Episodic And Autobiographical Memory Research Paper

The relation between episodic and autobiographical memory can be illustrated through the case study of a man, known as K.C., who became profoundly amnesic in 1981 following a serious closed head injury (Tulving et al.1991). As a result of the injury, K.C. suffered a complete loss of episodic memory, as he cannot recollect a single thing that has ever happened to him. Despite extensive questioning, he cannot remember even things that should be highly memorable, such as the accidental death of a brother to whom he was close. It follows that K.C. has a drastically reduced ability to recount his autobiography. Yet he can report autobiographical facts, such as his date of birth, the names of many of the schools he attended, the make and color of a car he once owned, and the fact that he has spent many summers at his parents’ cottage. K.C. is unable to remember any aspects of his life, although he knows many important facts about his past. In healthy individuals, autobiographical memory encompasses both what can be recollected episodically, and self-relevant knowledge that can be expressed through semantic memory.

1. A Psychological History

Psychological writing on the topic of episodic memory began with William James in his classic Principles of Psychology (James 1890). To James, the concept of memory was equivalent to what is now called episodic memory. The defining characteristic of memory was the conscious recollection that accompanied the retrieval of personally experienced events. ‘Memory requires more than the mere dating of a fact in the past. It must be dated in my past … I must think that I directly experienced its occurrence.’ (p. 612). Thus James distinguished between acts of episodic memory and other manifestations of learning and memory (semantic memory, classical and emotional conditioning), which do not involve conscious recollection of the personal past. Later philosophers and psychologists emphasized the role of such personal memories in defining an individual’s personal identity and autobiography (Shoemaker 1959).

Although James provided a strong foundation, there was very little research directed at memory for personally experienced events for the next several decades. Psychological research became dominated by the idea that observable behavior, and not conscious experience, was the appropriate subject matter for the field. The study of learned responses was not compatible with the study of the remembering of experiences.

Contemporary investigations began when Tulving (1972) proposed the distinction between episodic and semantic memory. This distinction can be characterized in two different ways, and it is important for these meanings to be differentiated and the meaning used in a particular context to be made clear. One distinction has to do with a particular kind of memory task or memory performance, while the other concerns different neurocognitive memory systems in the human brain. Episodic memory was originally defined as the variety of memory that receives and stores information about temporally dated episodes or single events, and the temporal–spatial relations among them, while semantic memory comprised organized knowledge about the world, including information about words, objects, and events. The episodic semantic distinction introduced the possibility that the remembering of single, personal events should be studied separately from factual knowledge. For many years, the study of memory for single events was classified as episodic memory research and it proceeded largely independently of research on semantic memory. Even today, the prototypical laboratory list-learning task in which participants are exposed to study material and then tested for what they have learned is often classified as an episodic memory task.

Over the intervening years, it became clear that the 1972 definition of episodic memory was no longer satisfactory. Performance of a so-called episodic memory task, such as free recall, can depend not only upon the type of conscious recollection that was described by William James, but also on general knowledge about single events from the past. Episodic memory now refers to a neurocognitive system that renders possible the conscious recollection of events as they were previously experienced. The defining characteristic of episodic memory is its dependence on a special kind of conscious awareness called autonoetic awareness. An individual with autonoetic (or ‘self-knowing’) awareness is capable of roaming at will in subjective time, by recollecting aspects of past experiences, or imagining future experiences through anticipations and daydreams. Semantic memory is de- fined as a neurocognitive system that makes possible the acquisition, retention, and use of factual information. The retrieval of semantic information is accompanied by noetic awareness, which is experienced when one thinks about something one knows. Noetic awareness does not involve any feeling of reliving the personal past; it represents knowledge about some aspect of the world.

While episodic remembering is necessarily auto-biographical, a person’s autobiography can be ex- pressed through systems of memory that are not episodic. The final criterion is the individual’s phenomenal feeling, not the contents, or veridicality, or age, or confidence of what is retrieved. Therefore, it is possible for individuals to recall autobiographical aspects of their lives, even single events in the recent past, via noetic awareness only.

2. Neural Bases

Research in cognitive neuroscience has made significant progress in identifying the brain structures that support episodic and autobiographical memory. Most of the relevant knowledge about the relation between neuroanatomy and memory has been gleaned from the study of patients with circumscribed brain injuries. Especially important has been the identification of patients with selective losses of episodic or autobiographical memory, with a relative sparing of other varieties of memory, such as semantic memory. The integrity of two brain regions, the medial temporal lobes and the prefrontal cortex, are critical for the normal operations of episodic memory. The former area is necessary for the establishment of episodic and autobiographical memories, and also participates in their retrieval for a limited time following encoding. The latter area underlies the ability to become autonoetically aware of the personal past and future.

2.1 Medial Temporal Cortex And Episodic Memory

It is now well established that several structures in the medial temporal cortex have a critical role in both the encoding and retrieval of episodic memory. This region, often referred to as the medial temporal lobe system, comprises a number of anatomically connected and functionally interrelated structures, including the hippocampi, entorhinal, perirhinal, and parahippocampal cortices. Midline diencephalic structures, such as the medial thalamus and the fornix, are also included in this system.

The importance of the medial temporal cortex in episodic memory was established in one of the classic case studies in cognitive neuroscience (Scoville and Milner 1957). In an experimental surgery, a patient known as H.M. underwent bilateral hippocampal excision with the goal of curing his severe epileptic seizures. While the surgery was effective in the prevention of seizures, it had the unintentional side-effect of rendering the patient amnesic for all postsurgical events. To this day, H.M. cannot remember anything that has happened to him since the surgery, a condition known as anterograde amnesia. Over the intervening years, many other patients with damage limited to the medial temporal lobe system have reinforced the relation between this brain region and anterograde amnesia for episodic and autobiographical memories. The medial temporal lobes receive highly preprocessed inputs from multiple, diverse cortical and subcortical brain regions. This makes the hippocampal system well-placed anatomically to bind or conjoin the inputs, and combine them into a single complex event or episode that could subsequently be stored and remembered (Cohen and Eichenbaum 1993). When the hippocampi and related structures are damaged, affected patients are unable to form a ‘memory trace’ from the various aspects of an ongoing event; it follows that the event cannot be subsequently remembered.

The hippocampal system participates in the retrieval of episodic memories for a limited time after encoding. Patients with anterograde amnesia typically are unable to remember events that occurred just before they became amnesic. The extent of this retrograde amnesia varies between patients yet there is typically a temporal gradient in which memory for recent episodes is affected more than that for remote episodes. As time passes after encoding, episodic and autobiographical memories gradually become reorganized, or consolidated. Over a period of several weeks or months, recollection becomes mediated by distributed areas in the neocortex.

Structures in the medial temporal lobes are critical for the encoding and retrieval of both episodic and semantic memory. The majority of patients with amnesic syndromes have an equivalent or roughly equivalent impairment in their ability to autonoeti-cally recollect their past (episodic memory) as well as noetically know about the past (semantic memory). Yet some recent reports have described a group of amnesic patients with profound losses of episodic memory, with little or no deficit in semantic memory (Vargha-Khadem et al.1997). Two distinctive features of the pathology were that: (a) it was confined solely to the hippocampus itself, while sparing the surrounding cortex, and (b) it was sustained very early in life, before the patients had the opportunity to build a base of autobiographical memories. While the patients are unable to remember any of their ongoing experiences, they have made satisfactory progress through school, and assembled a large body of semantic knowledge. The patients have some autobiographical knowledge, as they know many facts about their lives, although they cannot remember anything that has happened to them. An increasing body of knowledge suggests that episodic memory processes depend upon the hippocampus proper, while semantic memory is mediated largely by the perirhinal cortex in the temporal lobes (Aggleton and Brown 1999), although a definitive resolution of this proposal remains elusive.

2.2 Prefrontal Cortex And Autonoetic Awareness

The operations of episodic memory are dependent upon a sophisticated variety of conscious awareness, one that arguably does not exist in nonhuman animals. Autonoetic awareness refers to the ability to consider one’s subjective experiences in the past, present, and future. It is the human prefrontal cortex, defined as those aspects of the frontal lobes anterior to motor and premotor areas, which subserves the capacity for autonoetic awareness.

An extensive literature in clinical neuropsychology indicates that patients who have suffered injuries restricted to the prefrontal cortex often suffer from impoverished awareness of their own mental life. Though patients are fully awake, alert, and have full command of intelligence and language capabilities, they often appear either unwilling or unable to engage in self-reflection or introspection (Luria 1973). Reflection upon the personal past defines episodic memory. Multiple lines of research converge to show that the integrity of the prefrontal cortex is necessary for successful episodic recollection of past experiences. Although patients with large prefrontal lesions are not amnesic— they can learn from experiences and update their knowledge—recalled information does not have the subjective, personal quality that comprises episodic recollection. Similarly, as patients report about autobiographical experiences, they often discuss their own lives as if they are casual observers (Wheeler et al.1997). The prefrontal cortex confers humans with the ability to recollect prior experiences in a personal, subjective way. It follows that, when patients with prefrontal lesions attempt to recount their autobiography, it is the episodic component which is compromised, much more than the semantic component. Because of its reliance upon autonoetic awareness, episodic memory is related to other cognitive skills and behaviors that are not typically considered to be memory capabilities. Autonoetic awareness allows for self-reflection extending into both the past and future. Patients with large prefrontal lesions typically lack ambition, foresight, and initiative, and this brain area has been considered as important for ‘memories of the future’ (Ingvar 1985). As such, episodic memory shows a close family resemblance to some sophisticated mental achievements, like introspection, anticipation, and daydreaming, which require individuals to reflect upon personal experiences, past or present, that are not part of the immediate perceptual environment.

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