Paradigm and Stimulus Material

While the memory task itself was similar to the paradigm used in the first study, the overall structure of this experiment differed. Instead of two memory tasks and one nap break in between, the experiment now consisted of two periods of resting

Figure 11.1: Overview of the paradigm: Two resting periods were recorded before and after a declarative associative memory task. In this task, 12 emotionally negative and 12 emotionally neutral pictures were presented at specific positions on the screen, 24 times each. Participants were instructed to learn the position for each of the pictures. One memory test was conducted immediately after the memory task and a second memory test took place after the second resting period. A linear SVM was trained on the fMRI data of the learning task to discriminate between the 24 different pictures and then made predictions on fMRI volumes during the resting states. More evidence for reactivation of negative as compared to neutral stimuli was expected. In addition, correlation between reactivation frequency for single items and subsequent memory performance was expected for predictions in the resting state following but not preceding the task.

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state and a memory task in between. In addition, the two periods of resting state were now waking state only. Figure 11.1 provides an overview of the structure of the experiment.

Two resting periods of 30 minutes each preceded and followed the memory task (named “Rest 1” and “Rest 2”, respectively). During these periods, participants were instructed to lie still and relax. To ensure that they stayed awake during that time, a simple button press task was introduced: Every 40-80sa large red dot appeared on the screen and participants were instructed to press a button when this occurred, after which the red dot disappeared. After 5swithout button press, the dot began blinking to encourage reaction. Participants were told that the sole purpose of this task was to ensure that they stayed awake and that the reaction to the appearance of the red dot was not at all about speed. This explanation was meant to reduce stress for participants and to minimize the perceived task character of this resting period.

The main task, scheduled between the two resting periods, was a declarative, associative memory task again during which pictures were associated with a specific position. 24 different stimuli were used: 12 neutral and 12 negative pictures from the IAPS collection (Lang et al., 1999). The dimensions most interesting to us were arousal (ranging from 1=“low arousal” to 9=“high arousal”) and valence (ranging from 1=“low pleasure” to 9=“high pleasure”). The neutral pictures were selected so that they had low arousal and neutral valence (arousal 2.62±0.23 mean±std, valence 4.96±0.24 mean±std), while the negative pictures were selected to have high arousal and negative valence (arousal 6.69±0.36 mean±std, valence 2.03±0.38 mean±std).

In each trial, one of the 24 pictures was shown at a specific position on the screen.

The picture size was 150*100 pixels, which was large enough to get the gist of the scene. The picture was on the screen for 4s, then a fixation cross was presented until the beginning of the next trial. The inter trial interval was 5s, which again corresponded to the time needed for collection of two fMRI volumes. In total, each of the 24 pictures was presented for 24 times. The experiment was divided into twelve blocks which were separated by 1 minute of resting state. During each block, every picture was shown twice. The sequence of pictures was randomized within blocks. In total, the memory task lasted about 55 minutes.

Participants were instructed to judge for each picture in each trial whether they

thought the scene was outdoors or indoors and indicate their answer via button press.

Note that for some pictures this was an easy judgment while for some pictures it was completely arbitrary. Participants were instructed that, when in doubt, they should choose whichever they thought was more likely. Again, this was a task that was supposed to induce a deeper level of processing and was not analyzed further.

The instruction further stated that participants were supposed to look closely at the pictures in each trial and to memorize the position of each scene on the screen.

Participants were aware that two memory tests would follow the experiment in which they would be presented with the scenes and would have to indicate the position that they were shown at during the learning task.

One test immediately followed the learning task and the second test was admin-istered after Rest 2. Each memory test consisted of 24 trials. In each trial, one of the previously presented 24 IAPS pictures was shown in the center of the screen, a position at which none of the pictures had actually been presented during learning.

Participants then moved the picture with four buttons up, down, left or right until it was at the position that participants thought it had been at during the learning task. The duration of trials was self-paced as every trial automatically concluded eight seconds after the last button press was entered by the participant. Five seconds after the last button press, a bright red frame appeared around the stimulus picture to warn participants that the trial would end in three seconds if they did not press any button. If participants pressed any button when the red frame was present, the frame disappeared and the time limit of 8swas set to zero again. The order of the 24 stimuli was randomized across each memory test. In addition to distance error, reaction time was also calculated in these memory tests (an improvement over the first study, in which this had not been possible). Reaction time was defined as the time from presentation of the stimulus until the last button press.

The paradigm was presented using the software Presentation (http://www.neurobs.

com) inside the scanner with video goggles (NordicNeuroLab) with a resolution of 800x600dpi.

The memory task combined with the two resting periods and the memory tests lasted close to two hours in total. Since this is a duration which is too long to be scanned in one session (at least for most participants), a break was introduced after half of the memory task, which corresponded roughly to half of the experiment.

During this break the scanner was stopped and participants were taken out of the scanner to allow them to drink something, relax their muscles or take a bathroom break. After five minutes, they were taken inside the scanner again and continued with the rest of the experiment. This break was meant to prevent high drop-out due to excessive scanning length.

The break was placed so that a symmetric structure of the paradigm was pre-served. In the previous experiment it had become apparent that separate scanning sessions can have considerable impact on classifier output. With the chosen setup, half of the memory task was in the same scanning session as Rest1 and the other half of the memory task was in the same scanning session as Rest2.