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What is episodic memory if it is a natural kind?

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Abstract

Colloquially, episodic memory is described as “the memory of personally experienced events”. Even though episodic memory has been studied in psychology and neuroscience for about six decades, there is still great uncertainty as to what episodic memory is. Here we ask how episodic memory should be characterized in order to be validated as a natural kind. We propose to conceive of episodic memory as a knowledge-like state that is identified with an experientially based mnemonic representation of an episode that allows for a mnemonic simulation thereof. We call our analysis the Sequence Analysis of Episodic Memory since episodes will be analyzed in terms of sequences of events. Our philosophical analysis of episodic memory is driven and supported by experimental results from psychology and neuroscience. We discuss selected experimental results that provide exemplary evidence for uniform causal mechanisms underlying the properties of episodic memory and argue that episodic memory is a natural kind. The argumentation proceeds along three cornerstones: First, psychological evidence suggests that a violation of any of the proposed conditions for episodic memory amounts to a deficiency of episodic memory and no form of memory or cognitive process but episodic memory fulfills them. Second, empirical results support a claim that the principal anatomical substrate of episodic memory is the hippocampus. Finally, we can pin down causal mechanisms onto neural activities in the hippocampus to explain the psychological states and processes constituting episodic memory.

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Notes

  1. In psychology, the adjective-noun combination “false memory” is often used to refer to a false mnemonic representation. The use of this phrase by psychologists is sometimes interpreted as if memory in psychology would not be regarded as factive. However, this conclusion would be justified only if “false” were an intersective adjective, for which the inference from “x is AN” to “x is N” is valid. The more plausible interpretation, we think, is that “false” in “false memory” is a privative adjective like “false” in “false money” or “fake” in “fake gun”. For privative adjectives, the inference is not valid: false money is not money, a fake gun is not a gun and, likewise, false memory is not a case of memory. Moreover, in psychological research and, even more so, in forensic situations the question of whether a memory report of the form “I remember that...” truly is a case of memory or rather a case of confabulation or error often arises and is naturally regarded as a sensible question. This question would be pointless if memory were not generally regarded as factive.

  2. Our definition presupposes some version of event atomism, i.e., the assumption that there are primitive events that do not further divide into other events. We, however, leave open on which ontological level those events occur (on a microphysical or rather on a neuro-cognitive level). We regard this issue as inessential to the following argumentation of the paper.

  3. There is experimental evidence that humans segment their experience into distinct episodes. For instance, Ezzyat and Davachi (2011) reported that cued retrieval is more successful if cue and target were perceived to have occurred within the same episode as opposed to distinct episodes.

  4. For a neurophilosophical account of events and their participating objects, see Werning (2003).

  5. For a discussion on reliable memory traces, see (Martin and Deutscher 1966).

  6. It is astounding that in some linguistic cases even the use of the noun “knowledge” does apparently not imply factivity. In the quite frequent phrase “his knowledge was outdated” the predicate “outdated” coerces an interpretation of “knowledge” as denoting a set of propositions that are not true given the current state of evidence.

  7. In psychology, the terms “false episodic memory” or “false memory” are popular (Marsh et al. 2008). We use the broader term “improper episodic memory” to indicate that a mnemonic representation fails to be proper episodic memory if one or more of the conditions (S1)–(S7) is violated, even in cases where the content of the mnemonic representation is veridical. In other words, false episodic memories are improper episodic memories, but the reverse is not true.

  8. A major methodological hurdle in studying the potential dissociation between semantic and episodic memory is that the retrieval might always involve both types of memory (e.g., McCabe et al. 2011). However, irrespective of whether an experimental paradigm can be developed to dissociate the retrieval process behaviorally, there could be a conceptual and neural difference between the two types of memory.

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Acknowledgments

We thank Thomas Suddendorf for helpful discussions and Kevin Reuter for comments on the manuscript. This work was supported by a Grant (SFB 874, project B2) from the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) and a Grant from the Stiftung Mercator.

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Appendix

Appendix

In Sect. 3.4 we distinguish between an actual and temporally enduring mnemonic representation and a possible and only instantaneous temporally explicit mnemonic simulation. The mnemonic representation is a compositional representation in which the temporal succession of the events in the episode is encoded by some structure among the representational constituents. This can be formally accounted for in the following way:

  1. (R1)

    Let \({\mathbb {R}}=\left( {\hbox {R}, \{\hbox {s}\}} \right) \) be a representational structure, where \(\hbox {R}\) is the set of representational states and \(\hbox {s}:\hbox {R}\times \hbox {R}\rightarrow \hbox {R}\) a thereon defined binary and recursive structure building operation; if \(\hbox {r}^{{\prime }{\prime }}=\hbox {s}\left( {\hbox {r}, \hbox {r}^{\prime }}\right) \), \(\hbox {r}\) and \(\hbox {r}^{\prime }\) are called the representational constituents of \(\hbox {r}^{{\prime }{\prime }}\).

  2. (R2)

    Let \({\mathbb {V}}=\left( {\hbox {V}, \{\langle \cdot ,\cdot \rangle \}} \right) \) be an event structure where \(\hbox {V}\) is the set of events (including episodes) and \(\langle \cdot ,\cdot \rangle \) the previously defined binary and recursive episode building operation \(\langle \cdot ,\cdot \rangle :\hbox {V}\times \hbox {V}\rightarrow \hbox {V}\).

  3. (R3)

    Let \(\upmu :\hbox {R}\rightarrow \hbox {V}\) be a mapping from the set of representational states into the set of events. \(\upmu (\hbox {r})=\hbox {e}\) means that \(\hbox {e}\) is the event or episode represented by the primitive or complex representational state \(\hbox {r}\).

  4. (R4)

    Then the mapping \(\upmu \) is a homomorphism from the representational structure \({\mathbb {R}}\) into the event structure \({\mathbb {V}}\). That is, for every \(\hbox {r}, \hbox {r}^{\prime }\in \hbox {R},\,\upmu \left( {\hbox {s}\left( {\hbox {r}, \hbox {r}^{\prime }}\right) }\right) =\langle \upmu ( \hbox {r} ),~\upmu (\hbox {r}^{\prime })\rangle \).

Statement (R4) is equivalent to the claim that the representational structure is compositional: The content \(\upmu \) of a complex representational state \(\hbox {s}\left( {\hbox {r}, \hbox {r}^{\prime }}\right) \) is a structure-dependent function, namely \(\langle \cdot ,\cdot \rangle \), of the contents of its representational constituents \(\hbox {r}\) and \(\hbox {r}^{\prime }\), i.e., \(\upmu \left( \hbox {r}\right) \) and \(\upmu \left( {\hbox {r}^{\prime }}\right) \).

In a temporally explicit mnemonic simulation the temporal succession of events in the object domain is represented itself by a temporal succession of events in the representational domain. We can formally account for this in the following way:

  1. (Q1)

    Let \({\mathbb {V}}=\left( {\hbox {V}, \{\langle \cdot ,\cdot \rangle \}} \right) \) again be our event structure, where \(\hbox {V}\) is the set of events and \(\langle \cdot ,\cdot \rangle \) the episode building operation.

  2. (Q2)

    Let \({\mathbb {Q}}=\left( {\hbox {Q}, \{\langle \cdot ,\cdot \rangle \}} \right) \) be the simulational structure, which itself contains a set of events \(\hbox {Q}\), i.e., \(\hbox {Q}\subseteq \hbox {V}\), together with the operation \(\langle \cdot ,\cdot \rangle \), which builds temporally succeeding sequences of events. \({\mathbb {Q}}\) hence is a substructure of \({\mathbb {V}}\).

  3. (Q3)

    Let the mapping \(\upnu : \hbox {Q} \rightarrow \hbox {V}\) assign each primitive or complex simulational state the primitive or complex event it, by simulation, represents.

  4. (Q4)

    Then the mapping \(\upnu \) satisfies the following condition: For every \(\hbox {q},\hbox {q}^{\prime }\in \hbox {Q}, \upnu \left( \langle {\hbox {q}, \hbox {q}^{\prime }}\rangle \right) = \langle \upnu \left( \hbox {q} \right) , \upnu (\hbox {q}^{\prime })\rangle \).

In this account, the representational content of a temporal succession of simulational states is just the episode comprising the temporally succeeding representational contents of each simulational state. Mathematically speaking, \(\upnu \), unlike \(\upmu \), is not only a homomorphism but an endomorphism.

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Cheng, S., Werning, M. What is episodic memory if it is a natural kind?. Synthese 193, 1345–1385 (2016). https://doi.org/10.1007/s11229-014-0628-6

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