4.1.1 Conway’s autobiographical memory model
Conway provides a theoretical framework for the organization of autobiographical memory (Conway, 1993; 2000). He proposes three levels of abstraction of autobiographical memory. The highest-ranking level comprises ‘lifetime periods’ with their period-spanning particularly dominant feelings and evaluations as well as information about locations, activities, and persons (for example ‘at high school’). The next lower level consists of memories of ‘general events’. This refers to extended events or events that occurred repeatedly in one’s biography (like ‘going hunting’).
The lowest level refers to single events of usually short duration (minutes – hours) and the memory for these events contains highly specific, sensory-perceptual internal details (Conway, 2001). This type of memory is termed ‘episodic memory’.
Recollections of these memories can have ‘near-experience’ quality. Conway further concludes that these highly sensory memories are represented in the brain regions that were also involved in the processing that took place during the actual event. For visual experiences this means that the temporal cortex that contains the ventral visual processing stream is also involved in their recollection. The ventral stream is responsible for object recognition and is traditionally associated with the storage of visual long-term memory.
Bayley et al. (2005) studied episodic memory in patients with damage limited to the medial temporal lobe and in a group with significant additional damage to the neocortex. The more severe memory impairments of this latter group stress that the ability to recollect remote autobiographical events strongly involves the temporal and other distributed neocortical areas.
For a contextualized and structured recollection of these memories, not only temporal cortex is crucial, but also the hippocampus. According to the ‘Multiple Trace Theory’ of memory (MTT) (Nadel et al. 2000), the hippocampus involvement remains always necessary for the storage and also the retrieval of episodic, but not of semantic memories. Thereby the model distinguishes between the neural underpinnings of semantic and episodic memory. For episodic memory it is supposed that various parts of a particular episode (sights, sounds, smells,…) have representations in distributed cortical areas. These have to be ’bound’ for later retrieval. The MTT declares that the hippocampus is always needed for this integrative process and for a holistic memory retrieval. The hippocampus is assumed to be the crucial structure that binds content and context information like time, place, emotional content, and perceptual features that are required for the experience of recollection. Hippocampal memory traces are regarded as the basis for the reactivation of cortical traces. In case of flashbacks seen in PTSD, the disturbing
‘here-and-now’ quality of this phenomenon is due to a lack of contextual information that relates the memory content to the past history (see also Elbert, 2002). A hippocampal dysfunction at the time of encoding is supposed to be responsible for the detachedness of the “hot” from the context memory. Furthermore, in victims of multiple traumatic experiences the high number of events provides more and more conflicting information. Typically a person can only retrieve one context in which the fear network was previously activated. With increasing number of traumatic events the associative fear network expands and interconnections become stronger. In parallel the likelihood of the coactivation of the declarative memory system decreases, further contributing to the lack of contextual information (see also Elbert et al., 2006). Reduced hippocampal activity has been associated with more severe PTSD symptoms (Astur, 2006).
Evidence for the necessity of hippocampal involvement in the retrieval of context information comes for example from a fMRI study by Ryan et al. (2001).
Subjects had to retrieve remote autobiographical memories that occurred either within the last 4 years or more than 20 years ago. During recollection bilateral activation of the hippocampus was measured in both conditions. The authors further hypothesized that reactivation of remote events creates new memory traces within the hippocampus, and that possibly old ones are strengthened.
4.1.2 The role of the temporal lobe in autobiographical memory – evidence from brain damaged patients
Steinvorth et al. (2005) conducted an extensive declarative memory study with two amnesic patients with bilateral medial temporal lobe lesions and a healthy control group. Episodic memory was investigated employing the Autobiographical Interview developed by Levine et al. (2002) that is designed to see how well subjects are able to re-experience personal happenings from the past. Participants were required to recall one specific personal life event from each of five life periods: childhood, teenage years, early adulthood (for example wedding), middle age, and the year prior to the study. The most important characteristic of this test is that in addition to a spontaneous recall, subjects are asked in a standardized and structured manner to recall highly specific information such as sounds, sights, smells, or emotions. Such internal details reflect the episodic re-experiencing. Subjects were also asked to rate the vividness of each memory, the emotional change elicited by the event, and its personal importance. The test also includes the recollection of external details that are rather semantic than episodic (factual information). Evidence was gathered that remote semantic memory was virtually preserved in the two patients. Their results for semantic memory were comparable to those of the controls. Contrary, a severe impairment for remote episodic memory was uncovered for the patients. The controls provided significantly more internal details for all recalled life events. They also rated most of the recollections as more vivid compared to the patients. These results stress the role of the medial temporal lobe for autobiographical memories. The type of episodic memory required in the Autobiographical Interview can be regarded as experience-near, highly specific, sensory-perceptual details of personal life events.
Exactly this type of memory could not be recalled by the patients.
A recent MRI study by Gilboa et al. (2005) further stresses the important role of the medial and lateral temporal lobe in the retrieval of episodic memory. Patients with mild Alzheimer's disease with varying degrees of retrograde memory loss were studied. Autobiographical memory for episodes from childhood, adulthood, and recent past was assessed. Subjects had to provide the temporal and spatial context of certain events and their reports were rated for descriptive richness and specificity.
In addition, the number of reported details was scored. Impairments for all these episodes were associated with combined atrophy in bilateral medial temporal lobe
and anterior lateral temporal neocortex. The amount of tissue loss from anterior temporal cortex and medial temporal structures was strongly related to the severity of autobiographical memory impairments. The multivariate analysis technique of Partial Least Squares (McIntosh, 1996) was used to identify assemblies of brain regions that together covary in relation to behavioral measures. Results yielded a group of regions consisting of the bilateral medial temporal lobes and anterior temporal cortices, as well as the right lateral posterior cortex that was associated with the performance on autobiographical memory tasks. These finding stresses the involvement of both medial and lateral structures in the retrieval of episodic memory.
Spiers et al. (2001) employed a ‘real-world’-like experimental design in order to investigate recent autobiographical memory. In their study, subjects made experiences in a virtual 3-dimensional town. Later, context-dependent memory about virtual encounters with other characters in the town and topographical information were assessed. Subjects were patients with either left (LTL), or right (RTL) temporal lobectomy, and a healthy control group. The results can be summarized as follows:
compared to the healthy controls, both patient groups showed significant deficits in all memory tests. The RTL group was most impaired in all topographical memory tests and in object recognition, whereas the LTL group showed the most severe deficits in the memory tasks in which various aspects of the encounters were asked.
These findings demonstrate that bilateral medial temporal lobe structures are involved in topographical and autobiographical memory processes, but that some degree of lateralization exists. The right hemisphere seems to be more involved in topographical tasks and object recognition, whereas the left hemisphere is more concerned with context-dependent autobiographical memory.
Thus, the above lesion and lobectomy studies show that context-rich episodic memory depends on neuronal ensembles in bilateral medial temporal lobe – neocortical networks. Cortical structures are thereby supposed to be involved in the retrieval of sensory-perceptual memories.
4.1.3 Autobiographical memory in the temporal lobe – findings from PET and fMRI studies
The role of the temporal lobe for episodic memory was confirmed in a number of imaging studies. Fink et al. (1996) for example investigated the neural networks involved in the retrieval of personal episodic memory. During a PET scan, subjects listened to sentences containing affect-laden autobiographical (PERSONAL) information. Auditory information was presented that contained information from the subjects’ childhood, adolescence, and early adulthood (for example: “When you were 15 you took part in a swimming marathon…”). Subjects were required to imagine in detail what had happened. Information about significant events was gathered some weeks prior to the experiment. Further, a baseline REST condition was implemented without any auditory material. Comparing the two conditions, increases in neural activity were observed in the temporal lobes including the temporal poles, the medial and superior temporal gyri, and hippocampal, parahippocampal, and amygdaloid regions. Further, dorsal frontal and right posterior cingulate areas were active. Thus, these findings demonstrate the essential role of the temporal lobe in the retrieval of personal episodic memory.
Similarly, Piefke et al. (2005) investigated the functional neuroanatomy subserving episodic memory in a fMRI study. Subjects were required to remember as vividly and emotionally as possible personal biographical events with positive and negative valence. Compared to baseline, neural activity was increased in medial and lateral temporal areas that included parahippocampal and hippocampal regions.
These two studies are examples of investigations demonstrating a heightened neural activity in the temporal lobe when subjects retrieve autobiographical events. A complete overview of studies further supporting this observation can be found in Moscovitch et al. (2005).