حافظه اپیزودیک رتروگراد و احساسات: چشم انداز بیماران مبتلا به فراموشی تجزیه ای

With his recent definition of episodic memory Tulving [Tulving, E. (2005). Episodic memory and autonoesis: Uniquely human? In H. Terrace & J. Metcalfe (Eds.), The missing link in cognition: Evolution of self-knowing consciousness (pp. 3–56). New York: Oxford University Press] claims that this memory system is uniquely human and thereby distinguishes human beings from other, even highly developed, mammals. First we will define the term episodic memory as it is currently used in neuropsychological research by specifying the three underlying concepts of subjective time, autonoëtic consciousness, and the self. By doing so, we will strongly focus on retrograde episodic memory and its relation to emotion and self-referential processing. We support this relation with a discussion of autobiographical memory functions in psychiatric disorders such as dissociative amnesia. To illustrate the connection of emotion and retrograde episodic memory we shortly present neuropsychological data of two cases of dissociative amnesia. Both cases serve to point to the protective mechanism of a block of self-endangering memories from the episodic memory system, often described as the mnestic block syndrome. On the basis of these cases and supportive results from further cases we will conclude by pointing out similarities and differences of patients with organic and dissociative (psychogenic) amnesia.

On a content-based view, memory can be seen as a multidimensional concept. With respect to the kind of processed information long-term memory can be demarcated into four (Markowitsch, 2000a and Tulving, 1987), recently into five different memory systems: procedural memory, priming, perceptual memory, semantic, and episodic memory (Markowitsch, 2003; Reinhold, Kuehnel, Brand, & Markowitsch, 2006). These systems are hierarchically ordered with episodic memory at the top (cf. Fig. 1), reflecting their ontogenetic and phylogenetic development. Tulving, 1995 and Tulving, 2001 emphasized in his SPI-model that encoding of information occurs serially, storage in parallel, and retrieval is independent of the condition during encoding, implying that information, encoded as an episode, may be retrieved on the basis of the procedural memory system. Full-size image (49 K) Fig. 1. Sketched pictograms to exemplify the five content-based divisions of long-term memory with their corresponding levels of consciousness. The memory systems are hierarchically ordered with procedural memory as the hierarchically lowest memory system (right) and episodic memory at the top (left). Figure options Introducing the term ‘episodic memory’ in the early 1970s of the last century, Tulving (1972) summarized the ability to recall events of one's own life with a strong relation to time and space and with a clear emotional connotation. Tulving thereby extended Nielson's (1958, p. 25) view of ‘two separate pathways for two kinds of memories. The one is memories of life experiences centering around the person himself and basically involving the element of time. The other is memories of intellectually acquired knowledge not experienced but learned by study and not personal’. In a relatively young science like the neurosciences and neuropsychology those definitions commonly change and are refined throughout the research process. With respect to Tulving's early definition, some researchers used the term ‘episodic memory’ for word-list paradigms in laboratory tasks. This and other observations led Tulving (2005) to a renewal and redefinition of his episodic memory definition. Currently, episodic memory is characterized as a conjunction of three concepts, namely subjective time, autonoëtic consciousness, and the experiencing self. With this definition Tulving stresses the importance of the self-relatedness of episodic memories. This is supported by the results of Gilboa (2004) who reviewed prefrontal cortex activation in laboratory word-list paradigms and autobiographical memory tasks. Although detecting overlapping activations for both tasks he was able to also reveal differential activation pattern in ventromedial (primary left-sided) prefrontal activations that are solely associated with autobiographical memory retrieval. It is concluded that both memory retrieval processes can be differentiated with regard to their modes of post-retrieval monitoring and verification. Whereas pure autobiographical-episodic memory retrieval induces an intuitive ‘feeling of rightness’ and a relation to an activated self-schema, retrieval of episodic memories (e.g., word lists) requests a more conscious and sophisticated monitoring (Gilboa, 2004). Taking into account the discussion of episodic word-list learning and autobiographical-episodic memory, Tulving (2001) also strictly differentiates between the terms ‘remember’ and ‘know’. To ‘remember’ a specific event means that the memory has a strong self-relation (autonoësis) and emotional connotation and thereby goes far beyond of just ‘knowing’ that this event occurred (see also Gardiner & Richardson-Klavehn, 2000). For this reason, the distinction of ‘remember’ and ‘know’ is often part of studies dealing with autobiographical memory. For instance, Piolino et al. (2007) used the remember/know paradigm to detect developmental differences between semantic and episodic subcomponents of autobiographical memory in school-age children. With the age of the children the ‘remember’ responses which were justified by contextual information of the event increased. Thus, Piolino and colleagues concluded that the ability of mental time travelling in subjective time is the last feature of the autobiographical memory system that becomes fully operational. Tulving argues that remembering is a central function of episodic memory and thereby distinguishes this memory system from semantic memory. The experiencing person – the mental-time traveller – is referred to as the ‘rememberer’. Autobiographical memory is often used synonymously for episodic memory though it needs not to be same as autobiographical memory additionally comprises autobiographical-semantic facts (e.g., date of birth). Cabeza and St Jacques (2007) illustrated in their recent review article of functional neuroimaging of autobiographical memory a continuum from the purest episodic to the purest semantic memory. This also reflects the discussion of a possible distinction of semantic and episodic subcomponents constituting autobiographical memory. Retrieving facts related to one's own biography, like date and place of birth, relates to the semantic portion of autobiographical memory since we ‘know’ and do not ‘remember’ these facts. In contrast, when we ‘remember’ a specific personal event like our last birthday party, the memory is associated with a clear emotional connotation and further perceptual features like the taste of our birthday cake. These features make the difference to just knowing where the party took place and who baked the cake. The usage of the term ‘autobiographical-semantic memory’ when relating to self-related facts and semantic knowledge and ‘autobiographical-episodic memory’ when talking of specific self-related and emotionally laden events reflects this segregation. Both subcomponents (autobiographical-semantic and autobiographical-episodic) contain self-related information but their contents differ concerning their emotional colouring. Thus, self-related and emotional processes are separable when looking at autobiographical memory. To judge whether this hand belongs to me or another person is a clearly self-related but un-emotional decision. In the following we will use the term episodic memory when referring to memories of the autobiographical-episodic memory system, since episodic memory as defined by Tulving (2005) still constitutes the core facet of autobiographical memory. Returning to Tulving (2005) definition, we will have a closer look at the three concepts constituting episodic memory: subjective time, autonoëtic consciousness, and experiencing self. Tulving's concept of ‘subjective time’ enables the ‘rememberer’ to mentally travel to any given point of his or her individual lifeline. It is also referred to as chronaesthesia (Tulving, 2002). With the term ‘subjective time’, Tulving highlights the fact that the time in which events are remembered differs from physical time. Though episodic memory is often related to the past it also enables us to anticipate future events and consequences or needs. The ability of prospective mental time travelling is subject of some recently published papers (e. g., Addis, Wong, & Schacter, 2007; Buckner & Carroll, 2007; Szpunar et al., 2007) and is currently defined by: “allows us not only to go back in time, but also to foresee, plan, and shape virtually any specific future event” (Suddendorf & Corballis, 2007, p. 1; see also Tulving & Kim, 2007). However, in this manuscript we will focus on retrograde episodic memory and therefore on the past-related aspect of mental time travelling. The second concept, in which episodic memory is embedded, is autonoëtic consciousness. Related to each of the above-introduced five content-based memory systems there are corresponding levels of consciousness. Procedural memory and priming as the hierarchically lowest memory systems remain on an ‘anoëtic’ level of consciousness, meaning that processed information is not subject to conscious attention. The perceptual and semantic memory systems require mindful processing of information what is referred to as ‘noëtic’ consciousness. Forming or retrieving episodic memories involves self-related and thus ‘autonoëtic’ information processing. Tulving (2005) considers autonoëtic consciousness to be a uniquely human trait. The corresponding levels of consciousness can also be derived from Fig. 1. Autonoëtic (self-)consciousness is defined as “the capacity that allows adult humans to mentally represent and to become aware of their protracted existence across subjective time” (Wheeler, Stuss, & Tulving, 1997, p. 335). Next to subjective time and autonoëtic consciousness the experiencing self amounts to the concept of episodic memory. Building up autobiographical memories enables us to experience our self as an entity over time and therefore enables the formation of an identity or “personality”. Our personality relies on past experiences, is strongly associated with our personal beliefs and allows us to consistently behave with our own beliefs and desires. The experiencing self assures the autobiographical memory to be a continuous memory system that is built up along a subjective time line (Markowitsch, 2005b). Although the term ‘experiencing self’ can also be used to name the ‘rememberer’, the concept of the experiencing self implies the perceptual component of remembering a specific event from one's own biography with contextual information like time, space, emotional state. However, the experiencing self has to be distinguished from the concept of the self as the self constitutes a multidimensional (moral, physical, personal, social, etc.) and outlasting construct, while the experiencing self refers to a momentaneous state. Taken together, it is hard to look at the three components of episodic memory (subjective time, autonoëtic consciousness and experiencing self) separately since they strongly rely on the existence and full development of each other. Therefore, we will have a closer look especially at the neural interrelation of episodic memory, emotion, and the self in the following paragraphs. Their interdependencies are central for an understanding of the relevance of the phenomenon of ‘dissociative amnesia’ to the concept of episodic memory, which is also reflected in the current definitions of the concepts of dissociative amnesia and dissociative identity disorder in DSM-IV-TR (American Psychiatric and Association, 2000). We outlined before that episodic memory is characterized by self-related and emotional processing of information. In general, processing of information into different memory systems is frequently divided into several steps from information registration, encoding, consolidation and storage to the retrieval of information (but see Craik & Lockhart, 1972; Moscovitch, 2000). Dependent on the different processes and the kind of processed information, specific brain regions and structures are involved (for a comprehensive review of the neuroanatomy of memory see Markowitsch, 2005a). Formation and storage of autobiographical memories recruits mostly limbic structures, involving the Papez circuit, the amygdala and other structures of the so-called basolateral-limbic loop (Brand and Markowitsch, 2003 and Brand and Markowitsch, 2009) as well as connecting fibers. Even though also engaged in memory acquisition and consolidation, regions of the prefrontal cortex are especially involved in the retrieval of episodic memories. In a recent review, Brand and Markowitsch (2008) highlight the specific role of several parts of the prefrontal cortex in episodic memory, particularly the orbitofrontal cortex (Brand & Markowitsch, 2006). Due to reciprocal connecting fibres with limbic and diencephalic structures, primary functions of the orbitofrontal cortex can be seen in the integration of emotion and self-relatedness information which is indispensable for the retrieval of distinct episodic memories. Taken together, an orchestra of limbic, diencephalic and prefrontal structures as well as several fiber tracts collaborate on encoding, consolidating, storage, and retrieval of personal events which are emotionally coloured and have a strong relation to the self (Fujiwara & Markowitsch, 2006; Markowitsch, 2003). Additionally, there seems to be evidence that autobiographical memory is, among other features, organized in terms of its emotional valence (Piefke, Weiss, Zilles, Markowitsch, & Fink, 2003; Schulkind & Woldorf, 2005). Processing of self-related information is a crucial element of autobiographical-episodic memory function (Conway & Pleydell-Pearce, 2000; Wilson & Ross, 2003). This can also be seen in overlapping neural networks of self-related and autobiographical-episodic memory processing which comprise the prefrontal cortex (Johnson et al., 2002; Northoff & Bermpohl, 2004) and further cortical midline structures (Northoff et al., 2006). Next to shared neural networks both functions are usually emotionally coloured. Northoff et al. (2007) therefore tried to disentangle neural correlates of self-related and emotional processing by using parametric modulation to correlate neural activity and the degree of self-relatedness while subjects perceived emotional stimuli. They found opposite parametric modulation of self-relatedness and emotion in the dorsomedial prefrontal cortex and the ventral striatum/nucleus accumbens while further subcortical regions were modulated in the same direction. This led the authors to the conclusion that processing of the self and emotions is strongly related to subcortical regions. However, higher cortical regions as the dorsomedial prefrontal cortex allow us to easily discern the self from emotional processing and thereby essentially contribute to the ability of self-consciousness. The interaction of self-related processing and episodic memory as well as the relation of emotion and episodic memory is also observable in various neurological and psychiatric patients. For example, patients with schizophrenia often suffer from changes in autobiographical memory abilities (Corcoran & Frith, 2003; Danion et al., 2005). Additionally, autobiographical memory disturbances are often reported and discussed as part of the pathology in patients with predominantly impaired emotional processing capabilities (e.g., major depression) (Lemogne et al., 2006 and Williams et al., 2007). In patients with neural damage in limbic structures like the amygdala (e.g., in patients with Urbach-Wiethe or temporal lobe epilepsy) deficits in memory or other cognitive functions are also observable (Bengner & Malina, 2007; Hurlemann et al., 2007 and Markowitsch et al., 1994; Richardson, Strange, Duncan, & Dolan, 2006; Siebert, Markowitsch, & Bartel, 2003; Weniger, Bouscein, & Irle, 2004). In conclusion there is a strongly overlapping and integrative network of brain structures engaged in emotional and self-related processing which corporately contribute to the processing of episodic memories. Due to its complex structure the episodic memory system is highly vulnerable as apparent in the phenomenon of dissociative amnesia. Since this psychiatric disease is a relatively rare condition we shortly introduce its main characteristics before describing two cases with this condition.