Saturday, January 30, 2016

Brainless Intelligence: Memory in Physarum polycephalum

by JH
Slime mold
     “If I only had a brain...”, cooed the Scarecrow from the hit classic, The Wizard of Oz. A brain he longed for so that he could think, retain memories and reflect intelligence, just like Dorothy and the other complex beings he encountered on his journey down the yellow brick road. Thoughts and memory reside in the brain, but what if an organism doesn't have a brain? Can it still possess the qualities that constitute intelligence, such as memory? In the case of the slime mold, Physarum polycephalum, the answer is yes. Scientists have explored the ability of the single-celled protist to construct a form of external memory to navigate through complex environments (1). This behavior demonstrates a primitive version of brain function in P. polycephalum, despite their lack of a central nervous system (3).

     Physarum polycephalum is a unicellular protist of the class Myxomycetae, better known as a slime mold, which is most commonly found in the organic matter of the forest floor such as woody debris and leaf litter (2). P. polycephalum shares the same bright yellow coloring as the popular kid’s television character “SpongeBob SquarePants”. Unlike its famous doppelgänger, P. polycephalum is far from simpleminded. The slime mold enhances its navigational ability through the use of an external spatial memory system, in which the organism can distinguish between new and previously explored pathways. The purpose of this “memory” is to determine the most efficient and direct route to a food source as well as prevent the organism from revisiting areas of depleted nutrients (1). This ability to construct a spatial memory system even allows P. polycephalum to find its way through a complex maze!

     How does this organism achieve such intelligible feats? Researchers studying Physarum polycephalum believed this organism may use a type of avoidance technique to forgo areas of previous exploration in order to find the most efficient route to a food source. They hypothesized that the slime mold uses the presence of extracellular slime, which is a thick layer of cells secreted by the organism, as a spatial memory system to aid in this avoidance behavior (1). To observe the behavior of P. polycephalum, a team of Australian scientists set up an experiment modeling a classic test of navigation that is commonly used in robotics, called the U-shaped trap problem (1). This test required the slime mold, in its vegetative state called plasmodium, to reach a goal behind a U-shaped barrier. The test consisted of a petri dish with a U-shaped trap, made of a material that the organism could not stick to and grow over, that was placed between the slime mold and the goal, a solution of sugar. The team challenged the slime mold to reach the sugar solution by growing around the trap on two types of media: blank agar and agar containing extracellular slime.

     What they found was that the slime mold strongly avoids areas that contain the extracellular slime (1). They observed that the organism first explores its entire environment by extending temporary projections that help the organism move, known as pseudopodia. The organism then detects where the food source is via specific receptor molecules. This detection causes the projections to flow towards the food source while simultaneously retracting the pseudopodia from all of the areas that do not contain food (1). As the plasmodium retracts its pseudopodia, it leaves behind a thick extracellular slime. It can then use this slime as a memory system by “reminding” the organism upon contact with the slime, that these areas did not contain food. The slime mold then grows exclusively along the shortest path avoiding all of the areas containing extracellular slime.

     This behavior was also observed in an experiment where P. polycephalum was placed at the start of a complex maze, as shown in Fig.1 (3). Food sources were placed at the starting and ending points of the maze, and the slime mold was encouraged to find a route that leads to the food source at the end. The observation was that the organism extended its pseudopodia throughout all possible paths of the maze, even the dead ends. Once the organism had completely covered the maze, it began to retract its pseudopodia, avoiding the slime in the dead end areas, and began to grow only along the path that offered the shortest distance to the food source at the end (3).

Fig.1 Slime mold solving a maze;
finding the shortest distance to food source
     Another aspect of this experiment explored the behavior of P. polycephalum when extracellular slime from other organisms of the same species as well as members of different species of slime mold were deposited in its presence. The results of this experiment showed that the organism discriminates between the two groups (2). Because P. polycephalum naturally coexists with a variety of species of slime mold, which compete for similar yet slightly different food sources, this distinction between slime from different organisms is essential for maximizing foraging efficiency (2). A member of the same species of P. polycephalum will have nutrient requirements that overlap entirely therefore the detection of its extracellular slime suggests that the food source is already depleted. The experimental P. polycephalum thus will avoid these areas. The organism would then prefer to follow cues from a member of a different species, as this would indicate an environment of slightly higher quality and less depletion than that of a member of the same species (2).

     Maze solving, network construction, and species specific discrimination are used by the slime mold Physarum polycephalum to allow the organism to more efficiently focus its search for nutrients. The studies previously discussed are important in that they show empirical evidence of a spatial memory system in a non-neuronal organism to make optimal foraging decisions. These conclusions support the theory that externalized memory may be a functional precursor to the internal memory of complex organisms (1). Just like the Scarecrow, Physarum polycephalum proves that even without a brain, you can still find your way to the end of the yellow brick road.

  1. (1)  Reid, C.R., Latty, T., Dussutour, A., Beekman, M. 2012. Slime Mold Uses an Externalized Spatial “Memory” to Navigate in Complex Environments. PNAS vol. 109 no. 43
  2. (2)  Reid, C.R., Latty, T., Dussutour, A., Beekman, M. 2013. Amoeboid Organism Uses Extracellular Secretions to Make Smart Foraging Decisions. International Society for Behavioral Ecology vol. 24 ch. 4 pgs 812-818
  3. (3)  Nakagaki, T., Yamada, H., Tóth, A. 2000. Maze Solving By an Amoeboid Organism. Nature vol. 407 pg 470
  4. (4)  Nakagaki, T., Kobayashi, R., Nishiura, Y., Ueda1, T. 2004. Obtaining multiple separate food sources: behavioural intelligence in the Physarum plasmodium. The Royal Society

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