Lecture
Это окончание невероятной информации про мемристор.
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learned, and restore memory when reminded.
The behavior of the mold Physarum polycephalum - This single-celled organism is capable of devising nontrivial ways of navigating mazes, solving geometric puzzles, and even being part of a simple cyborg robot. For one of the papers on this mold, its authors even received an Ig Nobel Prize in cognitive science.
Slime mold is able to remember what it has been taught, forget the lessons, and restore the memory of them when reminded. Japanese scientists not only observed this in an experiment but also built a model of it. Tetsu Saigusa and Toshiyuki Nakagaki of Japan's Hokkaido University reported in the pages of Physical Review Letters how they discovered the mold's ability to learn. If the temperature and humidity are lowered for 10 minutes every hour, three times in a row, then an hour later the cell will freeze for 10 minutes in anticipation of unfavorable conditions — even if in reality the humidity and temperature remain optimal. Over time, the mold forgets what it was taught. But if it is reminded of the unpleasantness by lowering the humidity and temperature again, the memory returns: an hour later, the mold again freezes in anticipation of the worst, like a professional lawyer who always prepares for the worst.
Physarum Polycephalum experimental maze solving (from Nakagaki et al. [17])
The presence of memory in the mold was not entirely unexpected. Earlier maze experiments had shown that at each new junction, the mold remembers which direction its tendrils turned at previous junctions. What surprised scientists was specifically the ability to learn, forget, and restore memory upon a reminder. The Japanese researchers put forward a hypothesis to explain what was happening. According to it, within the giant cell of Physarum polycephalum there is a vast number of diverse biochemical "oscillators," each with a different period. Some oscillators speed up the mold's movement, others slow it down. But since they all run out of sync with each other, the cell crawls at a more or less constant speed.
But when external conditions change, the processes that speed up movement shut down: in cold and dry conditions, they consume too many resources. In the organism, only the oscillators that slow the pace remain active. This state persists for a while, and the slime slows down regardless of whether fate deals it further blows. Then another round of harsh conditions follows, and after a while the mold forgets the imposed rhythm. This hypothesis has one significant drawback: it does not explain how the mold restores a forgotten rhythm when reminded by a change in conditions.
A fundamentally new simulation model of the mold is an electrical circuit that you could even solder together yourself, given one exotic component — a memristor. Your only problem would be finding this very memristor. Otherwise, the circuit is a series oscillating circuit with losses. Why an oscillating circuit specifically? The point is that the circuit needed a source of oscillations. There's no getting around the resistor, since in the real world every element has some active resistance. The memristor was connected in parallel with the capacitor, since any other connection either produced nothing or made the model inadequate to the real behavior of the mold. The circuit's parameters were optimized and fitted to the behavior of the real mold.
Fig. Electrical circuit model of the intelligence of the slime mold Physarum polycephalum (A) and its behavior in the case of irregular (B) and regular (C) changes in external conditions (Pershin et al, 2008, arXiv.org)
The memristor has two main states — one with high conductivity and one with low conductivity. An applied voltage can switch it from one state to the other, which is equivalent to a change in the external conditions for the mold's development — temperature and humidity. A positive voltage corresponds to favorable conditions, a negative one to unfavorable conditions. The system's response — the speed at which the slime crawls in the model — is the voltage across the memristor. If a single negative pulse is applied to the input of the circuit — placing the mold in unfavorable conditions — the circuit responds with rapidly decaying oscillations, whose period is determined by the parameters of the LC circuit. In this case, the memristor's conductivity is at its highest. But when the input receives a series of pulses whose period roughly matches the period of the circuit's free oscillations, then at some point the voltage across the memristor reaches a threshold value, and it quickly switches to a high-resistance state. This is how the mold remembers a series of unfavorable experiences. Oscillations in a circuit that has "remembered" the charge decay much more slowly.
All it takes is to "remind" the circuit with one more negative pulse, and it immediately responds with a series of slowly decaying oscillations — it "remembers" what it was taught. Moreover, this memory turns out to be very long-lasting — it is restored even after a hundred periods of the LC circuit's oscillations. In the mold, the memory is shorter.
Of course, this is only a model. Exactly how the memory of a real mold works is still unknown. It is not clear what sets the rhythm in this cell, how it remembers signals, and what makes it forget them. However, general knowledge of the mold's physiology suggests that the role of the memristor is played by a system of channels that transport cellular fluid within the elastic outer membrane. The movement of the myxamoeba is essentially a constant back-and-forth flow of intracellular fluid, with a slight net bias forward. Continuous vibrations of actin-myosin protein fibers create a pressure difference that drives the fluid. The fibers themselves are attached to the cell membrane, which in turn is subject to friction against the surface over which the myxamoeba crawls. As a result, the frictional force partially counterbalances the fibers' response to the fluid's motion, and the cell as a whole moves forward. The fluid in the cell is present as ectoplasm and endoplasm. The former has a higher viscosity than the latter, and the endoplasm makes its way through channels in the ectoplasm, like mercury through glass. But when the pressure on the ectoplasm at some point exceeds a threshold value, its viscosity drops sharply — one could say that another channel opens up in the ectoplasm. Clearly, the more channels that are open, the faster the myxamoeba can move. And the number of open channels is, in turn, determined by the history of the movement — in exactly the same way that the memristor's resistance is determined by the history of the voltage across its terminals. Incidentally, if one grasps the physics of how a memristor works, one can notice a deep similarity between the two models.
Часть 1 Memristor: The Fourth Passive Element of Electrical Engineering
Часть 2 - Memristor: The Fourth Passive Element of Electrical Engineering
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