What are memories made of? A survey of neuroscientists

• Ariel Zeleznikow-Johnston, 
• Emil F. Kendziorra, 
• Andrew T. McKenzie

x

Abstract

Citation: Zeleznikow-Johnston A, Kendziorra EF, McKenzie AT (2025) What are memories made of? A survey of neuroscientists on the structural basis of long-term memory. PLoS One 20(6): e0326920. https://doi.org/10.1371/journal.pone.0326920

Editor: Stéphane Charpier, Sorbonne Universite UFR de Biologie, FRANCE

Received: February 26, 2025; Accepted: June 6, 2025; Published: June 24, 2025

Copyright: © 2025 Zeleznikow-Johnston et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: A list of the survey questions is available here: https://osf.io/agkrn The full set of participant response data is available here: https://osf.io/bas2u

Funding: Research funding for this study was supported by a CryoDAO grant (2024.1) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: I have read the journal's policy and the authors of this manuscript have the following competing interests: Andrew McKenzie is an employee of Oregon Brain Preservation, a non-profit brain preservation organization, and a director of Apex Neuroscience, a non-profit research organization. Emil Kendziorra is a shareholder and CEO of Tomorrow Biostasis, a biostasis provider, President of the Board of the European Biostasis Foundation, a non-profit research foundation, a shareholder and director at Oxford Cryotechnology, Inc., a cryopreservation research organization, and a board member at CryoDAO, a Swiss research association. This does not alter our adherence to PLOS ONE policies on sharing data and materials.

Abbreviations: ASC, aldehyde-stabilized cryopreservation; COSYNE, computational and systems neuroscience; LTM, long term memory

Introduction

Through what physical means do brains retain information for use weeks, months, or years later? Recall of long-term memories – be it recoiling from a distinct taste after a food-induced illness six months prior, or singing the lyrics of a song one hasn’t heard in years – enable an animal’s behaviors to be shaped by a lifetime’s worth of accumulated experience. For this to be possible, earlier experiences must create some kind of ‘memory trace’ in an animal’s brain, meaning there must be some enduring physical record that bridges the gap between an initial experience and its subsequent recall [1]. But among the many potential mechanisms for information storage in the brain, which are actually responsible for long-term memory retention?

There is evidence that robust, long-lived memories are likely retained by relatively stable and static aspects of neurophysiology. Long-term memories can be recalled even after periods of prolonged global neuronal depolarization and inactivity, for example as can occur in deep hypothermic circulatory arrest [2,3]. This suggests that ongoing global electrophysiological activity is not required for the retention of already formed long-term memories. Additionally, while a temporary period of protein synthesis inhibition disrupts memory formation when administered during or shortly after training, it has been found to have no effect on memory recall when administered six or more days after training [4]. This temporal distinction in the mechanisms between memory formation and recall suggests that long-term memory storage depends on stable structural changes rather than ongoing protein synthesis or dynamic neural activity, unlike working and short-term memory.

Over the past century, neuroscientists have suggested a variety of structural candidates for the physical basis of long-term memory. By ‘structural,’ we refer to relatively stable physical changes to neural components that persist independently of ongoing neural activity or metabolic processes, though these changes may initially require activity-dependent processes to be established. These structural changes exist on a spectrum from modifications of individual proteins and channel properties through to larger-scale alterations in neural connectivity.

A non-exhaustive list of structural candidates includes: synaptic strength alterations mediated by receptor insertion, removal, or subunit modification [5]; synaptogenesis [6]; intracellular phosphorylation, epigenetic, or other molecular modifications [7]; alterations to neuronal excitability through widespread modifications of voltage- or calcium-dependent ion channels, i.e., intrinsic plasticity [8,9]; changes in axonal myelination [10]; modifications of perineuronal nets and the extracellular matrix [11]; and many others. These candidates are not necessarily mutually exclusive, as long-term memory may well depend on a variety of these mechanisms and their interactions. Additionally, these mechanisms may be redundant, with multiple parallel processes capable of maintaining the same memory information even if some mechanisms become temporarily disrupted or degraded.

Breakthroughs in the past decade have seen the search for the neurophysiological substrates responsible for long-term memories, termed ‘engrams’, pursued with increased vigor. These advances were kicked off by the seminal 2012 publication of a method for artificially forcing the recall of a specific memory through tagging and optogenetically stimulating a subset of hippocampal neurons [12]. Subsequent experiments have since demonstrated selective memory erasure [13], artificial memory formation [14], and artificial memory linkage [15], among other impressive feats. Given that ensembles of hippocampal and cortical neurons have arguably shown to be both necessary (erasure experiments) and sufficient (forced recall and insertion experiments) for memory-related behaviors, it could be argued that the physical basis of long-term memories has now been established. Indeed, in the words of many of the scientists who have furthered engram research in the past decade, “There is a clear consensus on where the memory engram is stored—specific assemblies of synapses activated or formed during memory acquisition” [16].

But is it really true that there is now a consensus that long-term memories are stored in synaptic ensembles? For example, consider how dendritic spines on mature hippocampal neurons disappear when exposed to cold temperatures, yet reappear in the same locations upon rewarming [17]. This finding shows that temporary disruptions to fine neural architecture can occur during certain perturbations (e.g., hypothermia) without necessarily destroying long-term memories (as demonstrated in humans who have undergone deep hypothermic circulatory arrest). For some, this may cast doubt upon any claim that dendritic spine positions are the exclusive substrate for long-term memory maintenance.

Regardless, perhaps there is a common understanding among neuroscientists as to which aspects of neurophysiology are critical for long-term memory storage, and which are irrelevant. Some properties—perhaps the specific polymer subunit count of synaptic microtubules, or the exact positions of cholesterol molecules in the neuronal membrane—would presumably be deemed below the physical scale relevant for encoding memories. At the other extreme, some macroscopic neurophysiological properties, like total hippocampal volume, would likely be considered mere epiphenomena of the mechanisms actually responsible for memory storage. If a consensus truly exists, then neuroscientists must have some agreement that somewhere between these lower and upper bounds there lies a ‘critical scale’, wherein there exists the properties or mechanisms responsible for long-term memory.

To determine whether or not neuroscientists agree on where this critical scale lies, we performed a survey of both neuroscientists generally and neurophysiology-of-memory experts specifically. Expert surveys in other disciplines, such as philosophy [18,19] and artificial intelligence [20], have proven instrumental in ascertaining what is and is not consensus within an academic community. While there exists a survey of the general US population on their beliefs about how memory works [21], and there have been surveys of memory scientists on the psychological properties of memories [22], we are unaware of any previous surveys of the opinions of experts as to the mechanics of long-term memory storage.

This survey was divided into three major components. The first queried participants about whether they believed long-term memories were stored in structural aspects of neurophysiology, and if so, through which physical mechanisms and at which critical scale(s). The second explored what the implications would be of memories being stored structurally, by asking them for their probability estimates as to whether information contained in long-term memories could theoretically be extracted from static brains with ideal morphological preservation. The final section examined a theoretical test for whether long-term memories are stored structurally, by probing participants on whether readout and reinstantiation of these memory-storing neurophysiological structures in another physical form, such as through whole brain emulation [23], could someday provide proof that long-term memories are indeed stored structurally and at a particular critical scale(s).

For questions about the implications of static brain preservation for memory storage, we used aldehyde-stabilized cryopreservation (ASC) of a laboratory animal as a practical example of a preservation method that is thought to maintain ultrastructure with minimal distortions across the entire brain [24]. Additionally, we asked participants to imagine it was performed under ideal conditions and was technically successful, deliberately discarding the fact that procedural variation or errors in the real world may prevent this ideal from being routinely realised in practice [25]. Rather than focusing on these technical preservation challenges, which we acknowledge are immense, we deliberately asked participants to consider memory extraction under optimal preservation conditions to assess their beliefs about the structural basis of memory storage itself. With this approach, our aim was to specifically target participants’ views on whether static brain structures – i.e., non-dynamic physical aspects of the brain that persist independent of ongoing neural activity – may on their own contain sufficient information for memory retrieval, which is the central theoretical question underlying our study.

Methods

The survey was distributed from August-October 2024 to two separate cohorts of neuroscientists: (1) those who had published research papers directly related to the neurophysiology of memory (Engram Experts); and (2) any attendees with an abstract listed in the Computational and Systems Neuroscience (COSYNE) conference booklets from 2022−2024 (COSYNE Neuroscientists). COSYNE attendees are self-described as those interested in “the exchange of empirical and theoretical approaches to problems in systems neuroscience” [26]. The questions were focused on their beliefs about the physical basis of memory, as well as the implications of these beliefs in various theoretically plausible scenarios. The survey and its implementation were reviewed by the Pearl Institutional Review Board and received an exemption determination (#2024-0303).

Results

Discussion

This study provides the first formal survey of the neuroscience community’s beliefs on the physical basis of long-term memory. We found that the majority of participants endorse the claim that “the structural basis of long-term memories primarily consists of lasting changes in neuronal connectivity and ensembles of synaptic strengths, rather than in molecular or subcellular details”. However, there was no clear consensus on exactly which neurophysiological feature or scale is critical for memory storage, with only features at or below the level of ‘atomic-level features of biomolecules’ being declared ‘below the critical scale’ by a majority of participants. Correspondingly, there were a broad range of estimates for the probability of whether long-term memories could be decoded from brain structure alone. While the median participant estimate of whether memories could be extracted from a preserved brain was 40%, there was considerable variance and some evidence for a bimodal distribution of results. Similar results were seen for the question of whether a whole-brain emulation could theoretically be created from a preserved brain, with participants estimating even odds it will be possible to create these for a C. elegans by 2045, a mouse by 2065, and a human by 2125. No notable effects of participant background or expertise were observed, except for a small negative correlation between self-reported expertise in ‘neuroscience of memory storage’ and estimates of whether a whole-brain emulation could possibly be created from a preserved brain without prior in vivo recordings.

We did not find a unanimous view on which particular neurophysiological features are critical for memory readout. Participants did indicate some boundaries, with a majority stating that the atomic-level composition of biomolecules can be ignored, while subcellular structures at ~500 nm resolution would definitely need to be ascertained. Between these though, no clear finding emerged for which particular features between these two scales are crucial for memory storage.

This is perhaps unsurprising, as despite the progress made by neuroscientists in manipulating memories over the past decade, researchers have noted that the theoretical framework for precisely identifying and understanding engrams is still absent [29]. Demonstrations that long-term memories can survive periods of electrocerebral silence [2], or can be manipulated through optogenetic activation of particular neurons and erasure of particular synapses [12,13,30], provides strong evidence that memory is dependent on structural features of neurophysiology. Nonetheless, when a mouse recalls the route through a maze, it is still unclear if this representation ultimately depends on the specific densities of receptors on particular dendritic spines in particular circuits, the coarse-grained connectivity strength of several populations of neurons, intrinsic plasticity manifested through persistent modifications of voltage-gated ion channels that alter neuronal excitability [8,9], or some combination of these or other neurophysiological features working in parallel. Consider how in genetics, it is understood how DNA triplets (codons) encode protein sequences through the processes of transcription and translation. While memory neuroscientists agree that synapses are crucial, there is no agreement on which features (if any) are equivalent to a codon.

Despite an inability to identify which neurophysiological features are critical for long-term memory storage, a substantial proportion of participants indicated that memories could probably be decoded from the static structure of a preserved brain. This held even to the extent that the median estimate of whether ‘a brain preserved by aldehyde-stabilized cryopreservation could potentially be used to create a whole-brain emulation capable of reproducing the long-term memory behavior of an animal’ was 40%. This optimistic view on what information is retrievable from structure alone may have been influenced by the recent demonstration that a whole-brain model of Drosophila, reconstructed from preserved brain slices imaged using electron microscopy, partially reproduced the animal’s natural feeding behavior [31]. While this was merely an observation of instinctual behavior, rather than individual-specific long-term memory, it is possibly suggestive of why only 10.3% of survey respondents indicated there was a less than 10% probability of a human whole-brain emulation ever being achievable. Correspondingly, as the feasibility of creating more sophisticated models of animal brains increases to the point of being able to potentially create a faithful whole brain emulation, it should become easier to test whether an emulation can indeed recreate an animal’s long-term memories. If this eventually proves possible, manipulations of the emulation should further clarify which neurophysiological features are actually critical for memory.

Although questions about ‘memory extraction from preserved brains’ and ‘whole brain emulation’ are admittedly strange and speculative, we believe they provide a practical framework for understanding memory’s physical basis. These concepts create a direct link between theoretical views and their logical consequences. If memories are indeed encoded in stable structural elements like synaptic connectivity patterns - as 70.5% of our respondents indicated - then such memories should theoretically remain accessible in appropriately preserved neural tissue. Alternatively, if memories depend primarily on dynamic processes that cease at preservation, then extraction would be impossible. This approach is analogous to asking electrical engineers about data recovery from powered-down hard drives to understand their beliefs about how data is physically stored. We acknowledge, however, that while these questions provide valuable insights into neuroscientists’ views on memory storage mechanisms, they alone do not establish a scientific consensus on the precise physical basis of memory, as evidenced by the diversity of responses we received.

We note that should it become possible to decode memories from a preserved brain, let alone create a whole brain emulation, this would have profound moral, philosophical, and societal consequences. As this survey focused solely on establishing the potential feasibility of this possibility, we asked participants to disregard these considerations when providing their responses. However, the substantial number of neuroscientists who indicated that memories could likely be extracted from preserved brains suggests that examining the ethical implications of this technology should not be dismissed. For example, these include the potential for life extension through means of whole brain emulation [32,33], or privacy violations that could occur through inappropriate memory extraction from a person’s preserved brain [34]. Given the median participant response indicated a 10% probability of the first human brain emulation occurring around 2075, and 50% by 2125, these prospects and pitfalls need to be understood in the decades before these capabilities might become available.

There are several limitations with both the data collection and analysis of this study that warrant consideration. First, as we received responses from only a subsample of our target populations (Engram Experts: 12.1%, COSYNE Neuroscientists: 7.3%), our findings may be subject to response bias. However, this possibility is mitigated by the fact that our invitation email only revealed this was a survey on the neuroscience of memory (i.e., it did not provide any detail of the specific questions), as well as the high completion rate (~75%) of those respondents who began the survey.

Second, while we restricted the survey to questions about long term memory, we did not ask participants to provide separate responses for the various subtypes of long term memory. This precludes the possibility of determining whether participants have different views about the physical nature of engrams underlying episodic memory from those that form the substrate of procedural memories.

Third, we note that the COSYNE Neuroscientists comprise only one subsection of the neuroscientific community with a particular focus on computational and systems neuroscience, and that ideally this survey would be replicated with participants drawn from additional neuroscience subfields (such as attendees of the Society for Neuroscience or Federation of European Neuroscience Societies conferences). However, we note that the Engram Experts serve as an internal control, as they were a non-COSYNE cohort of largely behavioural neuroscientists who have published studies on the neuroscience of engrams.

Fourth, in terms of research background, those doing laboratory experiments can be distinguished into multiple types, including wet-lab experiments (patch clamping, animal behaviour, etc.) and neuroimaging techniques, which are substantially different. In future studies, it would be better to break down the participants into finer research background categories.

Fifth, although we attempted to provide background information through the preamble to the questions in our survey, this information was of course incomplete. Respondents have vastly different amounts and sources of knowledge relevant to this topic, which surely affected their responses to the questions.

Sixth, we note that our survey question assumes ASC would provide ideal-quality ultrastructural preservation, which may not be achievable routinely, especially outside laboratory settings, and depends on specific assumptions about ASC mechanisms. We note that some studies comparing preservation techniques have suggested that there are certain ultrastructural artifacts introduced by aldehyde fixation compared to cryofixed brain tissue [35,36]. These findings are mixed, as more recent work has shown comparable measurements between techniques [37], and the preponderance of evidence suggests that the ultrastructural appearance achieved with both aldehyde fixation and cryofixation is largely similar [38]. A valuable follow-up study would examine whether long-term memories might be preserved under more realistic conditions, such as when people donate their bodies to science. Unlike the ideal conditions assumed in our survey, these situations involve unavoidable brain deterioration due to the cessation of blood flow and associated tissue decomposition. Such circumstances typically include periods of ischemia prior to preservation, potentially resulting in cell death or synaptic loss over time.

Last, we should flag that our analysis of the data is incomplete, as participants provided additional information through hundreds of free-text comments that this report has not examined. Informal exploration of these comments reveals nuanced perspectives that quantitative data alone cannot capture. For instance, participants who agreed that memories are primarily stored in synaptic connectivity sometimes qualified their responses with significant uncertainty, noting it could “go either way” or represented “wild speculation.” Others highlighted definitional ambiguities, particularly regarding what constitutes “emulation” or how to interpret terms like “primarily.” While we chose not to exclude potentially contradictory responses to avoid introducing selection bias, these comments suggest that many seemingly-contradictory responses reflect sophisticated reasoning rather than misunderstanding. The qualitative insights underscore a central finding of our study: while there may be provisional agreement on broad principles of memory storage, substantial uncertainty remains about specific mechanisms and implications.

In conclusion, this survey reveals that while most neuroscientists believe long-term memories are stored in structural features of the brain, there is no clear consensus on which specific neurophysiological features or scales are critical for memory storage. The finding that a substantial proportion of participants believe memories could theoretically be extracted from preserved brains, despite the aforementioned uncertainty about the precise physical basis of memory, suggests an underlying confidence in the structural nature of memory storage. However, the lack of agreement on critical features and high variance in probability estimates for the feasibility of memory extraction or brain emulation, indicates that fundamental questions about memory’s physical basis remain unresolved. Despite this, as technological capabilities advance in the realm of whole brain emulation, we may gain empirical evidence to resolve these theoretical uncertainties. Future studies would benefit from examining different memory subtypes separately, as well as analyzing the rich qualitative data provided by neuroscience experts. Because understanding the physical basis of memory storage has consequences that extend far beyond the laboratory, the ethical and philosophical implications of our observations of the neuroscience community’s beliefs should be broadly examined before they become increasingly practically relevant.

Supporting information

Acknowledgments

Research funding for this study was supported by a CryoDAO grant. We would like to thank Kenneth Hayworth and John Smart for helpful personal communications.

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