Decision-making without a brain: how an amoeboid organism solves the two-armed bandit.

Chris R Reid,Simon Garnier,Tanya Latty,Hannelore Macdonald,James A R Marshall,Richard P Mann

doi:10.1098/rsif.2016.0030

Abstract

Several recent studies hint at shared patterns in decision-making between taxonomically distant organisms, yet few studies demonstrate and dissect mechanisms of decision-making in simpler organisms. We examine decision-making in the unicellular slime mould Physarum polycephalum using a classical decision problem adapted from human and animal decision-making studies: the two-armed bandit problem. This problem has previously only been used to study organisms with brains, yet here we demonstrate that a brainless unicellular organism compares the relative qualities of multiple options, integrates over repeated samplings to perform well in random environments, and combines information on reward frequency and magnitude in order to make correct and adaptive decisions. We extend our inquiry by using Bayesian model selection to determine the most likely algorithm used by the cell when making decisions. We deduce that this algorithm centres around a tendency to exploit environments in proportion to their reward experienced through past sampling. The algorithm is intermediate in computational complexity between simple, reactionary heuristics and calculation-intensive optimal performance algorithms, yet it has very good relative performance. Our study provides insight into ancestral mechanisms of decision-making and suggests that fundamental principles of decision-making, information processing and even cognition are shared among diverse biological systems.

Highlights

While less recognized than their animal counterparts, many non-neuronal organisms, such as plants, bacteria, fungi and protists, have the ability to make complex decisions in difficult environments
We examine decisionmaking in the unicellular slime mould Physarum polycephalum using a classical decision problem adapted from human and animal decision-making studies: the two-armed bandit problem
To challenge the slime mould with the two-armed bandit problem, we provided P. polycephalum plasmodia with a choice between two differentially rewarding environments

Summary

Introduction

While less recognized than their animal counterparts, many non-neuronal organisms, such as plants, bacteria, fungi and protists, have the ability to make complex decisions in difficult environments (for a full review, see [1]). The most incredible feats of problem-solving among non-neuronal organisms, many previously reported only in the so-called cognitive organisms, have been demonstrated by the unicellular slime mould Physarum polycephalum. This unicellular protist lacks a central nervous system and possesses no neurons, yet it has been demonstrated to solve convoluted labyrinth mazes [2], find shortest length networks and solve challenging optimization problems [3], anticipate periodic events [4], use its slime trail as an externalized spatial memory system to avoid revisiting areas it has already explored [5] and even construct transport networks that have similar efficiency to those designed by human engineers [6].

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Conclusion