicate the position of such a hypothetical control with a dashed line.
0.001, but no effects in the two conditions CO−TD=−3.60 ms, t(29) =−0.54,p=0.29 and CO−DD=0.20 ms, t(29) =0.024,p =0.48. For trials that incorporated a target color switch from the prime, the present effects are CO_SW−DT_SW=−28.07 ms, t(29) =−2.54,p<0.01, CO_SW−TD_SW=−21.23 ms, t(29) =−2.31,p<0.05 and CO_SW−DD_SW=−19.66 ms t(29) =2.63,p<0.01, no effect shows CO_SW−TT_SW=−5.30 ms, t(29) =−0.66,p=0.25.
The results are summarized in figure 6.7, details given in table A.10 in appendix A.4.
6.4.7 Discussion
The baseline experiment shows that it is easier to respond to a red target than to a green one, presumably because of the higher saliency of the color red. But despite the faster response in the red target block, priming effects did not differ. Further, the different saliency does not influence priming in the baseline experiment as it does in other settings (Tipper et al., 2002), see also sec-tion 3.3.3. We therefore pooled both parts and still found convincing priming effects, i.e. negative priming in the DT and TD condition and positive priming for TT and DD, see figure 6.6. The two condition pairs are each associated by a reoccurrence of an object in the same or in a different color, respectively. The results of the baseline experiment once more prove the suitability of our paradigm to access negative priming.
Introducing the task switch in the pre-cue experiment destroyed the effects in the TD and DD condition, as the influence of the probe distractor apparently is diminished. But the effects in the
6 Perception or Selection Effect
most interesting conditions, DT and TT, are even more pronounced than in the baseline experiment without task switch, see figure 6.7. Comparing the pre-cue experiment with the baseline reveals global switch costs visible in the general slowing of reactions after the introduction of the task switch. Comparing non-switch conditions in the pre-cue experiment with the switch conditions reveals specific switch costs of comparable order to the global ones. Trials following a task switch are slower than trials that require the attention to an identically colored target.
Remarkably the priming effects in the switch trials show an overall deceleration in the case of an object repetition. Only TT_SW trials are not distinguishable from CO_SW. The reoccurrence of an object in conjunction with a task switch seems to overload the subjects, such that they react slower than if no object is repeated from the prime trial. A closer look at figure 6.7 reveals that the relations between the priming conditions themselves look very similar to the conceptually matching non-switch trials. Seemingly, the repetition of an object in any color produces a constant offset into the processing of switch trials, but the particular priming effects remain.