As outlined above, the effect of emotion on deliberate memory has been relatively thoroughly investigated. The mechanisms underlying the development of intrusive memories and the relationship between intrusions and deliberate memory are less studied. In general, two alternative views about intrusion development have been suggested; one is a unitary account and the other is known as the dual representation theory.
1.4.1 Unitary account view
One view is a unitary theory suggesting that intrusions are supported by emotional memory enhancements in the same way as deliberate memory. The unitary account is based on the idea that traumatic memory is inherently the same as non-emotional episodic memory and that there is no qualitative differences in encoding and processing between trauma memories and non-emotional memories (Hall and Berntsen, 2008; Talarico et al., 2004; Berntsen and Rubin, 2014; see Figure 1.1). What distinguishes trauma memories from other memories is that trauma memories will be subject to emotional facilitation, resulting in stronger encoding and consolidation of the traumatic material (Hall and Berntsen, 2008). This facilitation
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concerns the memory as a whole and hence this theory assumes that both intrusive and episodic memory is dependent on the same neural systems (Rubin et al., 2008a; 2008b).
This view also emphasizes that spontaneous memories – emotional or not – are inherently a normal phenomenon, except that emotionally negative memories will be noticed more because they are more distressing (Berntsen and Rubin, 2008). In line with ordinary memories, it is argued that memory intrusions are not more sensory (Greenberg and Rubin, 2003) or emotional in nature than other autobiographical memory, which is already characterised by being highly visual and emotional (Berntsen and Hall, 2004; Hall and Berntsen, 2008). This view emphasizes facilitated retrieval in the occurrence of memory intrusions; because emotionally negative memories are more salient and are rehearsed more than other information, there will be more cues in the environment that are likely to trigger involuntary memories of the trauma compared to the number of cues for less salient information rendering highly negative memories more likely to be spontaneously retrieved. Also, within this view, it is argued that traumatic memories become landmark memories that shape other autobiographical memory.
Figure 1.1. Overview of the DRT and unitary accounts of intrusion development. Top panel shows the foundation for theories suggesting that
encoding during trauma is impaired, such as the dual representation theory. Bottom panel shows the foundation for theories proposing that memory intrusions rely on enhanced encoding, including the unitary theory. Adapted from Hall and Berntsen (2008).
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1.4.2 Dual representation account
In an alternative view, Jacobs and Nadel (1985) suggested that there were two memory systems involved in the encoding of memories; the taxon system, which encodes sensory and affective information related to an experience, and the locale system that places the memory in a spatio-temporal context and is dependent on the hippocampus (O’Keefe and Nadel, 1978). The authors hypothesized that a lack of encoding in the locale system is related to childhood amnesia and that this form of encoding is similar to the conditioning underlying phobias, which can be reinstated during times of high stress where the hippocampus – and consequently taxon encoding – becomes impaired. Subsequent to this early work, the theory has been elaborated as a more comprehensive theory of PTSD and intrusion development and assimilated with more recent research findings. Hence, based on the BBB model mentioned previously, Brewin et al (2010) proposed in this dual representation theory that during encoding of an event, two parallel representations will be formed; one system is the contextual representation system (C-reps), which supports voluntary and deliberate retrieval of declarative information (Brewin et al., 2010). The C-reps allow flexible manipulation of the stored information and, critically for episodic memory, places the event in a temporal and spatial context. It is believed that these processes are mediated by the classical MTL memory system (Brewin 2001; Brewin et al., 2010). In particular, C-reps comprise the spatial properties of the scene related to an episodic event, in the form of allocentric representations and reliant on the hippocampus and surrounding medial temporal lobes (Brewin et al., 2010; O’Keefe and Nadel, 1978; Byrne et al., 2007). When the memory is retrieved, an egocentric image of the memory is formed in short-term memory, a process supported by the precuneus (Burgess et al., 2002; see Figure 1.2).
In parallel with the encoding of C-reps, a more perceptually rich, lower-level representation of the event will be created as a sensation-based representation (S- rep). Under normal circumstances, the S-reps are only transient and will quickly become inaccessible (Brewin et al., 2010). Hence, the S-reps are closely related to the affective, sensory properties and egocentric representations of the memory and can be reactivated by both bottom-up mechanisms such as priming and top-down
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by associations. Under normal circumstances, moderately salient events will enhance both types of representations and ensure stronger encoding (Brewin et al., 2010).
Figure 1.2 Schematic presentation of the dual representation theory and proposed associated neural structures. According to the DRT, two memory
representations are formed following encoding of an event. A contextual representation (marked in green) is created in the perirhinal cortex and bound to its corresponding context in the parahippocampal gyrus in the hippocampus. When a memory is voluntarily retrieved, an allocentric representation of the scene in which the experience took place will be created through pattern completion in MTL structures and transposed with an egocentric viewpoint of the event in the precuneus. The other proposed representation is a sensory representation, believed to be facilitated for emotionally negative content. Here, sensory and affective aspects of the event are upregulated via the amygdala, resulting in highly sensory and emotional memories with poor contextual encoding, rendering these representations more likely to involuntarily intrude. Reproduced with permission from Bisby and Burgess (2017).
However, in situations in which the individual is exposed to extreme stress, S-reps will be facilitated via the amygdala and become more enduring while C-reps will be impaired via down-regulation of the hippocampus (Brewin, 2013). These more enduring S-reps may then be activated by associated sensory experiences or emotional states, leading to involuntary retrieval of perceptually rich and affective memories with a sense of reliving the traumatic experience (Brewin et al., 2010; see Figure 1.1). Whereas the unitary account is in line with what is known about the effect of emotion on deliberate memory (for items at least), the DRT is consistent
53 with the clinical presentation of PTSD where there is typically a discrepancy between the presentation of strong negative memory imagery without context (intrusions) on one side, and poor deliberate memory on the other. In addition, the dual representation theory is in line with findings from imaging studies showing decreased hippocampal processing and increased amygdala processing in PTSD (Rauch et al., 2006; Shin et al., 2006; Brewin et al., 2001).