Single-celled organism with no brain is capable of Pavlovian learning

A simple single-celled organism, without a brain or neurons, seems capable of an advanced form of learning.

The simplest form of learning, known as habituation, involves gradually reducing your response to a repeated, innocuous stimulus, such as a smell or noise. This is common in all animals and has even been observed in plants. This has also been demonstrated in some protists, which have complex eukaryotic cells like animals, land plants, and fungi, but are generally single-celled organisms, notably trumpet-shaped organisms. Stentor coeruleus and slime mold Physarum polycephalum.

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

Sam Gershman of Harvard University and his colleagues used similar conditioning experiments to show that Stentor also appears capable of associative learning.

These amazing organisms live in ponds and swim using lines of hair-like cilia that run along their sides. Measuring up to 2 millimeters long, they are giants among single-celled life. At one end they have an anchor called a holdfast to attach to a surface, while at the other is their trumpet-shaped feeding device.

“When they are attached, they simply filter their food. If they are disturbed, they will quickly contract into a sphere. During this time, they cannot feed, so it is ecologically advantageous not to react like this very often, unless they have to,” says Gershman.

He and his colleagues used this behavior to study the extent to which Stentor can learn. First, they vigorously tapped the bottom of Petri dishes containing cultures of a few dozen Stentor cells. In response, most organisms contracted quickly at first, but as the bangs continued every 45 seconds, for a total of 60 thuds, fewer and fewer organisms contracted. Stentor contracted, showing that they had become accustomed to the signal.

Then, the Stentor Crops felt low pressure – in response to which fewer organisms typically contract – 1 second before high pressure. The tap pairs repeat every 45 seconds, which is about how long it takes Stentor deploy again.

Over the course of 10 trials of this process, the risk of organisms contracting immediately after the gentle tap first increased and then decreased. “We saw this bump in the graph where the contraction rate initially increases before decreasing. If you just present the weak tap alone, you don’t see it,” says Gershman.

Researchers say this means Stentor associated the weak faucet with the larger faucet, making it the first known protist capable of mastering associative learning. “This raises the question of whether seemingly simple organisms are capable of cognitive aspects that we typically associate with much more complex multicellular organisms with brains,” says Gershman.

It also suggests an ancient evolutionary origin of 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 able to learn from their inputs in a way that does not depend on changing 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 things so complex that we thought required a brain, neurons and behavioral learning,” says Shashank Shekhar of Emory University in Atlanta, Georgia, who showed that Stentor can group together in ephemeral groups to feed more efficiently.

He thinks other single-celled organisms might also be able to learn by association. “My intuition is that if it’s there once, it’ll be there more,” he says.

If an organism learns, that means it must somehow store a memory. How does this happen in Stentor is not yet known, but Gershman suspects it involves receptors that respond to touch by letting calcium flow into the cell, thereby changing the tension inside and leading Stentor contract. It suggests that after repeated stimuli, certain receptors are altered in some way, acting as a molecular switch to stop contraction.