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A Would-Be Nervous System Made from a Slime Mold

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A Would-Be Nervous System Made from a Slime Mold A Would-Be Nervous System Andrew Adamatzky** Made from a Slime Mold University of the West of England Keywords Slime mold, nervous system, unconventional computing Abstract The slime mold Physarum polycephalum is a huge single cell that has proved to be a fruitful material for designing novel computing architectures. The slime mold is capable of sensing tactile, chemical, and optical stimuli and converting them to characteristic patterns of its electrical potential oscillations. The electrical responses to stimuli may propagate along protoplasmic tubes for distances exceeding tens of centimeters, as impulses in neural pathways do. A slime mold makes decisions about its propagation direction based on information fusion from thousands of spatially extended protoplasmic loci, similarly to a neuron collecting information from its dendritic tree. The analogy is distant yet inspiring. We speculate on whether alternative—would-be—nervous systems can be developed and practically implemented from the slime mold. We uncover analogies between the slime mold and neurons, and demonstrate that the slime mold can play the roles of primitive mechanoreceptors, photoreceptors, and chemoreceptors; we also show how the Physarum neural pathways develop. The results constituted the first step towards experimental laboratory studies of nervous system implementation in slime molds. 1 Introduction The plasmodium of Physarum polycephalum (order Physarales, class Myxomecetes, subclass Myxo- gastromycetidae) is a single cell, visible with the naked eye, with many diploid nuclei [63]. The plasmodium feeds on bacteria and microscopic food particles by endocytosis. When placed in an environment with distributed sources of nutrients, the plasmodium forms a network of proto- plasmic tubes connecting the food sources (Figure 1a). The topology of the plasmodiumʼs proto- plasmic network optimizes the plasmodiumʼs harvesting resource from the scattered sources of nutrients and makes more efficient the transport of intracellular components [49]. In [6] we have shown how to construct specialized and general-purpose massively parallel amorphous computers from the plasmodium (slime mold) of P. polycephalum that are capable of solving problems of com- putational geometry, graph theory, and logic. The plasmodiumʼsforagingbehaviorcanbeinter- preted as a computation [49, 51]: data are represented by the spatial distribution of attractants and repellents, and results are represented by the structure of the protoplasmic network [6]. The plasmodium can solve computational problems with natural parallelism, such as finding shortest paths [50] and hierarchies of planar proximity graphs [5], plane tessellations [56, 6], and planar ** University of the West of England, Bristol BS16 1QY, United Kingdom. E-mail: [email protected] © 2015 Massachusetts Institute of Technology Artificial Life 21: 73–91 (2015) doi:10.1162/ARTL_a_00153 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/ARTL_a_00153 by guest on 26 September 2021 A. Adamatzky A Would-Be Nervous System Made from a Slime Mold Figure 1. Plasmodium of P. polycephalum. (a) Oat flakes colonized by the slime mold are black irregular blobs. (b) A hypothetical neural network made of P. polycephalum. Every Physarum blob (oat flake colonized by Physarum) is a neuron, shown by a black disc. Protoplasmic tubes connecting the blobs are analogues of synaptic connections; they are shown by lines ending with small grey disks. shapes [12]; execution of logical computing schemes [66, 3]; and natural implementation of spatial logic and process algebra [58], unconventional hybrid wetware and hardware [11], and prototypes of microfluidic logic gates [16]. Nowadays Physarum has become a dominant popular substrate for designing future and emergent computing architectures. This is because the slime mold is easy to culture and handle, most experi- ments do not require sophisticated equipment, technical difficulties are minimal, and costs of proto- typing are very low. The plasmodium of P. polycephalum is a simple-to-maintain substrate that requires minimal equipment to experiment with. In [6] we developed a concept and designed a series of experimental laboratory prototypes of computing devices—Physarum machines [6]—based on P. polycephalum.APhysarum machine is a programmable amorphous biological computing device experimentally implemented in plasmo- dium of P. polycephalum.APhysarum machine is programmed by configurations of repelling and attracting gradients. The mechanics of Physarum machines is based on the following unique features of P. polycephalum [6]: • Physarum is a living, dynamical reaction-diffusion pattern formation mechanism. • Physarum may be considered as equivalent to a membrane-bound subexcitable system (excitation stimuli provided by chemoattractants and chemorepellents). 74 Artificial Life Volume 21, Number 1 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/ARTL_a_00153 by guest on 26 September 2021 A. Adamatzky A Would-Be Nervous System Made from a Slime Mold • Physarum may be regarded as a highly efficient and living micromanipulation and microfluidic transport device. • The induction of the pattern type is determined partly by the environment—specifically, nutrient quality and substrate hardness, dryness, and the like. • Physarum is sensitive to illumination and electric fields and therefore allows for parallel and nondestructive input of information. • Physarum represents results of computation by the configuration of its body. Physarum is thus a computational material based on modification of protoplasm transport by the presence of external stimuli. The range of laboratory prototypes of living slime mold devices is impressive: robot controllers [67], microfluidic logic gates [16], computational geometry processors [12, 6], and electronic ele- ments [17], to name but a few. We decided to go a bit further and to answer a question borrowed from the field of living technologies [18, 19]: “Can we make an artificial living neural system from the slime mold?” Rephrasing Bedau et al. [18], we can say that a nervous system made from Physarum is artificial in that it is created by our intentional activities, yet is natural in that it grows, responds to environmen- tal stimuli, and adapts according to its own biological laws. A model nervous system would be a spatial configuration of the Physarum neurons (slime mold blobs, such as sources of food colonized by Physarum; the blobs are interconnected with each other by Physarum neuron terminals), proto- plasmic tubes, and imitating axons and dendrites (Figure 1b). In this article we highlight some analogies between the slime mold and neurons, and demonstrate that the slime mold can play the roles of primitive exteroceptors such as touch (mechanoreceptors), and teleceptors such as sight (photoreceptors) and smell and taste (chemoreceptors). The article is structured as follows. We introduce methods of culturing and experimenting with the slime mold in Section 2. We discuss neuronlike features of Physarum electrical behavior in Section 3. Section 4 shows what types of mechanical, chemical, and optical receptive organs can be made from Physarum. In Section 5 we investigate the growth of Physarum neural pathways. 2 Methods The following experimental conditions were employed and equipment used in the experiments discussed. Plasmodium of Physarum polycephalum wascultivatedinplasticlunchboxes(withafew holes punched in their lids for ventilation) on wet kitchen towels and fed with oat flakes. The culture was periodically replanted to fresh substrate. In all recordings mentioned we used planar aluminum foil electrodes (width 5 mm, thickness 0.04 mm, sheet resistance 0.008 V/cm2). In experiments on sensorial properties two blobs of 2% non-nutrient agar (Select Agar, Sigma-Aldrich), 2 ml each, were placed on electrodes stuck to the bottom of a plastic petri dish (9 cm). The distance between proximal sites of electrodes was 10 mm in all experiments. Physarum was inocu- lated on one agar blob. We waited till the Physarum colonized the first blob, where it was inocu- lated, and propagated towards and colonized the second blob. When the second blob was colonized, the two colonized blobs of agar became connected by a single protoplasmic tube (Figure 2a). Electrical activity of plasmodium wasrecordedwithADC-24HighResolutionData Logger (Pico Technology, UK). The data logger ADC-24 employs differential inputs, galvanic isolation, and software-selectable sample rates, all contributing to superior noise-free resolution; its 24-bit A/D converter maintains a gain error of 0.1%. When necessary, a resistance was mea- sured with four wires using a Fluke 8846A precision voltmeter, test current 1.0000 ± 0.0013 lA. InoneoftheillustrativeexampleswestimulatedPhysarum with regular waveforms using a bench Tenma function generator. Artificial Life Volume 21, Number 1 75 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/ARTL_a_00153 by guest on 26 September 2021 A. Adamatzky A Would-Be Nervous System Made from a Slime Mold 3 Physarum Neurons We represent a Physarum neuron by a physically localized and almost everywhere isolated locus of Physarum, such as the blobs of agar colonized by Physarum in Figure 2a. There is no difference between axons and dendrites in the Physarum analogue models of a neural network, so we use a general term “connection” or “pathway.” A connection is a protoplasmic tube linking two Physarum neurons. An example is shown in Figure 2a,
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