Recent studies showcase that single-celled organisms, devoid of brains or neurons, can exhibit forms of advanced learning.

The most basic learning type is called habituation, where an organism gradually reduces its response to non-threatening stimuli like sounds or smells. This process is observed across various species, including animals and even plants. Habituation has also been demonstrated in some protists—complex eukaryotic cells that typically exist as unicellular organisms. For example, the trumpet-shaped blue spot stentor and slime mold poly skull.

Moving beyond habituation, associative learning evaluates how organisms connect multiple stimuli and predict events based on previous experiences. This concept was famously demonstrated by Ivan Pavlov, who showed that dogs could associate the sound of a bell with food, resulting in salivation at the mere sound.

Recently, Sam Gershman from Harvard University and his team utilized similar conditioning experiments to reveal that Stentor, a freshwater organism, is also capable of associative learning.

The stunning Stentor lives in freshwater habitats, using fine hair-like structures called cilia to navigate. Measuring up to 2 millimeters in length, it stands out among unicellular organisms. One end features a holdfast for surface attachment, while the opposite end has a trumpet-like feeding structure.

“When attached to a surface, Stentor primarily filters food from water. However, when disturbed, it retracts into a ball, making it temporarily unable to eat, which presents an ecological advantage,” Gershman notes.

To study Stentor’s learning capabilities, researchers conducted experiments by tapping the bottom of a Petri dish containing Stentor cultures. Most organisms initially contracted rapidly in response to loud taps, but this behavior diminished with repeated stimulation, indicating a form of habituation.

In subsequent experiments, the researchers introduced a weak tap followed by a strong tap. Typically, few microorganisms responded to the weak stimulus alone. However, the paired taps, executed every 45 seconds, gave Stentor sufficient time to re-extend, demonstrating associative learning over multiple trials.

After conducting over 10 trials, researchers noted an increased and then decreased probability of contraction following the weak tap, indicating a nuanced form of learning. “The observed pattern in the contraction rate signals a depth of cognitive ability previously underestimated in such simple organisms,” asserts Gershman.

The findings suggest that Stentor may be the first known protist capable of associative learning by linking weak and strong stimuli. “This raises compelling questions about the cognitive abilities of seemingly simple organisms compared to more complex multicellular entities,” adds Gershman.

Moreover, these revelations imply that associative learning could have ancient evolutionary roots, predating the emergence of complex nervous systems by millions of years. It echoes the way neurons in multicellular organisms learn through stimuli, drawing connections independent of synaptic changes, as described in previous research (here).

“The capacity of a single cell to perform complex tasks, once thought exclusive to organisms with brains, is quite remarkable,” concludes Shashank Shekhar of Emory University, who demonstrated Stentor’s ability to aggregate in short-lived groups for more efficient feeding.

“I suspect that other unicellular organisms may also possess similar associative learning capabilities,” he remarks. “Once such abilities arise, they may become prevalent across various organisms.”

While the mechanisms behind Stentor’s learning remain to be fully understood, Gershman posits that it may involve specific receptors allowing calcium influx, altering the internal voltage response to touch and thus influencing contraction behavior. Over time, repeated stimulation may modify these receptors, functioning as molecular switches to curtail contraction.