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Scientist’s have long marvelled at the “intelligent” accomplishments of the humble slime mold (and here). Noting that certain slime molds can make decisions, solve mazes, and remember things, Matthew Sims ponders what we can learn from the blob…

During the COVID-19 pandemic, some people took up baking, others decided to get a dog; I chose to grow and observe slime mould. The study in my partner’s flat in Edinburgh became home to two cultures of Physarum polycephalum, an acellular slime mould sometimes more casually referred to as ‘the blob’.

I began a series of experiments investigating how long it would take for two separated cell masses from the same bisected Physarum cell to stop fusing with one another upon reintroduction. Hours turned into days, and days into weeks, and, due to time constraints, the experiment eventually fizzled out around six weeks. This, however, was only the beginning. Over that following year (unbeknown to our unsuspecting neighbours), I conducted several more experiments. Although none of them were published, each inspired new philosophical questions – which to this day continue to shape my thinking. One of the core questions was: what can the behaviour of slime mould teach us about biological memory?

The differences between P polycephalum and humans may seem vast, but slime mould can reveal a remarkable amount about various aspects of how we remember. While many people might assume that our memories are primarily stored within our brains, some philosophers like myself argue that – along with some other aspects of cognition – memory can extend beyond the confines of the body to involve coupled interaction with structures in the environment. At least some of our cognitive processes, in short, loop out into our surroundings. Slime mould is an intriguing candidate to explore this idea because it doesn’t have a brain at all, yet in some cases can apparently ‘remember’ things without needing to store those associated memories within itself. In other cases, memories acquired via learning by one individual can even be acquired by a separate individual through physical contact. The behaviour of this strange form of life suggests that some of our ideas about how memories are acquired may need a rethink…

[Sims explains how slime mold “remembers”– via slime trails– and explores the questions that this raises…]

… So, what can slime mould teach us about biological memory? One lesson is that spatial memory needn’t be confined entirely within an organism (á la HEC). Moreover, what becomes memory traces when used (eg, extracellular slime) needn’t be the result of learning by the external trace-producer. Another takeaway is that, in some cases, an individual can acquire such memory without having engaged in learning itself. This raises an intriguing parallel in the human case. We do, after all, routinely read and act upon instructions, maps and manuals written by others, drawing on information acquired through their experiences, not our own. Although such externalised sources of information are typically declarative in structure – designed to represent facts explicitly – we often act upon them automatically, without needing to consciously recall or reflect on the information they convey. In this way, they guide behaviour in ways that functionally resemble non-declarative memory. While the analogy shouldn’t be pushed too far, both the human and slime mould cases illustrate how memory can become decoupled from individual learning, instead becoming accessible to others through environmental structures.

These conclusions, of course, remain contentious within traditional cognitive science and psychology where memory is often defined as the result of learning on the part of the same individual whose memory it is. Despite important concerns raised by the likes of Francis Crick in 1984, memory storage is still often attributed to synaptic plasticity – changes in strength of connection between neurons – quashing the very possibility of external memory traces. That said, some like the psychologist C Randy Gallistel – who has long argued that memory may also be stored in molecules like RNA within the brain – have remained vigilant in thinking outside the box. However, given the accumulating empirical evidence that memory-guided behaviour is exhibited in non-neuronal organisms like Physarum, then even this outside-the-box thinking remains firmly planted in traditional views about the requirements of brains for memory and the kind of strict internalism HEC suggests needn’t always be the case. Both HEC and memory without learning are not easy pills to swallow, but then again, neither is the very idea that a non-neuronal organism can learn in the first place – an idea that Physarum’sbehaviour unequivocally seems to support.

Whether it’s the subject of experiments carried out in a lab (or in a cramped study of an Edinburgh tenement flat) or it’s the subject of empirically informed, armchair philosophising, Physarum provides a valuable model organism to inspect, challenge and refine some of our most fundamental biological concepts – concepts like memory…

Fascinating: “Memories without brains,” from @philosobio.bsky.social‬ in @aeon.co‬.

* Rupert Brooke

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As we reckon with recall, we might send microscopic birthday greetings to Carl Woese; he was born on this date in 1928. A microbiologist and biophysicist. Woese is famous for defining, in 1977, the Archaea (a new domain of life, distinct from the previously-recognized two domains of bacteria and life other than bacteria). To accomplish this feat, he pioneered phylogenetic taxonomy of 16S ribosomal RNA, a technique that has revolutionized microbiology. Microbiologist Justin Sonnenburg of Stanford said “The 1977 paper is one of the most influential in microbiology and arguably, all of biology. It ranks with the works of Watson and Crick and Darwin, providing an evolutionary framework for the incredible diversity of the microbial world.”

Woese originated the RNA world hypothesis in 1967, although not by that name. And he also speculated about an era of rapid evolution in which considerable horizontal gene transfer occurred between organisms. With regard to Woese’s work on horizontal gene transfer as a primary evolutionary process, Professor Norman R. Pace of the University of Colorado at Boulder said, “I think Woese has done more for biology writ large than any biologist in history, including Darwin… There’s a lot more to learn, and he’s been interpreting the emerging story brilliantly.”

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