Dogs are used to responding to stimuli
A simple single-celled organism appears to be able to predict that one thing will follow another – suggesting that this type of associative learning arose long before multicellular nervous systems.
A study recently in bioRxiv revealed that a single-celled organism without brain or neurons is capable of an advanced form of learning.
The simplest form of learning, known as habituationconsists of gradually reducing how much you respond to a repeated and harmless stimulus, such as a smell or a noise. This is common in all animals and has also been observed in plants.
Much more difficult is learning to link different types of stimuli or events, and predicting that one is linked to another. This associative learning was most famously demonstrated when Ivan Pavlov associated the sound of a bell with the offering of food to dogs, resulting in animals salivating when they heard the bell ring.
Now, a group of scientists at Harvard University have used similar conditioning experiments to show that the Stentor coeruleusa brainless single-celled organism, also appears to be capable of associative learning.
As details, these amazing organisms live in ponds and swim using rows of hair-like cilia along their sides. Up to 2 millimeters long, they are giants among single-celled life.
“When they are fixed, they limit themselves to feeding by filtration. If they are disturbed, they quickly contract into a sphere. During this time, they cannot feed, so it is ecologically advantageous not to respond in this way very often, unless necessary”, explained, to the same magazine, the leader of the research, Sam Gershman.
The team used this behavior to investigate whether the Stentor can learn. First, they hit the bottom of Petri dishes containing cultures with a few dozen Stentor cells. In response, most organisms contracted quickly at first, but as the tapping continued every 45 seconds for a total of 60 beats, they contracted less and less, showing that they had become accustomed to the signal.
Then, the cultures Stentor they felt a weak blow — to which, in general, fewer organisms respond by contracting — 1 second before a strong blow. The pairs of blows were repeated every 45 seconds, which is approximately the time the Stentor takes time to extend again.
Over the course of 10 attempts at this process, the probability of the organisms contracting immediately after the weak beat first increased and then… decreased.
Scientists say this means Stentor associated the weak hit with the stronger hit, making him the first known protist capable of mastering associative learning.
“This raises the question of whether apparently simple organisms are capable of aspects of cognition that we normally associate with much more complex, multicellular organisms with a brain”says Gershman.
Furthermore, it suggests a very ancient evolutionary origin of associative learning, hundreds of millions of years before the emergence of multicellular nervous systems.
