Has there ever been an experiment performed that demonstrated a form of 'spatial memory' in a unicellular organism? I'm imagining something analogous to the classic 'rat in maze' experiments, but obviously on a much smaller scale. Possibly even something as simple as following a concentration gradient, but choosing to go either upstream or downstream based on some sort of prior reward in a similar circumstance.
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$\begingroup$ Several (if not all) motile microorganisms, such as E.coli, will follow chemical traces (usually food sources) but I am not aware of them having any memory in the sense they will simply follow it until the trace disappears. Another well know example are the immune system cells like this neutrophil chasing a bacteria: youtube.com/watch?v=NN9EHZugwl8. $\endgroup$– cagliari2005Jul 3, 2015 at 4:19
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1$\begingroup$ The closest I can get is slime moulds. They are not strictly unicellular, but they even have the ability to adapt to periodic stimulus, showing clearly that they are capable of both memory-like behaviour and some form of pattern recognition. $\endgroup$– busukxuanJul 4, 2015 at 15:33
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To build on busukxuan's answer, there are a lot of single-celled organism responses that kind of resemble memory:
Slime mold uses an externalized spatial 'memory' to navigate in complex environments - essentially, a slime mold leaves behind a trail, which it then avoids, allowing it to avoid where it has traveled in the past.
If you apply a periodic stimulus to Physarum, it will anticipate the next stimulus: Popular, openly accessible summary Paywalled journal article.
If the amoeba Dictyostelium sees an increasing cAMP gradient, it will follow that gradient - and then be less sensitive to a reversal in the gradient: Cellular memory in eukaryotic chemotaxis
In fact, even bacteria have some memory - they can figure out whether or not they are going up a chemoattractant gradient by making a comparison with the signal they observed a few seconds ago: Temporal comparisons in bacterial chemotaxis
