Lecture
Это окончание невероятной информации про мемристор .
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and Toshiyuki Nakagaki of Japan's Hokkaido University wrote in the pages of Physical Review Letters how they discovered slime's capacity for learning. If the temperature and humidity are lowered for 10 minutes each 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 trouble by again lowering the humidity and temperature, the memory returns: an hour later the mold will again freeze 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 fork the mold remembers which way its branches turned at previous forks. What surprised the scientists was precisely the ability to learn, forget, and restore memory upon a reminder. The Japanese researchers proposed a hypothesis to explain what was happening. According to it, inside the giant cell of Physarum polycephalum there is a huge number of diverse biochemical "alarm clocks" with a wide variety of periods. Some of these clocks 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 were speeding up movement shut down: in cold and dry conditions they consume too many resources. Only the clocks that slow the pace remain active in the organism. This state persists for some time, and the slime's pace stays slow regardless of whether fate deals it further blows. Then another bout 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 of it by a change in conditions.
A fundamentally new simulation model of the mold takes the form of an electrical circuit that you could even solder together yourself, given one exotic component — a pyzastor (ferroelectric memristor). Your only problem would be finding this pyzastor. 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 away from a resistor, since in the real world every element has some resistance. The pyzastor 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 tuned to match the behavior of real mold.
Fig. Electrical model circuit 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 pyzastor has two basic states — one of high conductivity and one of low conductivity. It can be switched from one state to the other by an applied voltage, which is equivalent to a change in the external conditions for the mold's development — temperature and humidity. Positive voltage represents favorable conditions, negative represents unfavorable ones. The system's response — the speed at which the slime crawls in the model — is the voltage across the pyzastor. If a single negative pulse is applied to the circuit's input — placing the mold in unfavorable conditions — the circuit responds with rapidly damped oscillations with a period determined by the parameters of the LC circuit. In this case the pyzastor's conductivity is at its highest. But when a series of pulses arrives at the input, with a period roughly equal to the period of the circuit's free oscillations, then at some point the voltage across the pyzastor reaches a threshold value, and it quickly switches to a high-resistance state. This is how the mold remembers a series of unfavorable impressions. Oscillations in a circuit that has "remembered" a charge decay much more slowly.
It's enough to "remind" the circuit with one more negative pulse for it to immediately respond with a series of slowly decaying oscillations — to "recall" what it had been taught. Moreover, the memory turns out to be very long-lasting — it is restored even after a hundred periods of the LC circuit's oscillations. The mold's 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, or what makes it forget them. However, general knowledge of the mold's physiology suggests that the role of the pyzastor is played by a system of channels transporting cellular fluid within an elastic shell. The movement of the myxamoeba is essentially a constant back-and-forth flow of intracellular fluid with a slight bias forward. Continuous vibrations of actin-myosin protein fibers create a pressure differential that pushes the fluid. The fibers themselves are connected to the cell membrane, which in turn experiences friction against the surface over which the myxamoeba crawls. As a result, the friction force partially counterbalances the fibers' reaction to the fluid's motion, and the cell as a whole moves forward. The fluid in the cell exists as ectoplasm and endoplasm. The former has a higher viscosity than the latter, and the endoplasm makes its way through channels in the ectoplasm, much 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. It is clear that the more channels are open, the faster the myxamoeba can move. And the number of open channels is ultimately determined by the history of movement — in exactly the same way that the pyzastor's resistance is determined by the history of the voltage across its terminals. Incidentally, if one grasps the physics of how the pyzastor works, one can notice a deep similarity between the two models.
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NOMFET
Organic field-effect transistor
Часть 1 Memristor: Theory and Implementation of the Fourth Passive Electrical Element
Часть 2 See also - Memristor: Theory and Implementation of the Fourth
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