Slime molds are fascinating: these are unicellular organisms that can display complex behaviors such as finding the shortest path in a maze and developing an efficient transportation network. Actually each of these two findings generated a high-impact publication (Science and Nature) and an Ignobel prize. In the latter study, the authors grew a slime mold on a map of Japan, with food on the biggest cities, and demonstrated that it developed a transportation network that looked very much like the railway network of Japan (check out the video!).

More recently, there was a recent PNAS paper in which the authors showed that a slime mold can solve the “U-shaped trap problem”. This is a classic spatial navigation problem in robotics: the organism is behind a U-shaped barrier and there is food behind it. It cannot navigate to the food using local rules (e.g. following a path along which the distance to the food continuously decreases), and therefore it requires some form of spatial memory. This is not a trivial task for robots, but the slime mold can do it (check out the video).

What I find particularly interesting is that the slime mold has no brain (it is a single cell!), and yet it displays behavior that requires some form of spatial memory. The way it manages to do the task is that it leaves extracellular slime behind it and uses it to mark the locations it has already visited. It can then explore its environment by avoiding extracellular slime, and it can go around the U-shaped barrier. Thus it uses an externalized memory. This is a concrete example that shows that (neural) representation is not always necessary for complex cognition. It nicely illustrates Rodney Brook’s famous quote: “The world is its own best model”. That is, why develop a complex map of the external world when you can directly interact with it?

Of course, we humans don’t usually leave slime on the floor to help us navigate. But this example should make us think about the nature of spatial memory. We tend to think of spatial memory in terms of maps, in analogy with actual maps that we can draw on a paper. However, it is now possible to imagine other ways in which a spatial memory could work, in analogy with the slime mold. For example, one might imagine a memory system that leaves “virtual slime” in places that have been already explored, that is, that associates environmental cues about location with a “slime signal”. This would confer the same navigational abilities as those of slime molds, without a map-like representation of the world. For the organism, having markers in the hippocampus (the brain area involved in spatial memory) or outside the skull might not make a big difference (does the mind stop at the boundary of the skull?).

It is known that in mammals, there are cells in the hippocampus that fire at a specific (preferred) location. These are called “place cells”. How about if the meaning of spikes fired by these place cells were that there is “slime” in their favorite place? Of course I realize that this is a provocative question, which might not go so well with other known facts about the hippocampus, such as grid cells (cells that fire when the animal is at nodes of a regular spatial grid). But it makes the point that maps, in the usual sense, may not be the only way in which these experimental observations can be interpreted. That is, the neural basis of spatial memory could be thought of as operational (neurons fire to trigger some behavior) rather than representational (the world is reconstructed from spike trains).