Encoding memory in tube diameter hierarchy of living flow network

Abstract

The concept of memory is traditionally associated with organisms possessing a nervous system. However, even very simple organisms store information about past experiences to thrive in a complex environment—successfully exploiting nutrient sources, avoiding danger, and warding off predators. How can simple organisms encode information about their environment? We here follow how the giant unicellular slime mold Physarum polycephalum responds to a nutrient source. We find that the network-like body plan of the organism itself serves to encode the location of a nutrient source. The organism entirely consists of interlaced tubes of varying diameters. Now, we observe that these tubes grow and shrink in diameter in response to a nutrient source, thereby imprinting the nutrient’s location in the tube diameter hierarchy. Combining theoretical model and experimental data, we reveal how memory is encoded: a nutrient source locally releases a softening agent that gets transported by the cytoplasmic flows within the tubular network. Tubes receiving a lot of softening agent grow in diameter at the expense of other tubes shrinking. Thereby, the tubes’ capacities for flow-based transport get permanently upgraded toward the nutrient location, redirecting future decisions and migration. This demonstrates that nutrient location is stored in and retrieved from the networks’ tube diameter hierarchy. Our findings explain how network-forming organisms like slime molds and fungi thrive in complex environments. We here identify a flow networks’ version of associative memory—very likely of relevance for the plethora of living flow networks as well as for bioinspired design.

Associative Conditioning Is a Robust Systemic Behavior in Unicellular Organisms: An Interspecies Comparison

“… Armus et al (2006) have identified in Paramecium caudatum that this protist is able to develop a preference for illumination level using a mild electric shock as a reinforce. Fungus-like organisms such as Physarum polycephalum has been shown to exhibit a primitive type of learning ( Boisseau et al, 2016 ; Boussard et al, 2019 ; Kramar and Alim, 2021 ). Moreover, this unicellular multinucleate plasmodium was observed to acquire new thermotactic behaviors when exposed to a temperature gradient and a source of nutrients.…”

Physarum polycephalum: Establishing an Assay for Testing Decision-making Under Shifting Somatic Boundaries

“…It is also possible that the Physarum could be remembering being a single entity and be motivated to return to that state as a kind of homeostasis. Physarum is known to be capable of remembering past experiences [21][22][23][24], and it is possible that the rapid merging is a kind of regenerative/repair response (anatomical homeostasis) that acts faster than individual pieces can acquire an independent identity.…”

Modelling ourselves: what the free energy principle reveals about our implicit notions of representation

“…Body design can be leveraged to alleviate on even replace control demands, as in passive (brain-and control-less) walking robots (McGeer, 1990). For a recent experimental and computational demonstration of how a slime mould's hierarchical network morphology may be seen as modelling its nutrient environment at fast timescales see Kramar and Alim (2021). See Sect.…”