|Endel Tulving and Karl K. Szpunar (2009), Scholarpedia, 4(8):3332.||doi:10.4249/scholarpedia.3332||revision #91236 [link to/cite this article]|
Episodic memory is the name given to the capacity to consciously remember personally experienced events and situations. It is one of the major mental (cognitive) capacities enabled by the brain.
In the prototypical act of exercising the capacity of episodic memory one may remember a recent trip to Paris, mentally reliving events that happened there, in the mind’s eye seeing again the places visited, sights seen, sounds heard, aromas smelled, and people met.
Memory is an umbrella term that covers a variety of different forms of acquisition, retention, and use of habits, skills, knowledge, and experience. Those who study memory have found it useful to assume that different forms of learning and memory are subserved by different memory systems--organized collections of neurocognitive components that work together to perform functions that other collections of components cannot perform, or cannot perform as well. An important objective of research has to do with the identification of these memory systems, specification of their properties, and delineation of the nature of the relations among them.
Historically, the most basic distinction is that between procedural memory (an action system that is expressed through behavior; e.g., when riding a bicycle) and declarative memory (a cognitive system that is expressed through propositional knowledge; e.g., when taking a classroom test). Both procedural and declarative memory are seen as consisting of a number of subdivisions (Eichenbaum & Cohen, 2001; Schacter & Tulving, 1994; Schacter, Wagner, & Buckner, 2000; Squire, 1992; Squire & Kandel, 1999; Squire & Zola, 1998). This article describes a theory of episodic memory, one of the two assumed subdivisions of declarative memory. However, because the theory of episodic memory can be only incompletely understood in isolation of the other assumed subdivision of declarative memory, semantic memory--the system that enables us to acquire and retain factual knowledge about the world (e.g., knowing that Paris is a nice city to visit in the springtime) and from which episodic memory is thought to have evolved, much of the discussion will focus on episodic memory in relation to semantic memory.
In this article, the term ‘episodic memory’ refers to a unique memory system (or capacity) of the brain. However, that is not the only meaning of episodic memory that one will find in the literature. For instance, the term is often used to describe the specific experience (content) that comes to mind when exercising the capacity of episodic memory and the accompanying feeling (phenomenology) that one is currently reliving that previous experience. In the interest of clarity, this article will refer to the contents of episodic memory as ‘remembered experiences’ and the phenomenological experience as ‘remembering.’ A similar issue exists in relation to the concept of semantic memory. Presently, the term ‘semantic memory’ also stands for a capacity of the brain. The structured contents of the semantic memory system are referred to as ‘knowledge’ and the phenomenological experience as ‘knowing’ (Gardiner & Richardson-Klavehn, 2000).
Relations between episodic and semantic memory
According to the theory of episodic memory, the assumed evolutionary sequence of episodic memory growing out of semantic memory is reflected in the global, monohierarchical relation between the two. That is, episodic memory shares with semantic memory many features that distinguish both of them (i.e., all of declarative memory) from other major subdivisions of memory, yet it also possesses features that it does not share with any other memory system, including semantic memory (Mishkin, Suzuki, Gadian, & Vargha-Khadem, 1997; Tulving, 1995). The monohierarchical relation also implies that episodic memory depends on semantic memory in its operations and cannot function without relevant components of semantic memory, whereas semantic memory does not depend on episodic memory in its operations and can function without episodic memory. This kind of a relation between the two memory systems mimics many other similar relations in the living world. As a single example, consider the relation between a visual system that has no sense of color and a visual system that does: The latter has everything that the former has, plus more.
What makes episodic memory special is that it makes possible mental time travel into the past, as well as into the future, as will be seen below. No other memory system has the same capacity, at least not in the sense that episodic memory does.
Some of the features (or “properties”) that episodic memory shares with semantic memory are:
- Both systems allow the organism to know about aspects of its world that are not immediately present.
- Encoding of new information [converting perceptual and cognitive input into ‘memory traces’ (engrams)] is fast and may occur on a single trial.
- Encoded information (memory traces) may be multimodal (polymodal).
- Storage of encoded information is transmodal: both remembered experiences and knowledge can be stored independent of the modality through which they were acquired.
- Storage of information is highly structured.
- Storage of information is highly sensitive to context.
- Stored information is representational (isomorphic) with what is or could be in the world.
- Access to stored information during retrieval is flexible, within limits.
- Behavioral expression of what is retrieved is optional and not obligatory. Thus, it is possible to hold the retrieved information online, and just contemplate it.
- Retrieval of information in both systems requires consciousness. It is not possible to directly retrieve information from either episodic or semantic memory nonconsciously. Of course, various processes that underlie the retrieval of remembered experiences and knowledge may take place beyond conscious awareness.
- The operations of neither system depend on language, although language may greatly facilitate them.
- The shared features of both systems are present in a wide range of animals; they are highly evolved in mammals and birds.
- The operations of both systems are subserved by shared, widely distributed, cerebral cortical and subcortical neural networks; especially critical are those in medial temporal lobes and the diencephalon.
Any one of these properties applies equally well to both episodic and semantic memory. It is their conjunction that allows us sometimes to classify both episodic and semantic memory together under the general label of declarative (also referred to as cognitive or explicit) memory, without further differentiating between them. In many situations, both in the laboratory and real life, such generalization is justifiable. In others, however, it is not, because episodic memory, in addition to the properties listed above, also possesses unique properties that are shared by neither semantic nor any other memory system.
Unique features of episodic memory
Some of the features (or “properties”) that are unique to episodic memory are:
- The key function of episodic memory is to allow the individual to remember personal past happenings as such; semantic memory is not capable of this function.
- This remembering takes the form of mentally “traveling” in subjectively experienced time. Semantic memory does not have anything special to do with time, other than the (trivial) fact that the knowledge that is brought to mind in the course of exercising this capacity was once learned in the past.
- Episodic memory, unlike semantic memory, is self-centered. The operations of episodic memory are predicated on one's conscious awareness of oneself as an independent entity that is separate from the rest of the world. In the absence of such awareness, episodic remembering is not possible.
- Episodic remembering expresses itself phenomenally through the medium of a distinctive form of conscious awareness that is familiar to all people in the sense that they know when they are remembering and not perceiving, or imagining, or daydreaming, or having any other kind of conscious experience. The conscious awareness accompanying semantic knowing has a different flavor, clearly distinct from that of remembering. The two kinds of consciousness involved in episodic remembering and semantic knowing have been named ‘autonoetic’ and ‘noetic,’ respectively (Tulving, 1985; Wheeler, Stuss, & Tulving, 1997).
- Episodic remembering requires the activation, by way of voluntary or involuntary processes, of a special kind of mental state that has been called ‘episodic retrieval mode.’ The operational default memory state is semantic, characterized by noetic consciousness.
- The ontogenetic development of episodic memory is delayed in relation to that of semantic memory: Children acquire a great deal of knowledge about the world they live in before they are aware of their own past personal experiences (Nelson & Fivush, 2004).
- Episodic memory tends to be more vulnerable to disease, injury, and the ravages of old age than is semantic memory. Brain damage is more likely to impair episodic remembering than semantic knowing, and in dementias such as Alzheimer's disease the impairment of episodic memory is frequently the first symptom to appear (Kitchener, Hodges, & McCarthy, 1998; Klein, Loftus, & Kihlstrom, 2002; Rosenbaum et al., 2005; Vargha-Khadem et al., 1997).
- Episodic memory that all healthy humans possess probably does not exist in other animals, although ‘episodic-like’ memory capacities have already been identified in several species (Clayton, Bussey, & Dickinson, 2003; Griffiths, Dickinson, & Clayton, 1999; Olton, 1984).
- Episodic memory is dependent on neural networks that extend beyond those that subserve the operations of semantic memory (Aggleton & Pearce, 2001; Nyberg et al., 2000).
Some of these features are more reasonable in light of empirical evidence than others. The whole enterprise of studying multiple memory systems is in an early stage. Therefore a great deal of further work and thought is required, and indeed is being spent, on many aspects of the problem (Dere, Easton, Nadel, & Huston, 2008; Szpunar & McDermott, 2008; Tulving, 2002a).
Today, a thumbnail description of episodic memory can be defined in terms of these unique features, against the backdrop of the shared features.
Episodic memory: 2009
Episodic memory is a recently evolved, late developing, and early deteriorating brain/mind (neurocognitive) memory system. It is oriented to the past, more vulnerable than other memory systems to neuronal dysfunction, and probably unique to humans. It makes possible mental time travel through subjective time--past, present, and future. This mental time travel allows one, as an “owner” of episodic memory (“self”), through the medium of autonoetic awareness, to remember one's own previous “thought about” experiences, as well as to “think about” one's own possible future experiences. The operations of episodic memory require, but go beyond, the semantic memory system. Retrieving information from episodic memory (“remembering”) requires the activation, by way of voluntary or involuntary processes, of a special mental set, dubbed episodic “retrieval mode.” The neural components of episodic memory comprise a widely distributed network of cerebral cortical and subcortical brain regions that overlap with and extend beyond the networks subserving other memory systems. The essence of episodic memory lies in the conjunction of three concepts--self, autonoetic awareness, and subjective time.
The remainder of this article summarizes some of the open issues concerning episodic memory, issues that remain in the focus of investigators.
Episodic memory is closely related to autobiographical memory. That term, however, most often is used in the sense of significant life experiences, either remembered or known. That is, people typically include facts such as when and where they were born as an important part of their life story although these are necessarily acquired through the semantic rather than episodic system. Episodic memory, on the other hand, has to do with remembered experiences alone (with regard to one's past). The distinction between episodic and semantic autobiographical memory receives support from relevant neuroimaging studies (Levine et al., 2004; Svoboda, McKinnon, & Levine, 2006).
Episodic memory in nonhuman animals
There exists essentially universal agreement among all practitioners of the science of memory that many species other than humans possess highly developed semantic memory systems. These allow them to acquire complex and intricate knowledge of various sorts about their own ecological niches (as well as other aspects) of the world.
Whether or not species other than humans possess episodic memory depends on how episodic memory is defined. If the definition is given along the lines of the properties of episodic memory that are “shared” with semantic memory, then the answer to the question is definitely positive. However, in terms of the extended definition, including both the “shared” and “unique” lists, the answer is negative--other species probably do not possess the kind of episodic memory that humans do. At least, we are not aware of any findings that unequivocally attribute autonoetic consciousness to nonhuman animals (Suddendorf & Corballis, 1997, 2007).
When it comes to humans, the question as to the survival value of episodic memory arises. Here it is possible to think of plausible evolutionary drivers that may have played a role in initiating and maintaining the ability to autonoetically reflect on what has happened in the past. One such thought begins with the assumption that subjectively apprehended time, a key feature of evolved episodic memory, extends not only backwards to the past, but also forwards to the future. One’s ability to imagine what the future might bring allows one to prepare for the various eventualities in a way that a more restricted projection of the past would not. Most specifically, if one can anticipate possible untoward happenings at a time that has not yet arrived, one can take measures to ward them off now, in the present (Tulving, 2002b, 2005).
The power of ‘episodic future thinking,’ as the ability has been dubbed, is most clearly visible in the kinds of cultures and civilizations human beings have created. Many aspects of these would not be possible in the absence of the ability to mentally envisage the future (Suddendorf & Corballis, 1997). Indeed, evidence continues to accumulate suggesting that episodic memory and episodic future thinking share similar neural correlates, that both are similarly impaired in specific cases of brain damage, and that both share similar ontogenetic trajectories and subsequent decline in aging (Atance & O’Neill, 2001; Schacter & Addis, 2007; Szpunar, in press).
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