Experiment 2: Negative list 1 and negative list 2

Experiment 2 was identical to experiment 1 but only unpleasant words were used.

Emotional material is better recalled compared to neutral material (overview: Hamann, 2001). Thus, based on this memory enhancing effect, directed forgetting should be reduced or eliminated for emotional material. On the other hand, directed forgetting might be unaffected by the emotionality of pure lists, thus, replicating the findings of Wessel &

Merckelbach (2006) who found list method directed forgetting for both pure neutral and pure unpleasant lists. Additionally, as in experiment 1 we explored the electrophysiological dynamics correlated with the list learning.

3.2.1 Method Subjects

In this study 29 (15 male and 14 female) subjects participated with a mean age of 23.6.

The data sets of two subjects were excluded due to technical problems while recording and another three sets due to extreme alpha waves leaving 24 participants (12 male and 12 female) for analysis. All subjects were right-handed (assessed according to Annett, 1979;

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

Material

80 unpleasant words were chosen which had mean valence scores of 2.36 (SE: 0.06) on a scale from 1 (very unpleasant) to 9 (very pleasant) and 5.58 (SE: 0.13) for arousal on a scale from 1 (very un-arousing) to 9 (very arousing). The words were arranged in four lists of twenty unpleasant words that did not differ in regard to valence, arousal, concreteness, frequency, and length. The lists were randomly assigned to experimental conditions and lists.

Design, EEG recording, data analysis, and statistical analysis

The design, EEG recording, data analysis, and statistical analysis were identical to those used in experiment 1. In regard to the data analysis, visual inspection of the data revealed again that between 450 and 620 ms list 2 of the F condition yielded more positive-going ERPs than list 2 of the R condition in a left frontal sensor group. This electrode group was chosen for statistical analysis (fig. 13).

3.2.2 Results Recall data

The mean proportions of recalled items for the different conditions are displayed in table 5. For the immediate recall data an ANOVA with the factors condition (F condition, R condition) and list (list 1, list 2) was calculated. This yielded a significant interaction between condition and list [F(1, 23)=9.61, p<.01, fig. 12 right] which was due to costs (p<.05) and benefits (p<.05) of directed forgetting. Additionally, the mean difference between list 1 and list 2 was negative in the F condition (-.055) reflecting a tendency of reduced PI (p=.11) and positive in the R condition (.089) indicating effects of PI (p<.05).

The scatterplot (fig. 12 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 5: Details for the recall data. Including mean proportion of recalled words (standard error in brackets) and significant results for mean differences (p<.05*).

mean difference

F condition R condition

F condition – R condition

list 1 (neg) .233 (.027) .308 (.029) -.075* (costs)

list 2 (neg) .288 (.029) .219 (.031) .069* (benefits)

mean difference

-.055 (p=.11)

list 1 – list 2 .089*

figure 12: 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 - 620 ms: A repeated measures ANOVA with the factors condition (F condition, R condition) and list (list 1, list 2) was calculated. A main effect of condition occurred [F(1, 23)=4.656, p<.05] reflecting more positivity during the F condition compared to the R condition. This main effect was accounted for by an interaction condition x list [F(1, 23)=6.34, p<.05, fig. 13] with list 2 of the F condition eliciting increased positive activity compared to the other conditions and lists (F list 1: p<.05; R list 1: p<.01; R list 2:

p<.001).

figure 13: Effect of the increased positivity during list 2 of the F condition compared to list 2 of the R condition illustrated by frontal electrodes and the difference plot (F list 2 – R list 2).

3.2.3 Discussion

In experiment 2, directed forgetting was found as an interaction between condition and list using unpleasant words. Significant effects of costs and benefits occurred replicating the findings of Wessel & Merckelbach (2006). Again, the findings are consistent with the assumption of a two-factor account of costs and benefits of directed forgetting (Sahakyan

& Delaney, 2005). Comparing recall of the two lists within conditions yielded significant effects of PI in the R condition and a tendency of reduced PI in the F condition. This reduction might be a result of mental context change initiated by the forget instruction.

Similar to experiment 1, a positivity enhancement (450 and 620 ms after stimulus onset) occurred during encoding of words following the forget instruction. Studies have shown that late positive components occur in frontal regions for items that are later remembered compared to forgotten ones when the items are encoded with an elaborative strategy (Fabiani et al., 1990; Fernandez, Weyerts, Tendolkar et al., 1998) while rote encoding is

Fernandez, Weyerts, Tendolkar et al., 1998). In experiment 1, the enhanced positivity occurred in centro-parietal regions during subsequent learning following to-be-forgotten neutral words. In experiment 2, the increased positivity occurred in left frontal areas during learning following to-be-forgotten unpleasant material. Thus, the topographical differences found here might reflect the implementation of different encoding strategies:

As subsequent learning after learning unpleasant material might require more effort compared to subsequent learning after neutral material, a more elaborate strategy might be used reflected in increased frontal positivity. Frontal regions have also been associated with processes of inhibition and suppression (Anderson et al., 2004; Konishi et al., 1999).

Similarly, keeping information out of mind recruits prefrontal regions (Bunge, Ochsner, Desmond, Glover, & Gabrieli, 2001). Thus, the increased frontal positivity might, alternatively to elaborate encoding strategies, reflect inhibitory and control mechanisms that are engaged during forgetting unpleasant content.