Episodic Memory: Definition and Theories
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- FRITZ Claudia
- KRENN Nora
- SCHALLEHN Anna
Episodic memory is defined as a memory for personal unique past experience. Experimental approaches with diverse species were done in order to test for the “what where and when” of some unique recent event. These experiments provide convergent evidence that processes similar to episodic memory occur in animals, which is the main focus of this essay.
The memory in general is divided into two parts – procedural and declarative memory. Declarative memory is further divided into semantic and episodic memory. Episodic memory is the latest kind of memory to develop and the first to degenerate with age. When it was first discussed by Tulving in 1972 he proposed a distinction between episodic and semantic memory, knowing versus remembering. Episodic memory was defined primarily as a memory for personal unique past experience that is what happened, where and when, also known as the WWW theory. According to Tulving “episodic memory receives and stores information about temporally dated episodes or events, and temporal-spatial relations among these events.” In contrast, semantic memory is the knowledge of general facts, a structured record of details, concepts and skills that we have acquired. It is used as an aid in placing episodic events in time. Semantic information is derived from accumulated episodic memory. Episodic memory can be thought as a map that ties together items in semantic memory. For example, memory for the experience of dinner at Luigi’s Restaurant last Saturday night is episodic whereas knowledge about what was involved in having dinner at a restaurant in general is semantic. Furthermore supported by evidence that some people with hippocampal damage have semantic but not episodic memory, the definition of episodic memory evolved to emphasize its conscious component, a feeling of re-experiencing the remembered event which is also known as autonoetic consciousness (Tulving 2002). It enables an individual to be aware of the self in a subjective time. It is further claimed to be part of a uniquely human faculty of “mental time travel,” the ability to mentally project oneself into the future as well as into the past. How does mental time travel - the reconstruction of past events work? Imagine the last presentation you gave? Do you remember the audience’s reaction? You might have snapshots of what the scene looked like. But this imagery does not tend to unfold in an orderly fashion from start to end. Extensive research on episodic memory clearly shows that we do not simply press the rewind button – we actively reconstruct past events from the gist or from visual snapshots, but draw heavily on our general semantic knowledge. We tend to reconstruct in ways that help justify our current attitudes. Let us get back to Tulving’s proposal. Do we agree on the aspect that humans are alone in their ability to recall about the past and imagine the future? Some argue that language is the key difference between humans and animals and that episodic memory can only emerge in an organism that possesses language. However, there have been efforts to demonstrate episodic memory in animals. In one attempt pigeons were taught a language in order to respond to an unexpected question. Furthermore, a fundamental question in comparative cognition is whether animals remember unique personal past experiences. It has long been argued that memories for specific events, known as episodic memory, are unique to humans. We know that animals from dolphins to monkeys can learn from single events. But do they revisit the events that shaped their past? Innovative work by Clayton and Dickson has produced perhaps the strongest case yet for the WWW memory in animals.
The animals’ memory is referred to as episodic-like because it satisfies the original definition of episodic memory for what, where and when but without any evidence of autonoetic consciousness. Clayton and Dickinson (1998) where the first ones who tried to demonstrate the discrimination of WWW in scrub jays (Aphelocoma coerulescens). The birds got to cache perishable peanuts on one side of a storage tray and non-perishable wax worms on the other side. Two caching phases were separated by 120 hours. The birds either cached peanuts followed by worms or the other way round. After four hours they were allowed to recover the stored items. Before the actual test phase, one group of the birds learned that worms decay over time, thus after the four hours gap they directed their first inspection to the worm side. After the 124 hours gap the worms had already perished and the birds turned to the peanut side first. To validate that the preference for the peanuts on the longer trial was not because the scrub jays simply forgot where the worms were, the group was compared to a second group. This group learned that the worms never decay as they where removed immediately after caching and replaced by fresh ones. As a result all birds of this group inspected the worm side of the tray in both the short and the long trial first. In conclusion the switch from worm side to peanut side by birds in the first group can only be explained by recalling what items were cached, where they were stored and when they were cached, which is evidence for episodic-like memory.
Not only scrub jays but also rats were used for experiments for testing the “WWW”-theory. This was done by Babb and Crystal (2005), using the circadian rhythm. Critics stated however, that this could not be seen as a proof for “when” since the rats’ biological rhythm is responsible for the “when”. As a consequence Babb and Crystal (2006) modified the experiment, controlling for the time of day. The rats however could still accomplish the trials. It is therefore called episodic-like memory. In an experiment by Roberts et al. (2008), rats were tested if their memory is based on when or how long ago a specific event occurred. The results clearly displayed that rats use elapsed time as a cue for when an event happened. In terms of the definition of episodic-like memory the conditions for “what-where-when” were fulfilled in these types of experiments. However in terms of the definition of episodic memory the evidence of autonoetic consciousness is missing. Subsequently one can argue that those experiments had long training phases and the animals learned therefore rules on how to behave in certain situations. This could be declared as semantic memory.
A different approach to test episodic-like memory in animals was done by Zentall et al. (2001). In this study, the aim was to avoid the possibility for the tested animals to solve the task because of a lot of experiences with the “rules” of the task and thus form semantic memory. This was done by asking them nonverbally unexpected questions. Therefore the eight tested pigeons were forced to travel back in their minds. To test the pigeons, Zentall et al. used a study design with multiple phases. At first, the pigeons had a language training where they learned to respond to the question “did you just peck?” They were shown one of two different symbols. At the symbol with vertical lines they would peck and at the one with horizontal lines they would refrain from pecking. The demonstrated symbol was followed by a red and a green light. After pecking the choice of the red light was reinforced. After refraining from pecking the choice of the green light was reinforced. Now the red light can be seen as the answer “yes” and the green light as the answer “no” to the question “Did you just peck?” In the next phase they learned to peck at one of a new pair of symbols without being asked about what they had just done. However in the test phase they were confronted for the first time with the red and green comparisons after the interaction with a new symbol. They were also tested with a symbol where they would spontaneously peck at and the absence of a symbol, so that they would not peck. And again they were asked whether they had just pecked. In both cases they chose the right answer in about seventy percent of the first four test trials, which is above chance. This study outcome speaks in favor of episodic-like memory in pigeons. Zentall et al. (2001) could demonstrate, that the pigeons were able to retrieve knowledge of recent experiences on unexpected request. Additionally, further research displayed, that the pigeons did not solve the task through proprioceptive cues (Singer and Zentall 2007, as cited in Crystal). However the pigeons had to remember only their actions for a few seconds, while humans are able to remember episodes for a long time or even a lifetime. So studies on what-where-when memory with integration of unexpected questions could lead to stronger evidence (Crystal 2010).
Experiments testing for episodic-like memory in animals were moreover done with Dolphins. Mercado et al. (1998, as cited by Zentall et al.) demonstrated that these animals were capable to remember their actions by asking them to perform a certain behavior they had done most recently or a performance they had not recently done. Studies with pigeons as well as with dolphins indicate therefore that processes similar to episodic memory occur in animals. Since the tests were done with diverse species, it might be that this form of memory represents a general capacity in animals.
All the experimental approaches discussed in our presentation involve testing for the what where and when of some unique recent event. As discussed, these experiments provide convergent evidence that processes similar to episodic memory occur in animals. This kind of memory is now known as “episodic like” memory rather than episodic memory because it satisfies the original definition of episodic memory defined by Tulving as a memory for what where and when of a unique experience but without any evidence of autonoetic consciousness. Additionally, the series of studies raised doubt about the claim, mental time travel being unique to humans. Therefore the variety of approaches that have developed might be taken as a welcome evidence of a search for convergent data but they also reflect the fact that no one approach to date has captured all aspects of human episodic memory in another species. Knowing now all the definitions and experimental approaches one could come up with the question “why is it important to have the ability of episodic memory?” The answer is that there is no selective advantage to reconstruct the past per se, unless it matters for the present or future. It is proposed that episodic reconstruction is just an adaptive design feature of the future planning system such as building a nest.
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