A single-celled organism without a brain is capable of Pavlovian learning

A simple single-celled organism without a brain or neurons appears to be capable of an advanced form of learning.

The simplest form of learning, known as habituation, gradually reduces how much you respond to a repeated, harmless stimulus, such as a smell or noise. This is common in all animals and has even been observed in plants. It has also been shown in some protists that have complex eukaryotic cells, such as animals, land plants and fungi, but are generally single-celled organisms, including tube shaped Stentor coeruleus and mucous membrane Physarum polycephalum.

It is much more difficult to learn to connect different types of stimuli or events and to predict that one is related to the other. Such associative learning was most famously demonstrated when Ivan Pavlov associated the sound of a bell with giving food to dogs, resulting in the animals salivating when they heard the bell rung.

Now, Sam Gershman at Harvard University and his colleagues used similar conditioning experiments to show this Stentor It also appears to be capable of associative learning.

These surprising organisms live in ponds and swim using lines of hair cilia running down their sides. At up to 2 millimeters in length, they are giants among single-celled life. At one end they have an anchor called a handle to attach to the surface, while at the other is their tube-like feeding apparatus.

“When they’re attached, they’re just filtering feed. If they’re bothered, they quickly curl up into a ball. They can’t feed during that time, so it’s ecologically beneficial not to react as often if they don’t have to,” says Gershman.

He and his colleagues used this behavior to find out just how much Stentor can learn First, they firmly glued to the bottom of Petri dishes containing several dozen cultures Stentor cells. In response, most organisms first contracted rapidly, but as the tapping continued every 45 seconds, a total of 60 beats, fewer and fewer Stentor they withdrew, indicating that they had become accustomed to the signal.

Further, Stentor cultures felt a weak tap—typically fewer organisms retracted in response—1 second before a strong tap. The double taps repeat every 45 seconds, which is about how long it takes Stentor develop again.

Over 10 trials of this process, the probability of organisms becoming infected immediately after a light tap first increased and then decreased. “We saw this bump in the graph where the contraction speed initially increases before it decreases. If you’re just presenting a weak tap by itself, you don’t see it,” says Gershman.

Researchers say it means Stentor paired a weak faucet with a larger faucet, making it the first protist known to be capable of associative learning. “It raises the question of whether seemingly simple organisms are capable of the knowledge we generally associate with much more complex, multicellular organisms with brains,” says Gershman.

It also suggests an ancient evolutionary origin for associative learning hundreds of millions of years before the emergence of multicellular nervous systems, he says. Further traces of this can still be seen in the way our neurons seem to be able to learn from their inputs in ways that it is not dependent on the adjustment of synapses or connections between neurons—which is how most learning is thought to work, he says.

“It’s fascinating that a single cell can do such complex things that we thought required a brain, required neurons, required behavioral learning,” he says Shashank Shekhar at Emory University in Atlanta, Georgia, who showed that I’m fightingr can aggregate into short-lived groups to feed more efficiently.

He thinks that other single-celled organisms may also be capable of associative learning. “My feeling is that once it’s there, it’s going to be there more,” he says.

If the organism learns, it means that it must somehow store memory. How it happens in Stentor is not yet known, but Gershman suspects that these are receptors that respond to touch by letting calcium flow into the cell, changing the voltage inside and conducting Stentor to conclude a contract. It suggests that after repeated stimuli, some receptors are somehow modified and act as a molecular switch to stop the contraction.

topics:

• neuroscience /
• microbiology