An associative learning experiment using the plasmodium of Physarum polycephalum

Abstract

The plasmodium of Physarum polycephalum is a unicellular and multinuclear giant amoeba that shows adaptive behaviors. To test the presence of memory and learning ability in the plasmodium, we performed an associative learning experiment using the unicellular organism. The plasmodium in this experiment seemed to acquire a reversed thermotactic property, a new preference for the lower temperature. The result implied a possibility of unicellular learning, though in a preliminary way. We also discuss a possible mechanism of learning by the organism.

Introduction

In this paper, we discuss the possible presence of memory and learning ability in the plasmodium of Physarum polycephalum. Physarum plasmodium is a unicellular and multinuclear giant amoeba, which is visible to the naked eye (Fig. 1). The plasmodium is formed by aggregation and fusion of a massive number of uninucleate amoebae at one stage in the life cycle of Physarum polycephalum. The cell body of the plasmodium roughly consists of two parts; a sheet-like locomotive front and a tubular rear part (Fig. 1). The amorphous sheet rigorously crawls on plane surfaces searching for food sources, at the same time the tubes connect all the parts of the cell body so as not to separate the unicellular body. The plasmodium shows some adaptive behaviors in its search for food sources and quest for survival, and recent studies are revealing the organism’s computational, memory and learning abilities. The plasmodium is thus attracting a lot of attention not only from a biological standpoint but in a variety of scientific fields such as physics, information science, systems science and cognitive science as well.

The recent studies on the computational ability of the plasmodium revealed that the organism or the experimental system using it can find a solution in a maze [16], [17], [12], optimize a network between multiple nodes [15], [13], [23], and solve some other graph theoretical problems [22], [24]. The solutions for these problems are given by the arrangement of tubes. For example, the maze-solving by the amoeba is performed by selection of the shorter tubes connecting two food sources at the entrance and the exit of the maze, after full search of the maze space [16]. For these tubular dynamics, some models have already been proposed [26], [27], [29], [28], [5], [6], [18]. Furthermore, by utilizing the locomotive and tactic activities of the sheets, the plasmodium can be used in some logical operations [1], [2].

Additionally, the plasmodium is not merely a wet central performance unit, but also seems to have a reservoir for memories. Saigusa et al. showed that the plasmodium can be synchronized with the intervals of rhythmic stimuli, and is able to keep the record for more than 10 h [21]. Furthermore, we demonstrated in our previous study that the cell motility of the plasmodium depends on its locomotive history [25]. Though the two studies above did not fully demonstrate the presence of memory in the organism, the possibility was strongly implied.

In this study, we performed an associative learning experiment to test the presence of memory and learning ability in the plasmodium. Associative learning is a type of learning concerning the relationship between two different stimuli. The famous Pavlovian conditioning experiment gives a good example of associative learning (Fig. 2). In this example, the conditioned dog that has performed associative learning shows the same response to one stimulus as to the other stimulus. This demonstrates that the dog has memory and learning ability. In a similar way, we performed an associative learning experiment using the plasmodium, to test the presence of memory and learning ability in the organism.

We performed the experiment utilizing the thermotactic property of the plasmodium. In the temperature range around 10–30 °C, the plasmodium shows thermotaxis to the higher temperature. In the setup illustrated in Fig. 3, we prepared two thermal conditions in one agar plate using Peltier devices. The thermotaxis of the plasmodium is so strong and the organism without learning always proceeds to the higher temperature region. However, the food sources in this setup are located only in the lower temperature region. Though the plasmodium has the thermotaxis to the higher temperature, after the full search for the higher temperature region, it moves to the lower temperature region and is fed for the first time there. The plasmodium is forced to feed itself only in the lower temperature, and here the lower temperature is a conditioned stimulus for this experiment, and the food source is an unconditioned one. In such a way we tested the associative learning ability of the plasmodium.