The Networks Of A Cell Without A Brain

AFew organisms have been asked to carry as much theory as Physarum polycephalum. The plasmodium of this slime mould is a single cell holding thousands of nuclei and not one neuron, and it spreads across damp bark as a fan of yellow tubes. Set on a map with oat flakes at the cities, it will within two days link them by a web resembling the rail network built there over a century. Such demonstrations are reported as problem solving, a phrase inherited, along with the mazes and the scoring sheets, from decades of work on rodents. The inheritance is awkward: those procedures assume an animal that senses a goal at a distance, holds a route in mind and then commits a body to it, whereas a plasmodium has no fixed body and nothing that could obviously do the holding. Whether the tidy networks record anything deserving the old vocabulary is the question the field cannot avoid.

BHelena Broz, a biophysicist at the Northfield Centre for Complex Flows, argues that the networks are the residue of a rule simple enough to need no memory at all. Inside every tube the cytoplasm surges back and forth about once a minute; a tube that happens to carry heavier flow dilates, lowers its resistance and draws still more flow, while idle neighbours narrow and are resorbed. Between 2016 and 2022 her group photographed 138 plasmodia at ninety-second intervals and measured some 40,000 tube segments. Given only the starting geometry of each arena and this one rule, their model reproduced 84 per cent of the final networks to within the width of a single tube. The model carries no term for the past: what a tube did an hour ago enters the calculation solely through how wide that tube is now. Broz calls this an exercise in accounting: a network that looks planned and a network that was planned may be built by identical arithmetic.

CAn account of that kind yields a prediction, and Broz has tested it. If a network is only the settled state of flows already running, the organism should be defeated wherever the profitable move means abandoning a tube that is currently paying, and defeated too wherever what to do next depends on something that happened an hour earlier. In two-stage arenas, where a large food source appeared only if a smaller one had been left untouched for forty minutes, her plasmodia succeeded on 19 per cent of trials, against 88 per cent on shortest-path mazes of comparable length. Success in the two-stage arenas was confined to runs in which both sources lay inside the ground the plasmodium had already covered. The contrast is instructive, since a rat finds the waiting task easy and the maze hard. Broz insists on presenting the figures as a boundary around one explanation rather than a measure of the organism.

DRafael Otero, a protistologist at the Calder Bay Laboratory, replies that a plasmodium starved on bare agar under constant light is performing in conditions its species never meets, and that a failure recorded there describes the arena at least as much as the occupant. His own cultures are raised for eight days on oat substrate under a regular cycle of light and darkness before any test begins, an arrangement now ordinary in the field but thought eccentric when he adopted it in 2017. Under that regime, 61 per cent of his plasmodia slowed their advance shortly before a cold, dry pulse that had been delivered every sixty minutes was next due, and went on doing so for two further cycles after the pulses had stopped. Otero grants two weaknesses. His twelve plasmodia came from a single strain, and the internal state he calls memory was inferred from the timing of contractions alone, because nothing yet allows it to be read directly.

EThe argument has spread well beyond the laboratory. Engineers have adopted the tube-thinning rule as a design principle: a 2019 study applied it to a proposed rail corridor and returned a layout 7 per cent shorter in total track than the surveyors' plan, although it fell into two disconnected halves whenever any single link was cut. Municipal authorities in three countries have commissioned similar exercises, and one was criticised for calling the output a decision reached by an organism. Broz points out, awkwardly for her own position, that nothing in her model bears on whether the plasmodium senses anything, because a mindless rule and an inner life are not competing options. Otero answers that the two questions cannot be kept apart in practice, since funding bodies read reports of skill as evidence that something is understood and reports of failure as evidence that nothing is. Both regret the habit of publishing one photograph of an elegant network rather than the distribution of the untidy ones.

FA settlement of sorts is taking shape in which the two accounts are given different work rather than ranked against each other. Where a task can be solved by flows finding their own level, the tubes appear to solve it with nothing carried from one moment to the next, which would explain both the speed of the performance and the organism's indifference to where it has already been. Where something must survive a gap in time, attention has turned outwards, to the film of spent slime the plasmodium leaves behind and afterwards avoids, a record kept outside the cell. What remains unresolved is whether that external record does any work in a forest, where rain and competitors erase it, and no field technique now available can decide the matter. What both researchers reject is the assumption behind the rail-map photographs: that a system without neurons must be either a poor imitation of a brain or nothing at all.