Experiment 1: Neutral list 1 and neutral list 2

This study was designed to explore brain dynamics underlying list method directed forgetting. Analogous to the study of Wessel & Merckelbach (2006) only neutral words were used in order to replicate their results and to further examine brain activity during a standard paradigm.

3.1.1 Method Subjects

22 subjects (11 male, 11 female) with a mean age of 25.27 years participated in this experiment. The data of one subject was excluded from analysis due to artifacts leaving 21 data sets for analyses. All subjects were right handed (assessed according to Annett, 1979;

Oldfield, 1971) and had normal or corrected to normal vision. They provided informed consent and received course credit or payment for participation (15 €).

Material

From a set of German nouns with ratings for valence, arousal, and concreteness and scores for length and frequency (the latter based on the CELEX database: Baayen, Piepenbrock,

& Gulikers, 1995) 80 neutral ones were chosen as stimulus material. The words had mean scores of 4.86 (SE: 0.13) for valence on a scale from 1 (very unpleasant) to 9 (very pleasant) and 3.58 (0.12) for arousal on a scale from 1 (very un-arousing) to 9 (very arousing). Four lists of twenty words each were built that did not differ in regard to valence, arousal, concreteness, length and frequency.

Design

Prior to the experiment the subjects were given verbal and written instructions: “Several word lists are going to be shown to you. You shall remember some of them for later. After each word list you receive a cue whether the previous list is one to be remembered or whether you can forget it”. The experiment consisted of two conditions: The F (forget) condition and the R (remember) condition. All subject received both conditions, with the order being counterbalanced. Each condition started with the presentation of list 1 which was subsequently followed by an instruction to either forget (F condition) or remember (R condition) all previous words. Then, in both conditions, list 2 was shown and followed by the instruction to remember all previous words. After that a distractor task was given to the subjects (counting backwards in steps of three from a three digit number for 2 min) and they were then asked to recall as many words as possible of all previously shown nouns irrespective of the initial instructions. After the 2 min free recall test, the R condition was presented to subjects that had started with the F condition and vice versa.

Using such a within-participants design in the list method is not expected to vary from using a between-participants design and therefore offers the possibility of investigating individual differences in the abilities of remembering and intentionally forgetting (Bäuml et al., 2008; Zellner & Bäuml, 2006). During presentation each of the words appeared for 2000 ms on the display followed by a fixation cross for 1500 ms. All instructions were shown for 4 s.

EEG recording

The EEG was recorded from 65 Ag/AgCl electrodes using Neuroscan (Scan, SynAmps, Compumedics, El Paso, USA) soft- and hardware. During recording impedances were kept below 5 kΩ. Data were acquired with a sampling rate of 500 Hz and online filters of DC-100 Hz. During recording, electrodes were referenced to Cz. The experimental stimulation was run under Presentation (Neurobehavioral Systems, Albany, USA). Prior to

the experiment vertical, horizontal, and blink related eye movements were systematically recorded for later eye correction of the experimental EEG data.

Data analysis

The recorded EEG data were processed with Brain Electrical Source Analysis (Besa ®, MEGIS Software GmbH, Gräfeling, Germany, www.besa.com) and offline re-referenced to average reference. The EEG was corrected for eye movements, artifact were rejected (EEG>140 µV), and the data was filtered with a 0.3 Hz highpass and a 30 Hz lowpass filter. The corrected and filtered data were then segmented into epochs (-200 ms-1000 ms), baseline corrected, and averaged. The ERP analyses of the resulting grand means of each condition were done using EMEGS (ElectroMagnetic EncephaloGraphy Software, Junghofer & Peyk, 2004). Of special interest were effects of the mid-condition instruction on list 2. Visual inspection of the data showed that between 450 and 660 ms list 2 of the F condition yielded more positive-going ERPs than list 2 of the R condition. As the topography of this difference change over time, two time windows were determined with different but overlapping sensor groups (fig. 11).

Statistical analysis

All statistical calculations were done with Statistica 6.1© (StatSoft, Inc. 2003, www.statsoft.com). The statistical analyses of the behavioral and electrophysiological data were done using repeated measure ANOVAs with the within-factors condition (F condition, R condition) and list (list 1, list 2). Single differences were calculated with the Fisher’s LSD. A significance level of .05 was used for all calculations. The directed forgetting effect was defined as an interaction between condition and list. Also, poorer recall of list 1 in the F condition compared to the R condition (costs) and better recall of list 2 in the F condition compared to the R condition (benefits) were calculated (mean differences) according to Sahakyan and Delaney (2005). Additionally, inter-condition comparisons (mean differences) of the two lists were calculated to assess a measure of PI and thus of induced context change initiated by both the forget instruction and the change of list valence. Under the assumption of effective directed forgetting, mean differences were expected to be positive in the R condition while being negative in the F condition.

3.1.2 Results Recall data

Table 4 displays the mean proportions of recalled words for the different conditions. A two-way ANOVA with the factors condition (F condition, R condition) and list (list 1, list

2) was calculated and yielded a significant interaction of condition and list [F(1, 20)=15.46, p<.001, fig. 10 right] reflecting benefits of (p<.01) and costs (p<.05) of directed forgetting. Mean recall proportion difference of list 1 and list 2 was negative in the F condition (-.107, p<.05) reflecting reduced PI and positive in the R condition (.105, p<.05) reflecting PI. The scatterplot (fig. 10, left) reveals that most subjects recalled more list 1 items in the R condition than in the F condition (costs, more black stars above the linear line) and that also most subjects recalled more list 2 items in the F condition than in the R condition (benefits, more grey dots beneath the line).

table 4: Details for the recall data. Including mean proportion of recalled words (standard error in brackets) and significant results for mean differences (p<.05*, p<.01**).

mean difference

F condition R condition

F condition – R condition

list 1 (ntr) .326 (.045) .412 (.048) -.086* (costs)

list 2 (ntr) .433 (.035) .307 (.028) .126** (benefits)

mean difference

list 1 – list 2 -.107* .105*

figure 10: Left: Scatterplot of the individual recall data. The line describes a hypothetically symmetrical recall of list 1 and 2 in the different conditions. Right: Bar plot of the interaction condition x list.

EEG data

450- 520 ms: A repeated measures ANOVA with the within-factors condition (F condition, R condition) and list (list 1, list 2) was calculated for a left centro-parietal electrode group and yielded a significant interaction condition x list [F(1, 20)=7.23, p<.05, fig. 11]. Post-hoc tests showed that list 2 of the F condition elicited increased positivity compared to the other lists and conditions (p<.05).

540-660 ms: The repeated measures ANOVA, calculated for a centro-parietal sensor group, yielded again a significant interaction of condition and list [F(1, 20)=5.22, p<.05, fig. 11]. Post-hoc comparisons revealed again that during list 2 in the F condition

figure 11: Effect of the increased positivity during list 2 of the F condition compared to list 2 of the R condition illustrated by centro-parietal electrodes and the difference plot (F list 2 – R list 2).

3.1.3 Discussion

Using neutral word material, list method directed forgetting was found as an interaction between condition and list. Also, costs and benefits occurred which is consistent with several other studies using neutral word material (e.g. Pastötter & Bäuml, 2007; Sahakyan

& Delaney, 2003). These results fit with the two-factor account of directed forgetting suggesting that the forget instruction initiates a change (1) in mental context leading to cost and (2) in the encoding strategy leading to the benefits (Sahakyan & Delaney, 2005).

Comparing the list recall within the conditions showed that recall of list 2 benefited from the forget instruction suggesting a reduction of PI from list 1 to list 2 which is probably secondary to the change in mental context. In the R condition, recall of list 2 was reduced compared to list 1 suggesting effects of PI and memory capacity limits. The ERPs showed an increased positivity following the crucial mid-condition forget instruction compared to the remember instruction. This is consistent with findings of late positive potentials in response to attentional demands (Johnson, 1988; Paller et al., 1987) and therefore supports assumption of focused attention (Conway et al., 2000) during encoding of list 2 in the F condition presumably associated with a change in encoding strategy (Sahakyan &

Delaney, 2003).