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, is to gradually reduce how much you respond to a repeated, harmless stimulus, such as a smell or noise. This is common to all animals and has even been seen in plants. It has also been detected in some protists, which have complex eukaryotic cells such as animals, land plants and fungi, but are generally single-celled organisms, including the trumpet-shaped Stentor coeruleus and the slime form Physarum polycephalum.

Much more difficult is learning to connect different types of stimuli or events, and to predict that one is linked to another. Such associative learning was most famously demonstrated when Ivan Pavlov paired the sound of a bell with feeding dogs, resulting in the animals salivating when they heard the bell ring.

Now Sam Gershman at Harvard University and his colleagues have used similar conditioning experiments to show it Stentor also seems capable of associative learning.

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

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

He and his colleagues used this behavior to investigate how much Stentor can learn. First, they vigorously tapped the bottom of petri dishes containing cultures of a few dozen Stentor cells. In response, most of the organisms contracted rapidly at first, but as the taps continued every 45 seconds, with a total of 60 bangs, fewer and fewer of Stentor contracted, showing that they had become accustomed to the signal.

Next, the Stentor cultures felt a weak pressure – in response to fewer of the organisms generally contracting – 1 second before a strong pressure. The tap pairs repeat every 45 seconds, which is about how long it takes Stentor to unfold again.

Over 10 trials with this process, the chance of the organisms contracting immediately after the slight pressure first increased and then decreased. “We saw this bump in the graph where the contraction rate basically goes up before it goes down. If you just present the weak tap by itself, you don’t see this,” says Gershman.

The researchers say this means Stentor has associated the weak tap with the larger tap, making it the first protist known to be able to master associative learning. “It raises the question of whether apparently simple organisms are capable of aspects of cognition 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. Other traces of this can still be seen in the way our neurons seem to be able to learn from their inputs in a way that doesn’t rely on modifying the synapses or connections between the 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, that required neurons, that required behavioral learning,” says Shashank Shekhar of Emory University in Atlanta, Georgia, who has shown that Stentor can gather in short-lived groups to feed more efficiently.

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

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