Summary and experimental motivation

The discussed topics gave a broad overview of the directed forgetting literature, concerning underlying cognitive and neuronal mechanisms as well as limiting and influencing factors. The common view on underlying processes of directed forgetting is that mainly selective rehearsal processes are involved in the item method and retrieval inhibition in the list method (Basden et al., 1993). However, the involvement of inhibitory mechanisms has also been suggested to play a role in item method directed forgetting by behavioral studies (Geiselman et al., 1985; Zacks et al., 1996) and by recent studies

al., 2004; Ullsperger et al., 2000; Wylie et al., 2007). Results of ERP and fMRI studies, which found for example frontal activities during encoding of to-be-forgotten items, support the assumption of inhibitory mechanisms contributing to item method directed forgetting (Paz-Caballero & Menor, 1999; Paz-Caballero et al., 2004; Ullsperger et al., 2000; Wylie et al., 2007). The list method on the other hand has also been connected to differential encoding by Sheard and MacLeod (2005), attention allocation (Conway et al., 2000; Wessel & Merckelbach, 2006) as well as to dual-process accounts by Sahakyan and colleagues (Sahakyan & Delaney, 2003; Sahakyan et al., 2004; Sahakyan & Kelley, 2002).

Sahakyan and her co-workers assumed that costs and benefits of directed forgetting are dissociable and are associated with different underlying processes: Costs are assumed to result from changes in mental context initiated by the forget instruction (Sahakyan &

Kelley, 2002) while benefits are thought to stem from a change in encoding strategy also initiated by the forget instruction (Sahakyan & Delaney, 2003).

The literature suggests that directed forgetting is a stable effect over time (Gardiner et al., 1994; MacLeod, 1975), independent of output interferences (Barnier et al., 2007; Conway et al., 2000; Kimball & Bjork, 2002; Zellner & Bäuml, 2006), and dependent on the strength of memory representations as directed forgetting decreases with increasing item-cue intervals (Hourihan & Taylor, 2006; Wetzel & Hunt, 1977). Further, it seems that directed forgetting can be modulated by some specific characteristics of the participants and specific conditions of the stimulus material. For example, subjects dissociation scores seems to affect the amount of directed forgetting (Elzinga et al., 2000). The higher a subject’s dissociation score, the smaller the directed forgetting effect. Additionally, directed forgetting is reduced in OCD (Tolin et al., 2002), BPD (Korfine & Hooley, 2000), and anorexia nervosa (Tekcan et al., 2007) when the stimulus material is related to the diagnosis. Otherwise, participants with a repressive coping style show reduced recall of unpleasant to-be-forgotten items when they were processed in relation to the self (Myers et al., 1998).

So far, directed forgetting has been shown to occur for a variety of material as for example words (Sego et al., 2006), single phone numbers (Gottlob et al., 2006), stereotypes (Araya et al., 2003), autobiographical memories (Barnier et al., 2007; Joslyn & Oakes, 2005), and line drawings (Basden & Basden, 1996; Lehman et al., 2001; Salthouse et al., 2006;

Schmitter-Edgecombe et al., 2004). Also, while several studies found directed forgetting for stimuli irrespective of emotional content (e.g. Power et al., 2000; Wessel &

Merckelbach, 2006; Wilhelm et al., 1996), others have found that emotional valence modulated directed forgetting (Geiselman & Panting, 1985; Power et al., 2000).

Many different issues have been investigated in relation with directed forgetting.

However, some topics received more attention than others and thus, some issues are still unclear and require more and deeper exploration.

One open question concerned directed forgetting of nonverbal material, for example of complex colorful pictures. Using nonverbal material in directed forgetting experiments is desirable as photographic items seem be more naturalistic and more similar to real-life than words. Concerning potential future application of colorful pictures in directed forgetting experiments to explore impaired memory processes in specific populations (e.g.

intrusions in PTSD), exploring directed forgetting of complex pictures in healthy controls first is necessary. As mentioned, directed forgetting has been found for various stimuli as words (Sego et al., 2006), single phone numbers (Gottlob et al., 2006), stereotypes (Araya et al., 2003), autobiographical memories (Barnier et al., 2007; Joslyn & Oakes, 2005) that are directly connected to verbal processing. Directed forgetting also occurred for line drawings (Basden & Basden, 1996; Lehman et al., 2001; Salthouse et al., 2006; Schmitter-Edgecombe et al., 2004), which can be very easily verbalized as they all depict single objects. Also, while words offer few cues, colorful complex pictures contain information about color and space. Such additional information can enhance the memory representations of the items (Rossion & Pourtois, 2004; Tanaka & Presnell, 1999), thereby presumably reducing directed forgetting. Although there is one study on list method directed forgetting of complex pictures (Payne & Corrigan, 2006), the conclusions are limited as all pictures were accompanied by a short verbal description and thus, it is difficult to disentangle the individual influences of verbal and pictorial processes. In chapter 1 the issue of item method directed forgetting of complex colorful material was explored: In experiment 1, item method directed forgetting of neutral complex pictures and in experiment 2, influences of emotional content of pictures on item method directed forgetting and corresponding electrophysiological activity were investigated.

Although many studies implemented emotional material in the list method and found effects of directed forgetting (e.g. Power et al., 2000; Wessel & Merckelbach, 2006;

Wilhelm et al., 1996), there are also other studies suggesting some modulation of directed forgetting depending on emotional content (Geiselman & Panting, 1985; Power et al., 2000). The majority of those studies focused on differences between control and (sub-)

clinical participants, thus not always offering detailed information about effects for control subjects. Therefore, it was difficult to draw conclusions about directed forgetting of emotional material in healthy control subjects. Thus, the influence of emotional words on list method directed forgetting in healthy control subjects was systematically investigated in chapter 2. Emotionality (neutral versus unpleasant) of the lists was varied across four experiments. Additionally, electrophysiological activity was recorded during encoding with the intention to better understand the underlying processes of list method directed forgetting.

2. Chapter 1: Item method directed forgetting of