Abstract
Changes in synaptic strength are described as a unifying hypothesis for memory formation and storage, leading philosophers to consider the ‘synaptic efficacy hypothesis’ as a paradigmatic explanation in neuroscience. Craver’s mosaic view has been influential in understanding synaptic efficacy by presenting long-term potentiation as a multi-level mechanism nested within a multi-level structure. This paper argues that the mosaic view fails to fully capture the explanatory power of the synaptic efficacy hypothesis due to assumptions about multi-level mechanisms. I present an alternative approach that emphasizes the explanatory function of unification, accounting for the widespread consensus in neuroscience regarding synaptic efficacy by highlighting the stability of synaptic causal variables across different multi-level mechanisms.
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Notes
Argument patterns consist of schematic sentences (where nonlogical terms are replaced by variables), filling instructions (which are rules for substituting these variables with specific values), and classifications (which define which sentences serve as premises and which ones act as conclusions). For example, consider the sentence ‘Organisms homozygous for the sickling allele develop sickle cell anemia.‘ This can be restructured as ‘Organisms homozygous for A develop P,‘ where ‘A’ and ‘P’ are variables. The filling instructions, in this case, would direct us to replace ‘A’ with a specific allele’s name and ‘P’ with a phenotypic trait.
I will use the terms interchangeably.
Woodward (2021, p. 264, ft. 41) acknowledges the tight connection between invariance and unification but dismisses it due to the standard characterization of unification being considered non-causal. In contrast, the perspective on unification presented here, which treats it as a norm of causal explanation, effectively addresses Woodward’s concern.
Invariance offers a means of interpreting what minimizing the ratio of explanantia to explananda implies within the context of causal explanations, specifically those within the interventionist framework. Notably, unification need not be conceived in the same terms for non-causal explanations or under a non-interventionist understanding of causal relations.
What the subjects of these experiments are supposed to remember, and subsequently forget is not entirely independent from the mechanisms in which synaptic efficacy is nested. Synaptic efficacy determines whether memories are formed, it does not specify which memories are formed.
This impression might be conceived as overriding considerations of generality. The following discussion questions whether the diversity of synaptic mechanisms indeed hinders the feasibility of a generalized mechanism. However, my focus in this section is on identifying similarities through abstraction. It is crucial not to conflate abstraction with scope. A single mechanism can have a narrow scope yet remain highly abstract, or the other way around.
In this regard, my approach is similar to approaches that emphasize the role of kinding in hypothesis formation (Colaço 2022). However, one difference is that my approach focuses more on how existing evidence plays a role in mechanism characterization rather than concentrating on how kinding guides and justifies investigative and classificatory practices. In this sense, compared to other approaches, my approach is more backward-looking than forward-looking.
A potential worry is that the mechanistic model as described here is possible in principle but may not be an adequate depiction of scientific practice. According to Craver and Kaplan (2020), it is often “unwieldy to create a single ‘uber-model’” (p.309). However, attempts to create a general model that conjoins multiple models of synaptic efficacy by appealing to STDP are part of a live research program. On this approach, STDP is a multifactorial rule in which the magnitude of LTP and LTD depends jointly on spike timing, presynaptic firing rate, postsynaptic voltage, and synaptic cooperativity (Feldman 2020).
Sullivan’s position is too complex to be fully captured here. Sullivan (2009, 2016) argues that the diverse LTP-inducing stimulation protocols lead to differences in the cellular and molecular mechanisms responsible for LTP, making it unclear whether different labs are examining the same phenomenon or distinct ones. This diversity, according to Sullivan, indicates that the sciences of memory are not geared toward coordination. Consequently, it is unlikely that a single type of LTP will be discovered, given the nature of their investigative projects. Nonetheless, Sullivan acknowledges that unification is possible, at least in principle.
Whether these deeper patterns say something about the causal structure of the world rather than the epistemic characterization of these mechanisms is an open question. Craver does not deny that there are abstract patterns in the causal structure of the world. Nevertheless, this view faces various issues in accommodating the relation between token and type mechanisms (See, e.g., Sheredos 2016). I return to this point in Sect. 5.
Craver’s mutual manipulability account is partially responsible for this expectation, but mechanists are not necessarily committed to this account, and several alternatives have been proposed (e.g., Serban & Holm, 2019). Alternative views propose that constitutive relevance relations are, at most, abductive since they can be sensitive to interventions that may falsify the relation, like the connection between hippocampal (or striatal) level and synaptic level. Consequently, higher-level constraints do not necessarily require splitting synaptic mechanisms. This is consistent with the view of neuroscientists who consider synaptic efficacy as the primary, but not the sole, causal mechanism explaining memory formation.
This normative conception is compatible with the mechanistic approach to explanation. Recognizing that some mechanistic models may be more explanatorily powerful does not diminish the importance of mechanistic explanatory strategies, such as local decomposition. The synaptic efficacy hypothesis does not represent a global model for all explanatory practices concerning memory formation. Moreover, the primacy of synaptic efficacy does not imply a return to reductionism. As I have emphasized throughout, memory phenomena cannot be explained exclusively by synaptic efficacy without considering other neural systems.
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Najenson, J. LTP Revisited: Reconsidering the Explanatory Power of Synaptic Efficacy. Rev.Phil.Psych. (2023). https://doi.org/10.1007/s13164-023-00694-w
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DOI: https://doi.org/10.1007/s13164-023-00694-w