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Brainless slime molds 'think' their way through the environment

Slime molds don't have brains, but researchers say that their ability to interact with their environment sheds light evolution and the development of cognition. Photo by QuinceMedia/Pixabay
Slime molds don't have brains, but researchers say that their ability to interact with their environment sheds light evolution and the development of cognition. Photo by QuinceMedia/Pixabay

July 15 (UPI) -- Can you think without a brain? According to a new study, slime molds can.

Slime molds are without central nervous systems, but they are able sense tactile, chemical, and optical stimuli.

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Recently, researchers observed the slime mold Physarum polycephalum convert mechanical cues into electric pulses and perform computations -- something approximating "thinking" -- to determine in which direction to move.

Unlike previous experiments, the latest study -- published Thursday in the journal Advanced Materials -- did not rely on food or chemical signals to influence the slime mold's behavior.

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"People are becoming more interested in Physarum because it doesn't have a brain but it can still perform a lot of the behaviors that we associate with thinking, like solving mazes, learning new things, and predicting events," first author Nirosha Murugan said in a press release.

"Figuring out how proto-intelligent life manages to do this type of computation gives us more insight into the underpinnings of animal cognition and behavior, including our own," said Murugan, an assistant professor at Algoma University in Canada.

Slime molds are amoeba-like eukaryotic organisms that feature an aggregate of cells floating inside a membrane-wrapped blob of cytoplasm.

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They feed on microorganisms found among decaying plant matter -- rotting logs, mulch and dead leaves.

Slime molds often exist in single-cell forms when there is plenty of food, but come together to form visible molds when resources are scarce.

When slime mold cells congregate, they can exhibit behavior associated with multicellular organisms -- altering their shape and functions in order to adapt to different environs.

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"With most animals, we can't see what's changing inside the brain as the animal makes decisions," said Murugan, who led the study while working at the Tufts University's Allen Discovery Center.

"Physarum offers a really exciting scientific opportunity because we can observe its decisions about where to move in real-time by watching how its shuttle streaming behavior changes," Murugan said

In the lab, scientists placed slime molds in the center of gel-coated Petri dishes. Researchers also placed wither one or three small glass discs next to each other on opposite sides of each dish. The molds were then left to grow overnight in the dark.

For the first 12 hours, the molds grew evenly in all directions.

After that, 70 percent of the molds extended a long arm and began expanding toward the three disks, gravitating toward the larger object without having previously navigated the environment.

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To find out how, scientists began tweaking the experimental setup.

They found that when they stacked the three disks atop one another, the molds lost their ability to distinguish between one and three disks -- half the molds grew toward one side of the disk, while the other half opted for the opposite side.

To better understand how the molds perceived their environment, researchers developed a model to analyze how the mass and arrangement of disks would influence the stress placed on the gel coating in each dish.

The model confirmed that the increased mass of three disks altered the stress and strain placed on the gel, but the simulations also showed that the arrangement of the disks altered the perceptible strain, or deformation, pattern.

"Imagine that you are driving on the highway at night and looking for a town to stop at," said Richard Novak, lead staff engineer at the Harvard University's Wyss Institute. "You see two different arrangements of light on the horizon: a single bright point, and a cluster of less-bright points."

"While the single point is brighter, the cluster of points lights up a wider area that is more likely to indicate a town, and so you head there," Novak said.

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"The patterns of light in this example are analogous to the patterns of mechanical strain produced by different arrangements of mass in our model. Our experiments confirmed that Physarum can physically sense them and make decisions based on patterns rather than simply on signal intensity," Novak said.

The findings showed the molds were not simply perceiving the relative mass of the disks, but utilizing data related to the distribution of mass to perceive their environment and make decisions about where to move.

Researchers hypothesized that the molds were able to perceive the strain pattern by pulsing and pulling on the gel with each contraction.

In other animals, including humans, this type of tactile sensing is mediated by cell membranes called TRP-like proteins.

When scientists gave a strong TRP channel-blocking drug to the slime molds, they lost their ability to disguise between the disk arrangements.

"Our discovery of this slime mold's use of biomechanics to probe and react to its surrounding environment underscores how early this ability evolved in living organisms, and how closely related intelligence, behavior and morphogenesis are," said co-author Mike Levin, Wyss researcher and director of the Allen Discovery Center.

He noted that Physarum grows out to interact with the world, and uses changing its shape as its behavior -- similar to strategies used by cells in more complex animals, such as neurons, stem cells and cancer cells.

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"This work in Physarum offers a new model in which to explore the ways in which evolution uses physics to implement primitive cognition that drives form and function," said Levin, a Wyss researcher and director of the Allen Discovery Center.

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