8 October 2012

Brainless slime mould navigates using ‘memory’

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A brainless, single-celled organism called slime mould has been shown to employ a form of ‘memory’ to solve a complex task originally designed to test robot navigation.
Slime mould

A petri dish of slime mould (Physarum polycephalum) in an agar solution. The brainless single-celled organism has revealed some surprising capabilities that make it a fascinating object of study for biologists. Credit: Gemma Black / COSMOS

SYDNEY: A brainless, single-celled organism called slime mould has been shown to employ a form of ‘memory’ to solve a complex task originally designed to test robot navigation.

Researchers at the University of Sydney, Australia, challenged a unicellular species of slime mould called Physarum polycephalum to a ‘U-shaped trap test’, in which it successfully navigated around a U-shaped obstacle to find a food source (see video below).

“This is just another cool thing that slime mould can do,” said biologist Chris Reid, lead author of the paper published in the Proceedings of the National Academy of Sciences.

“Slime mould shouldn’t be able to do a lot of things because it’s just a single cell and it doesn’t have a brain,” he said. “It’s supposed to be acting on this very primitive level, but repeatedly, research has shown it can solve complex tasks.”

Slime trail: externalised spatial memory

The U-shaped test involves placing slime mould in a petri dish of gel at the mouth of a piece of U-shaped plastic. The plastic forms a barrier for the slime mould, because it is too dry for it to move over. On the other side of the U is a small well of glucose and water, which gradually diffuses through the gel. The slime mould follows the gradient of diffused glucose straight towards the food source – until it becomes trapped inside the U-shaped plastic.

“If it wants to get out of the trap, it has to move against that gradient,” explained Reid. “To do that, it has to have some other mechanism by which to navigate.”

That mechanism, the researchers discovered, is a form of ‘externalised spatial memory’ similar to that used by ants – which leave behind a trail of pheromones to inform collective decisions in the colony.

Slime mould ‘chooses’ when to avoid its trail

Slime mould leaves behind a trail of extracellular slime, which it avoids moving over more than once – enabling it to more efficiently explore an area for food.

This was proven using a Y-shaped ‘maze’, with food sources at the top of each arm of the Y. If one arm is covered in slime, the mould will always choose the other path. Interestingly, if both arms are covered in slime, only then will the organism move over the slime.

“If the slime mould was heavily programmed not to go on the slime, or if there was something dangerous about the slime, it’d just sort of stay there,” said Reid. This suggests the avoidance of its own slime is something of a ‘choice’, the researchers said in the paper.

Early form of memory

Storing information outside an organism for recalling later is a form of ‘externalised memory’ that was “probably used by primitive cells before there was even multi-cellularity,” said Reid.

While this form of memory may not have been a direct evolutionary forerunner for the memory now used by higher-order organisms such as humans, it is certainly a “functional precursor”, Reid said.

“It allows [slime mould] to solve problems that these days we use big brains to solve, but they didn’t – and still don’t – need them to solve similar problems,” he said.

Biomimicry: nanorobots

“This [research] adds to the quite impressive display of abilities that Physarum polycephalum could boast about – if it could boast,” said Bernd Meyer, a computer scientist from Monash University in Melbourne, Australia, who works with biologists on collective systems in nature including ant colonies and slime mould.

Meyer, who was not involved in the research, said the findings could contribute to work on the development of nanorobots that perform medical tasks in the bloodstream.

“Due to their size, such robots have very limited sensory and processing capabilities, much like the amoeboid studied here,” he explained. “From this perspective it is important to learn how simple organisms in nature have learned to deal with complex environments. Simply emulating natural behaviour is certainly not the goal, but the inspiration that we can gain from simple solutions to complex problems that evolution has shaped over enormous time scales should not be underestimated.”

Maze solutions, travel planning

Slime mould has been the subject of two IgNobel Prizes – annual awards that satirise the prestigious Nobel Prize by recognising weird and funny science.

In 2008, researchers from Hokkaido University in Japan won the prize for demonstrating slime mould’s capacity to solve complex mazes by ‘mapping’ the entire maze and then contracting to form the most efficient route – the solution to the maze – between food sources at each end.

Then, in 2010, the same researchers used slime mould to test the efficiency of Tokyo’s rail system. They placed a food source at each major centre and rail station on a map of Tokyo, and the slime mould then grew into a complex network connecting each station to form a web almost identical to Tokyo’s actual rail system.

Reid said he currently has another paper under review that demonstrates slime mould’s ability to distinguish and react differently to the slime of different species of slime mould.

His colleagues at the University of Sydney have also previously demonstrated slime mould’s capacity to make irrational decisions – something previously thought to be the exclusive domain of big-brained organisms.

Plasmodium solving U-shaped trap problem on substrate of blank agar.
Credit: Christopher Reid, The University of Sydney

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